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GAW Report No. 185

Guidelines for the Measurement of Methane and

Nitrous Oxide and their Quality Assurance

WMO/TD - No. 1478

© World Meteorological Organization, 2009

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WORLD METEOROLOGICAL ORGANIZATION GLOBAL ATMOSPHERE WATCH

GUIDELINES FOR THE MEASUREMENT OF METHANE AND NITROUS OXIDE AND THEIR

QUALITY ASSURANCE

WMO/TD-No. 1478

Table of Contents

1. INTRODUCTION................................................................................................................................................................... 1 2. TERMINOLOGY AND DEFINITIONS................................................................................................................................... 2 2.1 Rationale ............................................................................................................................................................................... 2 2.2 Glossary ................................................................................................................................................................................ 2 2.3 Recommendations ................................................................................................................................................................ 9 2.3.1 Precision, Repeatability and Reproducibility............................................................................................................. 9 2.3.2 Accuracy ................................................................................................................................................................... 9 2.3.3 Uncertainty of a measurement.................................................................................................................................. 9 2.3.4 Degrees of freedom.................................................................................................................................................. 9 2.4 References.......................................................................................................................................................................... 10 3. DATA QUALITY OBJECTIVES FOR CH4.......................................................................................................................... 10 3.1 Previous WMO/GAW data quality objectives for CH4 ......................................................................................................... 10 3.2 Analytical precision ............................................................................................................................................................. 10 3.3 Relevant range of CH4 mole fractions................................................................................................................................. 11 3.4 Uncertainty of the standard scale........................................................................................................................................ 11 3.4.1 Summary of the NOAA CH4 standard scale ........................................................................................................... 11 3.4.2 Hierarchy of standards............................................................................................................................................ 11 3.4.3 Uncertainties........................................................................................................................................................... 12 3.5 Uncertainty of ambient CH4 measurements ........................................................................................................................ 12 4. DATA QUALITY OBJECTIVES FOR N2O ......................................................................................................................... 12 4.1 Relevant range of N2O mole fractions................................................................................................................................. 13 4.2 Instrumental precision ......................................................................................................................................................... 13 4.3 Uncertainty of the standard scale........................................................................................................................................ 13 4.3.1 Summary of the NOAA N2O calibration scale......................................................................................................... 13 4.3.2 Hierarchy of standards............................................................................................................................................ 13 4.3.3 Uncertainty limits .................................................................................................................................................... 14 4.4. Uncertainty of ambient N2O measurements........................................................................................................................ 14 5. REPRESENTATIVENESS CRITERIA FOR TRACE GAS MEASUREMENTS.................................................................. 15 6. MEASUREMENT GUIDELINES FOR CH4 ......................................................................................................................... 16 6.1 Introduction ......................................................................................................................................................................... 16 6.2 Procedural........................................................................................................................................................................... 17 6.2.1 Scope and application ............................................................................................................................................ 17 6.2.2 Summary of method ............................................................................................................................................... 17 6.2.3 Interferences........................................................................................................................................................... 18 6.2.4 Personnel requirements.......................................................................................................................................... 18 6.2.5 Facility requirements............................................................................................................................................... 18 6.2.6 Safety Requirements .............................................................................................................................................. 18 6.2.7 Apparatus ............................................................................................................................................................... 18 6.2.8 Handling high-pressure cylinders ........................................................................................................................... 19 6.2.9 Calibration............................................................................................................................................................... 19 6.2.10 Analysis procedures ............................................................................................................................................... 20 6.2.11 Calculations ............................................................................................................................................................ 20 6.2.12 Quality control and selection of data....................................................................................................................... 20 6.2.13 Data management .................................................................................................................................................. 20 6.2.14 Reporting data ........................................................................................................................................................ 20 6.2.15 Corrective action..................................................................................................................................................... 21 6.2.16 Maintenance ........................................................................................................................................................... 21 6.3 Quality control ..................................................................................................................................................................... 21 6.3.1 Internal quality control (QC) checks and frequency................................................................................................ 21 6.3.2 Intercomparison experiments ................................................................................................................................. 21 6.3.3 Reporting intercomparison results .......................................................................................................................... 21

7. MEASUREMENT GUIDELINES FOR N2O......................................................................................................................... 21 7.1 Introduction ......................................................................................................................................................................... 22 7.2 Procedural........................................................................................................................................................................... 23 7.2.1 Scope and application ............................................................................................................................................ 23 7.2.2 Summary of method ............................................................................................................................................... 23 7.2.3 Interferences........................................................................................................................................................... 23 7.2.4 Personnel requirements.......................................................................................................................................... 24 7.2.5 Facility requirements............................................................................................................................................... 24 7.2.6 Safety requirements................................................................................................................................................ 24 7.2.7 Apparatus ............................................................................................................................................................... 24 7.2.8 Handling of high-pressure cylinders ....................................................................................................................... 25 7.2.9 Calibration............................................................................................................................................................... 25 7.2.10 Analysis procedures ............................................................................................................................................... 26 7.2.11 Calculations ............................................................................................................................................................ 26 7.2.12 Quality control and selection of data....................................................................................................................... 26 7.2.13 Data management .................................................................................................................................................. 26 7.2.14 Data reporting ......................................................................................................................................................... 26 7.2.15 Corrective action..................................................................................................................................................... 27 7.2.16 Maintenance ........................................................................................................................................................... 27 7.3. Supplementary information for the initiation of N2O measurements ................................................................................... 27 7.3.1 Gas chromatographic instrumentation.................................................................................................................... 27 7.3.2 Selection of peripheral devices............................................................................................................................... 27 7.3.3 Quality of the carrier gas......................................................................................................................................... 27 7.3.4 Columns.................................................................................................................................................................. 27 7.3.5 Valve configurations ............................................................................................................................................... 28 7.3.6 Software for GC control and peak processing ........................................................................................................ 28 7.3.7 Conditioning of a new N2O system ......................................................................................................................... 28 7.4. Quality control ..................................................................................................................................................................... 28 7.4.1 Internal quality control (QC) checks and frequency................................................................................................ 28 7.4.2 Intercomparison experiments ................................................................................................................................. 29 7.4.3 Reporting of intercomparison results ...................................................................................................................... 29 8. CONCEPTS FOR AUDITS AT WMO/GAW SITES ............................................................................................................ 29 8.1 Introductory remarks ........................................................................................................................................................... 29 8.2 General concepts ................................................................................................................................................................ 29 8.3 Documents for system and performance audits of atmospheric trace gas measurements at WMO/GAW sites ................ 30 8.3.1 SOP for audits ........................................................................................................................................................ 30 8.3.2 Questionnaire for audits.......................................................................................................................................... 31 9. REFERENCES.................................................................................................................................................................... 32 Annex A: Abbreviations and Acronyms...................................................................................................................................... 35 Annex B: List of Contributors and Reviewers ............................................................................................................................ 36

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1. INTRODUCTION

The comparability of data obtained for individual atmospheric parameters at stations around the globe (operated by different institutions or laboratories) is a prerequisite for the use of archived data from the WMO/GAW data centres. Therefore quality assurance (QA) is of major importance for the GAW network. In view of this, special emphasis has been devoted to this issue within GAW during recent years. This is in particular reflected by the WMO-GAW Reports No. 142 "Strategy for the Implementation of the Global Atmosphere Watch Programme (2001 - 2007)", No. 156 "Addendum for the Period 2005 - 2007 To the Strategy for the Implementation of the Global Atmosphere Watch Programme (2001 - 2007), GAW Report No. 142", and No. 172 "WMO Global Atmosphere Watch (GAW) Strategic Plan: 2008 – 2015". As noted in Report No. 142 (p. 51), the earlier concept of Quality Assurance Project Plans (QAPjP) has been abandoned in favour of individual documents defining Data Quality Objectives (DQOs) and Standard Operating Procedures (SOPs).

With respect to QA, major tasks listed in the updated Strategic Plan (GAW Report No. 156, p. 16) are, amongst others:

• To report the international terminology related to QA/QC of GAW measurements • To develop measurement guidelines and, when appropriate, SOPs • To develop guidelines for GAW station system audits

Section 2 deals partly with the ISO (International Organization for Standardization) terminology and provides definitions for terms relevant to trace species measurements. The terms are defined in a glossary. Some of the terms and related ISO recommendations might be less familiar to the reader, and in - some cases - might even require a revision of the usage of terms that are still widely accepted within the scientific community.

As foreseen by the GAW strategy, DQOs for methane and nitrous oxide, both important greenhouse gases, have been developed and were approved by the Scientific Advisory Group for Greenhouse Gases (SAG GG). The goals set out by the DQOs are detailed in sections 3 (CH4) and 4 (N2O).

Ways to achieve these goals are addressed in sections 6 and 7, which contain "Measurement Guidelines" for the two gases. Moreover, section 8 describes the concept of audits at GAW stations.

In WMO/GAW Report No. 142, the development of Standard Operating Procedures (SOPs) is requested for all parameters for which no SOP has existed so far. In the case of CH4 and N2O, the measurements require a complex analytical procedure, which may vary in several of its details from one laboratory or station to another. In view of this, the SAG GG felt uncomfortable with the concept of an SOP (as given in GAW Report 142, p. 51), which thoroughly defines techniques and steps of actions to be followed. Therefore the SAG GG suggests using the term "Measurement Guidelines (MG)" instead. This is in line with the terminology used in the Addendum to the Strategic Plan (GAW Report No. 156, p.16). Consequently, in order to adhere to the recommendation of the SAG GG, sections 6 and 7 of this report are referred to as "Measurement Guidelines". These form part of the general GAW quality assurance concept.

It is noted that the MGs for CH4 and N2O within the GAW Network are mainly intended for use at stations where measurements of these parameters have recently been added to the programme or will be in the foreseeable future. The MGs are not meant to interfere with procedures at stations with experienced personnel and where pioneering work on CH4 and N2O has been performed for many years.

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2. TERMINOLOGY AND DEFINITIONS 2.1 Rationale

The evaluation and characterisation of data obtained from measurements made within GAW involve a number of statistical parameters and specific terms to characterise the data quality. Several of these terms (e.g. precision), are frequently used with different meanings by different people. Efforts for standardization have been made in the past, involving contributions from a number of international organizations, and are coordinated under the umbrella of ISO (www.iso.ch).

With the aim of ensuring the comparability and consistency of measurements, the WMO/GAW Strategic Plan [5] recommends adoption and use of internationally accepted methods and vocabulary to deal with measurement uncertainty as outlined in various ISO publications [1-3, 5, 6]. Since each term should have the same meaning for all of its users, efforts are called for to familiarize all individuals involved in WMO/GAW and the associated scientific community with the relevant terminology. The following glossary is intended as a step in this direction. GAW members are encouraged to use these terms in their own publications and to suggest their use when reviewing manuscripts of others. 2.2 Glossary

Note: This glossary is also available on the WMO/GAW homepage, where it will be updated depending on future input from its users. Accuracy of measurement Closeness of the agreement between the result of a measurement and a true value of the measurand [1]. NOTES 1) Accuracy and precision are qualitative concepts and should be avoided in quantitative expressions. Accuracy cannot be expressed as a numerical value. 2) The term 'accuracy of measurement' should not be used for trueness of measurement and the term 'measurement precision' should not be used for 'accuracy of measurement'. 3) Accuracy is inversely related to the combination of systematic error and random error that occur in a single measurement result. 4) Accuracy is concerned with the difference between a single measurement result and a true (or the conventional true) value [7]. Assigned value (of a quantity) Synonym for conventional true value [1] NOTE The term 'assigned value' will be preferred in this document. Audit 1) Performance audit: Voluntary check for conformity of a measurement where the audit criteria are the data quality objectives (DQOs) for the specific parameter. In the absence of formal DQOs, an audit will at least involve ensuring the traceability of measurements to the Reference Standard [5]. 2) System audit: More generally defined as a check of the overall conformity of a station with the principles of the GAW system [5]. Bias (of a measuring instrument) Systematic error of the indication of a measuring instrument [1] NOTE The bias of a measuring instrument is normally estimated by averaging the error of indication over an appropriate number of measurements [1]

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Calibration Set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system and the corresponding values realized by standards [1]. NOTE The result of a calibration permits either the assignment of values of measurands to the indications [of a measuring instrument] or the determination of corrections with respect to indications [1]. Central Calibration Laboratory (CCL) Within the WMO/GAW network, laboratory responsible for maintaining the standard scale for the species under consideration. Combined standard uncertainty (of a measurement) Standard uncertainty of the result of a measurement when that result is obtained from values of a number of other quantities, equal to the positive square root sum of terms, the terms being variances or covariances of these other quantities weighted according to how the measurement result varies with changes in these quantities [3]. Comparability Mean difference between two (or more) sets of measurements, which should be within predefined limits. NOTES 1) For the WMO/GAW network, the dispersion of measurements of the same standard by different laboratories (within-network comparability). 2) See also trueness. 3) The term may also be used with reference to measurements by different laboratories in different places. 4) Moreover, the term may be used to describe the difference between a measurement of a species in a discrete sample and an averaged in-situ measurement for a period that includes the time in which the discrete sample was collected. 5) In the case of significantly different variances of two sample sets, the mean difference may not be meaningful. The Wilcoxon-Mann-Whitney test can be used to test for statistical significance. Conventional reference scale (reference-value scale, standard scale) for particular quantities of a given kind, an ordered set of values, continuous or discrete, defined by convention as a reference for arranging quantities of that kind in order of magnitude [1] NOTES 1) The scale is based upon a number of primary standards and a measurement procedure to interpolate other values. 2. Within WMO/GAW, the conventional reference scale refers in particular to the calibration scale used within the GAW network. In the case of CO2, CH4, N2O and CO, this scale is implemented as a family of gas cylinders maintained at the CCL (NOAA). Conventional true value (of a quantity) Value attributed to a particular quantity and accepted as having an uncertainty appropriate for a given purpose [1]. NOTE: In this report a conventional true value is preferably called assigned value. Coverage factor Numerical factor used as a multiplier of the combined standard uncertainty in order to obtain an

expanded uncertainty [3].

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NOTE: Coverage factors, k, are typically in the range 2 to 3. Data Quality Objectives (DQOs) Qualitative and quantitative statements that clarify the objectives of observations, define the appropriate type of data, and specify tolerable levels of uncertainty. DQOs will be used as the basis for establishing the quality and quantity of data needed to support decisions (adapted from [6]). NOTE Decisions in this context include scientific decisions (e.g. significance testing of trends) as well as decisions of political or societal dimension. Error (of measurement) Result of a measurement minus a true value of the measurand [1] NOTE Since a true value cannot be determined, in practice a conventional true value is used [1] Expanded uncertainty (of a measurement) Quantity defining an interval about the result of a measurement that may be expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the

measurand [3]. NOTE In practice the interval is obtained by multiplying the combined standard uncertainty by a coverage factor, the choice of which is based on the level of confidence desired. In other words, expanded measurement uncertainty is usually a stated multiple of the standard measurement uncertainty of a measurement result [1]. Laboratory standard standard of highest rank at an individual laboratory or station traceable to the WMO/GAW standard scale Measurand particular quantity subject to measurement [1] Measurement set of operations having the object of determining a value of a quantity [1]. Measurement Guideline (MG) Written instruction that provides basic information on various issues related to the measurement of a specific quantity. It usually covers major aspects ranging from instrumental set-up to obtaining final data and metadata of known quality. NOTE MGs permit more flexibility for the way the measurements are conducted than SOPs. Therefore MGs are used in the case of complex systems that can be differently set up and operated in practice. Measurement procedure Set of operations, described specifically, used in the performance of particular measurements according to a given method [1].

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NOTES 1) A measurement procedure is usually recorded in a document that is sometimes itself called a 'measurement procedure' and is usually in sufficient detail to enable an operator to carry out a measurement without additional information [1]. 2) Within WMO/GAW, a measurement procedure can also be referred to as a standard operating procedure (SOP) or can be replaced by measurement guideline (MG). Precision Degree of internal agreement among independent measurements made under specific conditions [2]. NOTE Precision is a qualitative concept. Related quantitative concepts are repeatability and reproducibility. Precision must not be confused with accuracy. Primary standard Standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity [1]. NOTES 1) In particular with respect to trace gases, standard with assigned mole fraction based on absolute

calibration, i.e. gravimetric or equivalent method. 2) Within WMO/GAW, the primary standards for CO2, CH4, N2O and CO are maintained at NOAA. Quality assurance All planned and systematic actions necessary to provide adequate confidence that a product, process or service will satisfy given requirements for quality [4] Quality control Operational techniques and activities that are used to fulfil given requirements for quality [4]. Random error Result of a measurement minus the mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions [1] NOTES 1) Random error is equal to error minus systematic error. 2) Because only a finite number of measurements can be made, it is possible to determine only an estimate of random error. Reference-value scale Synonym for conventional reference scale Reference standard Standard, generally having the highest metrological quality available at a given location or in a given organization, from which measurements made there are derived [1] NOTE 1) The term 'reference standard' was used in the WMO/GAW Reports 142 [5] and 156 to indicate the WMO/GAW primary standards or the organisation that maintains them. In the new GAW Strategic Plan, the term primary standard is preferred, in keeping with the ISO definition. Repeatability (of results of measurements) Closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement [1].

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NOTES 1) Repeatability conditions include: the same measurement procedure; the same observer; the same measuring instrument, used under the same conditions; the same location; repetition over a short period of time [1]. 2) Repeatability may be expressed quantitatively in terms of the dispersion characteristics of the results [1]. 3) For the dispersion characteristics, information on the level of confidence should be provided, e.g. '±1 standard deviation' or 'alpha = 0.95'. Reproducibility (of results of measurements) Closeness of the agreement between results of measurements of the same measurand carried out under changed conditions of measurement [1]. NOTES 1) A valid statement of reproducibility requires specification of the conditions changed [1]. 2) The changed conditions may include: principle of measurement; method of measurement; observer; measuring instrument; reference standard; location; conditions of use; time [1]. 3) Reproducibility may be expressed quantitatively in terms of the dispersion characteristics of the results [1]. 4) For the dispersion characteristics, information on the level of confidence should be provided, e.g. "±1 standard deviation" or "alpha = 0.95". Result of a measurement

Value attributed to a measurand, obtained by measurement [1] NOTE A complete statement of the result of a measurement includes information about the uncertainty of measurement. Secondary standard Standard whose value is assigned by comparison with a primary standard of the same quantity [1]. NOTE For trace gas measurements within WMO/GAW, this refers to a standard (natural air or synthetic gas mixture) with mole fractions for target species that are obtained from comparisons made by the Central Calibration Laboratory with primary standards kept at its laboratory. (Measurement) standard Material measure, measuring instrument, reference material or measuring system intended to define, realize, conserve, or reproduce a unit or one or more values of a quantity to serve as a reference [1] NOTE 1) In the case of trace gas measurements, generally any gas (natural air or synthetic gas mixture) with assigned mole fractions traceable to an accepted standard scale. 2) Within WMO/GAW, the standard scales for CO2, CH4, N2O and CO are maintained by NOAA. Standard Operating Procedure (SOP) A written document that details the method for a programme, operation, analysis, or action with thoroughly prescribed techniques and steps, and that is officially approved as the method for performing certain routine or repetitive tasks [6] NOTE In WMO/GAW, the term is understood to refer to a document that describes the measurement and quality assurance processes involved in obtaining the value of a quantity in as much detail as necessary to be able to achieve stated data quality objectives. For a similar term, see ' measurement procedure'.

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Standard scale Synonym for conventional reference scale Standard uncertainty (of a measurement)

Uncertainty of the result of a measurement expressed as a standard deviation [3]. Surveillance cylinder Synonym for target cylinder (target gas) Systematic error Mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions minus a true value of the measurand [1]. NOTES 1) Systematic error, and its causes, can be known or unknown. Correction should be applied for systematic error, as far as it is known. 2) Systematic error is equal to error of measurement minus random error [1]. 3) Systematic error may be constant or depend on the value of the measurand. 4) For a measuring instrument, see also bias. Target cylinder (target gas) Cylinder containing natural air or a synthetic gas mixture with assigned trace gas mole fractions that is treated as an (unknown) sample in a sequence of analyses. NOTE The target cylinder, or target gas, is used for quality control measures. In the hierarchy of standards the target gas is usually on the same level as a working standard. Tertiary standard Standard calibrated at the CCL by comparison with secondary standards NOTE For trace gases, it is the CCL (NOAA) tertiary standards that are used as laboratory standards by the World Calibration Centres (WCC), GAW stations and participating laboratories. Traceability Property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties [1]. NOTES 1) The unbroken chain of comparisons is called a traceability chain [1]. 2) Institutes should maintain as direct a path as possible between their laboratory standards and the CCL. Transfer standard Standard used as an intermediary to compare standards [1]. NOTES 1) For trace gas measurements within WMO/GAW, this refers in particular to gas (natural air or synthetic gas mixture) for use at different locations with assigned mole fraction of one or more trace species resulting from comparisons with laboratory standards by an approved laboratory, such as the WCC. 2) The term transfer standard is often used in the same sense as travelling standard. Here, 'transfer standard' is reserved for purposes when the standard scale is actually transferred.

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Travelling standard Standard intended for transport between different locations [1], i.e. for making comparisons between standards at different locations. NOTES 1) For trace gas measurements within WMO/GAW, this refers in particular to gas (natural air or synthetic gas mixture) for use at different locations with an assigned mole fraction of one or more trace species resulting from comparisons with laboratory standards by an approved laboratory, such as the WCC. 2) For stable gases, this is usually a high-pressure cylinder calibrated at the CCL or WCC for the purpose of audits or round-robin experiments. True value (of a quantity) Value consistent with the definition of a given particular quantity [1]. NOTE This is a value that would be obtained by a perfect measurement. True values are by nature indeterminate [1]. Trueness Closeness of agreement between the mean of a series of test results (calculated value) and the conventional true value (expected value). NOTES 1)Trueness cannot be expressed as a numerical value. 2) Trueness is inversely related to systematic error only. 3) The term 'trueness of measurement' should not be used for accuracy of measurement. 4) Trueness is an important parameter in comparisons, e.g. to determine if measurements at different sites are on the same scale. Uncertainty of measurement Parameter, associated with the result of a measurement, that characterises the dispersion of the values that could reasonably be attributed to the measurand [1]. NOTES 1) The parameter may be, for example, a standard deviation (or a given multiple of it), or the half-width of an interval having a stated level of confidence [1]. 2) The concept of 'uncertainty' is explained in detail in GUM [3]. In practice the term 'error (measurement error)' seems to be often used when actually 'uncertainty' is meant. An error is viewed as having two components, a random and a systematic component [3]. As further stated in this reference, 'error' is an idealised concept and errors cannot be known exactly. 'Error' and 'uncertainty' are not synonyms, but represent completely different concepts. World Calibration Centre (WCC) Part of the GAW network, responsible for quality assurance measures for one or more components, by way of audits and intercomparisons. NOTE For each component under consideration, the WCC refers to the calibration scale maintained by the CCL designated by GAW. Working standard Standard that is used routinely to calibrate or check material measures, measuring instruments or reference materials [1].

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NOTES 1) For stable gases, any gas (natural air or synthetic gas mixture) with assigned mole fractions of one or more trace species obtained from comparisons with the laboratory standard(s) of an individual laboratory or station, or from comparisons with transfer standards provided by another laboratory, such as the WCC. 2) For measurement of stable gases, usually gas cylinders denoted as working standards are employed as calibration cylinders for routine measurements. 2.3 Recommendations

In the following, some of the terms defined above are put into context for the practitioner who struggles with switching from the classical notion of 'systematic and random error' to the modern understanding of 'values, error, and uncertainty'. The reader is encouraged to study Figure D.1 and Figure D.2 in the GUM [3]

2.3.1 Precision, Repeatability and Reproducibility Use the term precision only in relative terms, e.g., to indicate that method X is more

precise (produces less spread among the results) than method Y. For repeated observations of the same analyte (sample) with the same instrumentation under unchanged conditions, use the term

repeatability to quantify the spread, e.g., the gas chromatograph X allows determination of methane in a given flask with repeatability of 0.1 % (1 standard deviation). To compare observations of the same analyte (sample) using different instrumentation/methodology or observations made at significantly different times, e.g. on different days, use the term reproducibility to quantify the spread, e.g., the dry mole fraction of methane in sample X was determined with a reproducibility of 0.5 % (1 standard deviation) using three independent instruments, namely two different GC-FIDs, and a GC-MS.

2.3.2 Accuracy Use the term accuracy only in relative terms, e.g., to indicate that method X is more

accurate (produces less bias) than method Y. To quantify the deviation of an instrument or analyses from an expected true value, use the terms 'deviation' or bias, e.g., instrument X is biased (or: deviates) by -1.5 % in comparison to the reference instrument Y.

2.3.3 Uncertainty of a measurement To express the uncertainty of (a) measurement (i.e., the degree to which a measured

result is unknown), use the terms standard uncertainty (to express the uncertainty in terms of 1 standard deviation), combined standard uncertainty (the positive square root of the sum of a number of terms contributing to the uncertainty), and expanded uncertainty (similar to, but not strictly identical to a confidence interval; obtained by multiplying a combined standard uncertainty with a coverage factor). It is recommended to express a measurement result (e.g. of methane in an air sample) in the following way: x = (1793+/-8) ppb (dry mole fraction, k=2, v=3), where k is the coverage factor selected (k=2 is roughly equivalent to expressing a 95 % confidence interval), and v is the number of degrees of freedom.

Additionally, more information should be provided (see Chapter 7 of reference [3]). In

particular, the way in which the standard uncertainty for each input quantity has been evaluated (see Chapter 4 of reference [3]) and the way in which the combined standard uncertainty for the output quantity has been determined (see Chapter 5 of reference [3]), should be indicated.

2.3.4 Degrees of freedom A simple application of the number of degrees of freedom is its use as denominator when

computing the experimental variance of a set of independent observations (to get an unbiased estimate of the variance), instead of the number of independent observations (see paragraph 4.2.2 and 4.2.6 in reference [3]). The number of degrees of freedom, v, for the mean of a series of independent repeated analyses is simply the number of analyses minus 1.

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For a least-squares fit of m parameters to n data points, v = n - m, as can be seen in paragraph G.3.3 in reference [3] (to be taken into account when computing the variance of the residuals departures from the fit). The determination of the number of degrees of freedom can, however, be more complex. The Welch-Satterthwaite formula [3] provides an estimate of the effective degrees of freedom, v_eff.

2.4 References [1] ISO Publications, International vocabulary of basic and general terms in metrology, 2nd edition, International Organization for Standardization (Geneva, Switzerland) (1993). The abbreviation of this title is VIM. [2] ISO Publications, ISO 3534-1, Statistics - Vocabulary and symbols - Part 1: Probability and general statistical terms, International Organization for Standardization (Geneva, Switzerland) (1993). [3] ISO Publications, Guide to the expression of uncertainty in measurement, International Organization for Standardization (Geneva, Switzerland), ISBN 92-67-10188-9, 110 p. (1995). The abbreviation of this title is GUM. Equivalent guide: American National Standard for Calibration - U.S. Guide to the Expression of Uncertainty in Measurement, ANSI/INCSL Z540-2-1997, NCSL International, Boulder, USA, 101 p. (1997). [4] ISO Publications, ISO 8402, Quality Management and quality assurance - Vocabulary, International Organization for Standardization (Geneva, Switzerland) (1994). [5] WMO (2001), Strategy for the Implementation of the Global Atmosphere Watch Programme (2001 - 2007), GAW Report No. 142, World Meteorological Organization, Geneva, Switzerland. [6] U. S. Environmental Protection Agency, EPA Quality System, Glossary, http://www.epa.gov/quality/glossary.htm (accessed 2006-11-30). [7] de Leer, E. (2006), personal communication.

3. DATA QUALITY OBJECTIVES FOR CH4

3.1 Previous WMO/GAW data quality objectives for CH4

Data quality objectives for CH4 have not been previously defined by GAW. The objectives listed in WMO/GAW Report No. 80 (Table 4-4) are insufficient for high-quality global monitoring of CH4, given the requirements of the scientific community. Therefore these preliminary objectives are superseded by the following, newly defined DQOs.

3.2 Analytical precision

The relative repeatability (1 standard deviation) of a gas chromatograph with a flame ionization detector (FID) for determining CH4 mole fractions in ambient air is typically better than ±0.3% (±5 ppb). A value of at most ±0.2% (±3 ppb) should be the goal for all GAW stations. With high-quality equipment and chromatographic gases (carrier, H2, and oxidizer), a precision corresponding to a relative standard deviation of ±0.08% (±1.4 ppb) can be achieved (see e.g., Cunnold et al., 2002).

Note: Here all quantitative expressions of repeatability or reproducibility refer to a

dispersion of ± 1 standard deviation (s.d.). If an approximately 95% confidence interval is of interest, the value for 2 standard deviations should be used.

Precision (repeatability), expressed in terms of relative standard deviation, should be

determined from multiple, interspersed analyses of a gas of constant CH4 mole fraction (e.g.

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working standard) during routine operation. Repeatability of 0.2% (1 s.d.) is adequate to meet scientific demands.

3.3 Relevant range of CH4 mole fractions

Typical background values for atmospheric CH4 are in the range 1600 to 2000 ppb. Since FIDs are usually linear over five orders of magnitude, single point calibrations of the instrument are adequate for monitoring background CH4 abundance, once detector linearity has been verified. Using more standards to correct for non-linearity in FID response will make the calibration more accurate.

3.4 Uncertainty of the standard scale

3.4.1 Summary of the NOAA CH4 standard scale The NOAA04 CH4 scale is based on 16 gravimetrically-prepared standards in the nominal

range 300-2600 ppb (Dlugokencky et al., 2005). Prior to 2004, the CH4 scale was based on a pair of standards purchased in 1983. Samples measured relative to the NOAA04 scale give CH4 mole fractions that are 1.24% greater than the previous scale.

The NOAA04 scale is derived from a single per cent level standard using three dilution

techniques; agreement among the three techniques was within experimental errors. Standards were prepared in 5.9 L aluminium cylinders. NOAA04 standards were prepared using Scott-Marrin zero air. Although CH4 was not found in the zero air within our detection limits at the time (about 5 ppb), subsequent analysis of the full range of standards revealed 5 ppb CH4 in the zero air. A 5 ppb CH4 correction was therefore applied to the final assigned values of the gravimetrically-prepared standards.

Analysis of the gravimetrically-prepared CH4 scale was done on a HP6890 (currently Agilent 6890) GC with flame ionization detector using the following parameters: Column: 1/8" o.d., HayeSep Q, 2 m FID: HP, 150°C Oven: 40°C Carrier: N2 (35.5 mL min-1) H2 flow rate: 35 mL min-1 Oxidizer flow rate: 250 mL min-1 Sample loop: 5 mL Gas sampling valve: Valco 6-port Typical repeatability 0.5-1.5 ppb (1 s.d.)

The precision of this system with a single porous polymer column is about a factor of two better than previously-used two-column (silica gel pre-column and molecular sieve 5Å main column) chromatographic systems. Much of the improvement in precision comes from improved (more narrow) peak shape.

3.4.2 Hierarchy of standards The NOAA primary CH4 standard scale is based on the set of gravimetrically-prepared

standards briefly described in section 3.4.1. Secondary standards are 29-L aluminium cylinders filled with dry natural air at Niwot Ridge, CO, USA and are in the nominal range 1700-1900 ppb. Secondary standards are used to calibrate tertiary (working) standards for use in Boulder, at NOAA observatories, and at other GAW stations.

Each laboratory operating a GAW station should maintain a set of at least 3 tertiary standards as their highest level, laboratory standards. These should be calibrated by the CH4 CCL (NOAA, Boulder, Colorado) every 6 years, and they should be used only for infrequent calibrations of working standards. Maintaining 3 standards builds in redundancy in case one cylinder is accidentally blown down, it allows for long-term continuity of standards, and it allows some internal

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means to test for potential drift. Working standards at each laboratory can be either dried natural air compressed into high-pressure aluminium cylinders (preferable) or appropriately prepared synthetic gas mixtures.

3.4.3 Uncertainties Useful observed spatial gradients in background atmospheric CH4 can be as small as a few

ppb. Therefore, it is important that the CCL’s CH4 scale can be propagated with an uncertainty of <±1 ppb. A statistical analysis of CH4 calibrations in 2004 suggests that this goal of less than 1 ppb uncertainty on a CH4 calibration can be met by the CCL. 3.5 Uncertainty of ambient CH4 measurements

The uncertainty in an ambient CH4 measurement has contributions from: (a) Analytical repeatability. (b) Random or systematic air sample collection, handling, and storage effects. (c) Uncertainty in high-level standards. (d) Uncertainty in propagating the scale to the working standards used at a site.

Absolute accuracy in the CCL CH4 scale is less relevant than the ability of the CCL to propagate the scale to participating GAW laboratories/stations, as long as all laboratories are on the WMO scale. Scale offsets between labs will be assessed by intercomparisons of calibrated cylinders of natural or synthetic air circulated within a round-robin experiment, by intercomparisons as part of a field audit performed by the WCC-CH4, and by systematic intercomparisons of measurements of the same discrete samples with another lab. Comparison of, e.g., weekly, discrete samples measured in a central lab with in-situ measurements from the GAW site is a good quality assurance measure to check for internal consistency.

Systematic offsets revealed during intercomparisons will determine if different institutions are on the GAW CH4 standard scale.

With respect to the resulting uncertainty, the following Data Quality Objectives have been defined and approved by the SAG GG:

(i) The repeatability of CH4 measurements should be ≤ 2.0 ppb, and the reproducibility ≤ 3.0

ppb. (ii) The combined standard uncertainty for an ambient CH4 measurement calculated using

reproducibility (±3 ppb), uncertainty in high-level standards (±1 ppb), and ability of a lab to propagate high-level standards to working standards (±2 ppb) should be no larger than ±3.7 ppb. Using optimized modern analytical methods should give uncertainties on order ±2.2 ppb at the 66% confidence level.

(iii) The uncertainties defined above will determine the network or interlaboratory comparability of GAW CH4 measurements.

4. DATA QUALITY OBJECTIVES FOR N2O

The uncertainty associated with ambient N2O measurements can be separated into

contributions related to:

(a) Analytical repeatability (instrumental precision) and reproducibility. (b) Random or systematic air sample collection, handling, and storage effects (flask sampling). (c) Uncertainty associated with the standard scale. (d) Uncertainty in propagating the scale to the working standards used at a site. (e) Systematic analytical deficiencies, such as interference of coeluting compounds (CO2, SF6)

in the case of insufficient peak separation in the chromatogram.

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Data quality objectives for N2O have not been previously defined by GAW in the past. The objectives listed in WMO/GAW Report No. 80 (Table 4-4) are insufficient for high-quality global monitoring of N2O, given the requirements of the scientific community. Therefore the preliminary objectives are superseded by the following, newly defined DQOs.

4.1 Relevant range of N2O mole fractions

For basic calibrations of the analytical system and for intercomparisons, five (or better even six) different N2O mole fractions ranging between 290 and 350 ppb should be used. This will determine the response curve of the ECD for N2O. The response curves are quadric in some cases. For ambient measurements, the most important range is between 310 and 330 ppb, for which more stringent objectives are set out than for the upper and lower wings (see 4.4).

4.2 Instrumental precision

For most N2O systems, the repeatability (1 standard deviation) of the gas chromatographic method under ambient sampling is expected to be better than ± 0.3% (± 1 ppb). With high-quality equipment, a precision corresponding to ± 0.04% (± 0.15 ppb) can be achieved (see e.g. Prinn et al., 2000; Hall et al., 2007). Note: Here all quantitative expressions of repeatability or reproducibility refer to a dispersion of ± 1 standard deviation (s.d.). If an approximately 95% confidence interval is of interest, the value for 2 standard deviations should be used, and this should be clearly declared.

Precision (repeatability), expressed in terms of relative standard deviation, should be determined from multiple, interspersed analyses of a gas of constant N2O mole fraction (e.g. working standard) during routine operation. The target value, as driven by scientific requirements, is 0.1 ppb (0.03% at ambient levels). 4.3 Uncertainty of the standard scale

4.3.1 Summary of the NOAA N2O calibration scale The NOAA-2006 N2O scale was developed based on 13 gravimetrically prepared

compressed gas standards. The 2006 scale supersedes the 2000 scale, which was based on 17 standards (Hall et al., 2007). The 2006 scale is 0.19 ppb lower than the 2000 scale at 320 ppb N2O. Gravimetric standards with N2O dry air mole fractions ranging from 260-370 ppb were prepared in 5.9-L aluminium cylinders (Scott-Marrin Inc., Riverside, CA) from four ppm-level standards prepared from 99.9% N2O (Scott Specialty Gases). All gravimetrically prepared standards contain CO2 (360 - 380 ppm) and SF6 (1 - 10 ppt). Analysis was performed by gas chromatography with electron capture detection.

The NOAA-2006 N2O scale compares well with other scales used by the atmospheric

science community. The NOAA scale is within 0.75 ppb of that defined by 300 and 330 ppb NIST SRMs (no longer available), and within 0.2 ppb of the SIO-98 scale (Hall et al., 2007).

4.3.2 Hierarchy of standards At NOAA, primary standards are prepared gravimetrically as static serial dilutions of pure

compounds. Secondary standards are samples of free tropospheric air obtained at Niwot Ridge, Colorado, USA, for which concentrations are determined by reference to the primary standard curve. Secondary standards are used to calibrate tertiary standards for distribution to NOAA sites and laboratories. It is the NOAA tertiary standards that are used as laboratory standards by the World Calibration Centre (WCC) and any participating laboratories.

A set of laboratory standards with at least five different N2O mole fractions calibrated by

NOAA should be obtained by each GAW station and should serve as the station's highest-level standards. These are to be safeguarded, used only for infrequent calibrations of working standards or reference gas, and they should be recalibrated by NOAA every 3 years. Although N2O at ambient levels is normally stable in high-pressure cylinders, the assigned mole fractions should be

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checked by recalibration to minimise uncertainties, to insure cylinder contents are not fractionated when gas is extracted, and as a general quality assurance procedure. Working standards at each laboratory can be either appropriately prepared synthetic gas mixtures or dried ambient air compressed into high-pressure aluminium cylinders. Besides N2O, synthetic mixtures should contain atmospheric levels of N2, O2, and CO2 as a minimum. For the use at a GAW station these should be calibrated by comparison with the station's set of laboratory standards or an equivalent set of standards traceable to the NOAA scale.

4.3.3 Uncertainty limits The mean interhemispheric difference in N2O mole fraction is around 1 ppb and the pole-to-

pole difference is 2 ppb. These global differences are 0.3 - 0.6% of the recent mean mole fraction of N2O in the atmosphere, which requires not only high precision of measurements, but also high consistency among assigned values for standards. Ideally, the expanded uncertainty (coverage factor of 2) would be ± 0.1 ppb or better, but this may prove too difficult a goal to meet in the short term.

NOAA maintains its scale by analysing 13 primary standards annually, and five secondary

standards over the 260-350 ppb range twice monthly. Repeatability (1 standard deviation) normally varies between ± 0.02 and ± 0.06%. An analytical repeatability of ± 0.02% produces an uncertainty in predicting an unknown from a 5-standard curve of ca. ± 0.1 ppb near ambient values and ± 0.13 and . ± 0.15 ppb, respectively at 250 and 350 ppb. However, at present (year 2007), the reproducibility does not hold at 0.02% over the long term. Based on weekly analysis of a 313 ppb tertiary standard along with several others analyzed 1-2 years apart, the reproducibility is estimated to be ± 0.2 ppb. 4.4 Uncertainty of ambient N2O measurements

A small bias of the CCL N2O scale is not relevant. It is the ability of the CCL to propagate the CCL N2O scale (which is the designated scale for WMO/GAW) to participating GAW laboratories that is important, as long as all laboratories are on the WMO/GAW CCL scale. Scale offsets between labs will be assessed by

• Intercomparisons of calibrated cylinders of natural or synthetic air circulated within a round-

robin experiment • Intercomparisons as part of a field audit performed by the WCC-N2O • Systematic intercomparisons of measurements of the same discrete samples with another

lab.

Comparison of, e.g., weekly, discrete samples measured in a central lab with in-situ measurements from the GAW site is a good quality assurance measure to check for internal consistency.

Systematic offsets revealed during intercomparisons will determine if different institutions are on the GAW N2O standard scale.

With respect to the resulting uncertainty, the following Data Quality Objectives have been defined and approved by the SAG GG.

Target Data Quality Objectives are: Maximum standard uncertainty 0.1 ppb for the entire range of 290 to 350 ppb (cf. WMO/GAW Reports No. 161 and 168). Note: The range of 290 to 350 ppb, as selected, will make an instrument more stable than a narrow range of N2O standards, so long as uncertainties in assigned values for individual standards are comparable across the range.

Uncertainties (coverage factor = 1) larger than 0.5 ppb in the range 310 - 330 ppb, and 0.8

ppb for the upper and lower wings can be included in the database, but should be flagged.

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The above-defined intercomparison objectives will determine the network or interlaboratory comparability of N2O measurements. 5. REPRESENTATIVENESS CRITERIA FOR TRACE GAS MEASUREMENTS

For any time-series of measurements, it is important to ask, “what spatial and temporal scales do the data represent?” This is especially important when comparing observations with models, where “model-data mismatch” (errors introduced because the spatial and temporal resolution of the model is different from that of the data) must be assessed to properly estimate uncertainties in calculated gas fluxes. To answer this question, several related issues must be addressed:

(a) Are the data traceable to the WMO/GAW standard scale (maintained by NOAA, Boulder,

Colorado, USA for N2O and CH4)? (b) Are there quality assurance rules in place to insure the measurements are free of sampling

and measurement artefacts? The significance of these first two questions is largely addressed in these measurement guidelines.

(c) How removed is the sampling site from anthropogenic and natural trace gas fluxes? (d) What is the impact of local meteorology on the measurements? This can manifest itself in

many ways including upslope/downslope flow regimes at mountain sites, diurnal land-/sea-breeze at coastal continental sites, and potential pollution sources in one wind sector, but not in others.

(e) What is the frequency of sampling? To calculate time-averages (e.g., monthly means), sampling must be frequent enough.

(f) For low-frequency sampling of discrete samples, is there a sampling strategy used (i.e., are samples collected only under specific meteorological conditions)?

(g) Have the data been “selected” based on meteorological conditions, another species (e.g., radon-222 or CO), or trajectories after the measurements were made?

Determining the representativeness of data is necessary to insure it is used properly in scientific analyses.

GAW classifies sampling and measurement sites as global and regional. The WMO Global Atmosphere Watch (GAW) Strategic Plan: 2008-2015 (GAW Report No. 172) lists the essential characteristics of GAW Regional and GAW Global Stations in its boxes 9 and 10, respectively (p. 23). The following considerations are not meant to interfere with the definitions given there. They serve as complementary information that provides a somewhat broader view on the issues related to representativeness.

Global stations are situated in locations representative of large geographic areas, have low levels of anthropogenic pollutants, and are designed for continuous measurements of a broad range of atmospheric parameters over decades. It is important to note that global stations must sample air that is free of the effects of local and regional pollution for substantial periods of the year. Regional station measurements are usually representative of smaller geographic regions than global station measurements. Regional sites are often chosen so they are not affected by nearby sources of pollution such as vehicles, industry, or agricultural activities. Although these classifications may seem useful in assessing the representativeness of data from specific sites, they are only indicators of how extensive the measurement programmes at a given site are. Consider a station where trace gas variations are sometimes large and rapid, so air sampled at these times represent only short time and small spatial-scales. This site is likely close to emissions (or sinks), so concentrations vary according to the local meteorology. If this site has a wind sector that samples background air part of the time, it may be considered a GAW global station, if there are sufficiently large measurement programmes there. Careful filtering of the data based on wind direction can be used to select the data for background conditions. Even then it is difficult to assess what are regional signals compared to larger scale signals.

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Further information on the representativeness of measurements at a certain site can be obtained from comparisons of trace gas variations with neighbouring stations. In the case of smaller-scale variations, comparisons will only be meaningful over distances of less than a few hundred kilometres. However, if background air masses are sampled at remote locations that are several thousand kilometres apart, concurrent variations on longer time scales may indicate representativeness even on a hemispheric scale.

Often when data are compared with global model results, time-averages are used. In these cases, one must assess whether or not low frequency measurements capture signals on the time scales of interest. For example, if a goal is to calculate monthly mean mole fractions from weekly samples, will the uncertainties be so large as to make the monthly means scientifically useless? At sites that are far from emissions and sinks (e.g., Antarctic sites), weekly or twice-monthly sampling will give monthly means with reasonable uncertainties. At other sites, where background trace gas levels change significantly on synoptic time scales, as weather systems pass with a period of a few days to a week, weekly or higher sampling frequency is required to define monthly means. At these sites, quasi-continuous measurements are preferred, because high-frequency measurements provide smaller uncertainties on monthly means than low-frequency measurements, and modern 3-D models use “real” meteorology that can extract more information about emission rates and sinks using continuous measurements than with low-frequency discrete measurements.

6. MEASUREMENT GUIDELINES FOR CH4

With the aim of achieving uniformly high-quality data within the GAW network, development of Measurement Guidelines (MG) and, when appropriate, Standard Operating Procedures (SOP), was foreseen in the Strategy for the Implementation of the Global Atmosphere Watch Programme (2001 - 2007). In line with the goals defined in WMO/GAW Reports No. 142 and 156, this chapter contains guidelines for the measurement of atmospheric CH4. 6.1 Introduction

Basic information about CH4 and the scope of its global monitoring has been summarised in the Global Atmosphere Watch Measurements Guide (WMO/GAW Report No. 143) from which a portion of the following has been used as an introduction to the Measurement Guidelines. Importance

Methane (CH4) has natural (e.g., wetlands) and anthropogenic (e.g., rice agriculture, fossil fuel exploitation, landfills, ruminant animals, and biomass burning) sources, and it contributes about 20% of the direct enhanced greenhouse effect. The atmospheric burden of CH4 has increased by a factor of 2.5 since 1750, and, although the rate of increase has slowed over the past two decades, the causes for this slowing are still not completely known. Methane reacts with hydroxyl radical, thus affecting the oxidizing capacity of the atmosphere. In the stratosphere, it is a sink for Cl atoms and, as a result, impacts ozone there. For these reasons, CH4 is included in the recommended measurement programme for global GAW stations. Siting requirements

Measurements of CH4 concentrations can be performed at GAW Global, Regional or Contributing stations, with the location being chosen such that it is regionally representative and is normally free of the influence of significant local pollution sources. In particular, background levels of CH4 can be representatively determined at GAW global stations. Moreover, measurements at regional stations may give useful new insights into the global CH4 budget. Methods of measurement and sampling frequency

A gas chromatograph equipped with flame ionization detector (FID) is used to separate and detect CH4 in ambient air. The FID signal is recorded by an integrator or computer, which uses an algorithm to quantify peak response (reported as heights and areas). Sample air is fed either directly from an intake to the analytical system (in situ) or by collecting discrete samples of air in

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flasks that are returned to a central laboratory for analysis. The typical frequency for discrete sampling is 1 per week. Such sampling requires relatively stable atmospheric composition. If substantial variability is expected, only in-situ (quasi-continuous) analyses are recommended.

A new generation of CH4 instrumentation based on spectroscopic techniques has become available since about 2005. GAW participants are encouraged to evaluate the performance and communicate their experience to other stations and laboratories.

Mole fraction calculation

CH4 abundance in ambient air is determined by comparing the peak response of a sample to that of a standard of known CH4 abundance. The quantity measured is “dry-air mole fraction”, expressed in nmol mol-1 (or ppb). The sequence of working standards (W) and ambient samples (A) may vary between one standard per hour and sequences such as "WW / AA / WW …" or "W / AA / W …". Optimal working conditions should be determined individually at each site. Quality control and selection of data

The data are screened for periods when the instrument was not operating optimally and for periods when the levels do not reflect background conditions. The check of instrumental performance should involve the parameters baseline noise, stability of the retention time as well as peak start and peak end. Moreover, the quality of the target gas results should be evaluated. Ancillary measurements

These include meteorological data (wind speed and direction, temperature, dew point and pressure) and tracers to define background conditions (e.g., carbon monoxide). Archiving procedures

CH4 data are archived at the WMO World Data Center for Greenhouse Gases (WDCGG), operated by the Japan Meteorological Agency in Tokyo (http://gaw.kishou.go.jp/wdcgg/). The World Data Center for Greenhouse Gases Data Submission and Dissemination Guide (GAW Report No. 174) should be referred to for details. Application of results

GAW CH4 measurements are used primarily to constrain the global CH4 budget and calculate its radiative forcing. 6.2 Procedural

6.2.1 Scope and application The methods described in these MGs are intended for quasi-continuous monitoring of CH4

in ambient air, but they are also applicable to analyses of discrete samples.

6.2.2 Summary of method For CH4 measurements at GAW stations, gas chromatography (GC) with a flame ionization

detector (FID) is typically used. The analytical set-up can vary within a wide range depending on details such as type (manufacturer) of GC, chromatographic separation scheme, carrier gas (e.g., N2 or He), data acquisition, system control hardware and software, and peak integration system. Consequently the operating procedures for the individual systems will vary.

The following subsystems are integral parts of the analytical system: • Sampling system • Drying system • Gas handling • Gas chromatograph equipped with FID • Compressed gases • Data acquisition and control hardware and software

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• Peak integration software New analyzers for atmospheric measurements of CH4 based on optical methods match the

GC for repeatability, and preliminary tests suggest that they are also robust and suitable for field operation (status as of 2007). Although these instruments, which also measure water vapour, often come advertised as not needing calibration or sample drying, attendees at the 13th meeting of CO2 experts (Boulder, CO, USA, September, 2005; WMO/GAW Report No. 168) strongly recommend that the analyzers are calibrated routinely and that air samples are dried to a dew point of ~ -40 °C.

6.2.3 Interferences It is easy to devise chromatographic schemes that resolve CH4 from potential interfering

peaks. Samples must be sufficiently dried (dew point <-40°) to insure water does not affect column performance or dilute ambient sample relative to dry standards.

GCs for air analysis are often equipped with both FID and ECD. ECDs are often operated

with argon/methane mixtures as carrier gas, which is a potential source of contamination for CH4 measurements.

6.2.4 Personnel requirements Personnel operating this instrumentation should be detail-oriented and conscientious,

familiar with general principles of trace gas sampling, and they should have a detailed understanding of gas chromatographic systems. Access to someone when needed with good electronics and computer-programming skills, including database management, is also required.

6.2.5 Facility requirements Depending on national regulations for the handling of flammable gases (H2 for FID fuel),

special laboratory facilities may be required. In addition, a clean temperature-controlled (± 2 °C) environment should be maintained.

6.2.6 Safety Requirements

6.2.6.1 Compressed gases General safety rules for the handling of compressed gases, particularly H2, must be

respected. For details, check national regulations, gas company information bulletins, or perform a web search (keywords: compressed gas safety, general safety guidelines)

6.2.7 Apparatus The basic analytical tool is a gas chromatograph equipped with FID. CH4 is typically

separated isothermally with a porous polymer packed column (e.g., Porapak Q or HayeSep Q). Injected sample volumes are typically between 5 and 15 mL, depending on the volume of column used.

6.2.7.1 Installation requirements Minimum equipment shall consist of: Sample (and standard) handling system consisting of: • Intake mounted on a tower (≥10 m) upwind of buildings and other sources of contamination • Sample line running from intake to pump • Pump capable of 7 to 10 L min-1 flow rate (using contamination-free materials) • Drying system (e.g., cryogenic cooler or Nafion tube setup) • Stream selection valves (multi-position) to select sample and standard flows • Cylinder containing CH4 standard and regulator

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Gas chromatographic system consisting of: • Gas chromatograph with FID and pressure control of column head pressure • Sample valve (2-position) to inject sample onto chromatographic column • High-pressure cylinders of carrier gas, H2, and oxidizer of sufficient purity with regulators Data acquisition and control system consisting of: • Computer with auxiliary storage device • System control hardware and software • Data management software and strategy • Chromatographic integration software

6.2.8 Handling high-pressure cylinders General instructions given in GAW publications on trace gas measurements are applicable.

Read and follow all safety procedures for handling high-pressure gas cylinders. Details on standard gas cylinder handling have been compiled by Lang (1998). Read these instructions carefully. Major points are: (a) Because of their light weight and their ability to maintain the integrity of dry air, aluminium is

the most commonly used material for cylinders containing calibrated CH4 standard. (b) Pressure regulators should be clean and oil-free. Dedicate a regulator to each laboratory

standard gas cylinder. To install the regulator on the cylinder, follow basic cylinder/regulator installation procedures as indicated in the instructions sent with each regulator. After the regulator is attached to the cylinder, check for leaks.

(c) Flush high and low sides of a two stage regulator (pressurise and vent) after initial installation and before each use. Pressurise and vent the regulator at least four times. Allow the regulator to fully depressurise before re-opening the cylinder valve. At the end of the day, or when a series of measurements is complete, close the cylinder valve and leave the regulator pressurised (high side).

(d) The regulator should be conditioned with standard air for at least one week before use. Failure to condition the regulator may result in incorrect measurements.

6.2.9 Calibration

6.2.9.1 Calibration scale The world CH4 standard is maintained by NOAA (WMO/GAW Report No. 142). A

description of the CH4 scale as of 2004 is given in the above chapter on Data Quality Objectives.

6.2.9.2 Laboratory standards Three standards with different CH4 mole fractions calibrated by the CCL should be

maintained as a lab’s highest-level laboratory standards. These are to be safeguarded, used only for infrequent calibrations of working standards, and they should be recalibrated by NOAA every 6 years.

6.2.9.3 Working standards Working standards can be synthetic gas mixtures or dry (<10 ppm water vapour) natural air

(preferable) compressed into high-pressure aluminium cylinders. The matrix of synthetic mixtures should be closely matched to natural air with atmospheric levels of N2 and O2. Working standards used at GAW stations should be calibrated using the institution’s laboratory standards. It is suggested that working standards are intensively calibrated over ~1 year, e.g., with determinations made every other month, before they are installed at the station.

6.2.9.4 Instrument calibrations It is recommended that injections of standards and unknown samples should alternate.

Depending on the individual systems, running a standard after several (maximum 4) ambient samples can still be acceptable. Sample mole fractions are usually calculated assuming linear

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response with zero intercept by comparing the peak response of the sample with the mean of the bracketing standard injections.

6.2.10 Analysis procedures At stations, the method is an in situ real-time procedure, but it can also be used for analysis

of grab sample. At stations, air is drawn with a pump from a dedicated air intake or from a manifold of the main sample air intake. Tubing is made from PFA, stainless steel, or equivalent, and it is optionally supplemented with a dust filter.

6.2.11 Calculations Before CH4 mole fractions can be calculated, the detector signal (voltage) from an injection

must be digitized and integrated to determine the peak response (area and height) for that injection. Ambient CH4 mole fractions are then calculated by comparing the peak response from ambient air to the mean of the peak responses from the working standard bracketing the ambient samples. As an example, consider the following raw peak information: REF 2004 10 01 00 16 7 1.916550e+06 1.083089e+07 MLO 2004 10 01 00 24 1 1.942990e+06 1.096653e+07 REF 2004 10 01 00 31 3 1.917043e+06 1.082295e+07

Lines 1 and 3 are from injections of standard gas (REF), and line 2 is from an injection of ambient air at Mauna Loa Observatory, Hawaii (MLO). Following the 3-letter code is the date and time of the injection (UT), valve information, peak height, and peak area. The CH4 mole fraction from the injection at 0024UT on 01 OCT 2004 calculated from peak areas is: 1.096653x107/((1.083089x107 + 1.082295x107)/2) x 1739.8 nmol mol-1 = 1762.2 nmol mol-1. The same calculation with peak heights gives 1763.6 nmol mol-1, in good agreement with the calculation from peak areas. When peak shapes are Gaussian, either height or area calculations can be used. The choice will depend on which one gives better repeatability for the system’s chromatographic and integration schemes.

6.2.12 Quality control and selection of data Data obtained during periods when the analytical system was not operating optimally are

identified by placing a “flag” (code) in the database. Variations in values derived from the chromatography (e.g., peak response, retention time, peak width) or engineering data may be used to assess the validity of data. It is also typical for programme scientists to use their knowledge of local conditions to select the data for conditions that reflect sampling of well-mixed volumes of the atmosphere. Potential information to base these judgements on are wind speed and direction or another tracer such as CO. A different flag is used here than for editing to alert users that the measurement is valid, but it may not be useful if they are seeking data representative of background conditions.

6.2.13 Data management A carefully-designed data management strategy is necessary from the start of a

measurement programme to handle the large volumes of data that will be produced. Do not use a spreadsheet in place of a database! An example strategy is outlined in GAW Reports No. 129 and 150. As part of this strategy, researchers should have tools in place to look at their measurement results and engineering data at the start of the measurement programme.

6.2.14 Reporting data Ambient CH4 mole fractions from quasi-continuous measurements are typically reported to

the WDCGG as 1-hour (or 1/2-hour) averages in dry-air mole fraction, e.g., nmol mol-1 (often abbreviated ppb for “parts per billion” (109) by mole). The position of the time stamp relative to the averaging interval (begin, middle or end) should be indicated in the meta data.

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6.2.15 Corrective action Troubleshooting and repair will be done whenever the instrument performance exceeds the

data quality objectives (DQO) during "calibration" or one of the analytical system parameters falls outside normal operating limits or the limits recommended by the manufacturer. The nature of the problem will dictate the type of corrective action which will be taken.

6.2.16 Maintenance Because of the complexity of the gas chromatographic system, no general maintenance

schedule will be given as part of these MGs, except that all traps (sample drying and gas purification) should be exchanged on a regular basis. The instrument and system should be maintained according to the instrument manufacturer's instructions and according to experience gained with the particular system at a site or upon need. It is recommended to define specific limits for the system parameters of each system. The parameters and any exceedance of the limits should be monitored. Any work performed should be noted on a maintenance log sheet or in a dedicated notebook.

6.3 Quality control

6.3.1 Internal quality control (QC) checks and frequency The following are useful quality control measures for in-situ measurements at GAW

stations:

(a) Compare working standards with laboratory standards once per year. Furthermore, comparisons are necessary whenever working standards are exchanged.

(b) If discrete samples are collected at the site in addition to in-situ measurements, compare measurements of CH4 in those with the in-situ measurements from the hour in which the discrete sample was collected.

(c) Look at the measurements and engineering data weekly. Develop an algorithm that looks at engineering data (pressures, temperatures, flow rates, peak parameters (retention time, peak width, peak area), etc,) and alerts you to problems. This is facilitated by having a good data management strategy.

(d) Run a target cylinder, also known as a surveillance cylinder, of known CH4 mole fraction daily, but not at the same time every day (e.g., every 23 hours).

(e) Initiate comparisons of data with other laboratories.

6.3.2 Intercomparison experiments Establish an on-going intercomparison experiment (ICP) with another laboratory. The most

effective of these is when two labs measure air from the same discrete sample (flask-ICP). In an alternative approach, in situ measurements of CH4 from a site can be compared with a different lab’s measurement of CH4 in a discrete sample collected at the time of the hourly average.

Participate in GAW-sponsored round-robin intercomparisons organized by the CCL or WCC. Intercomparisons for atmospheric CH4 are usually combined with a CO2 intercomparison and are summarized by an external referee.

6.3.3 Reporting intercomparison results For audits, the results of the intercomparisons performed by the station operator will be

reported to the WCC-CH4, where a summary report will be prepared. In the case of round-robin experiments, the procedure will be determined depending on the overall organisational structure of the experiment.

7. MEASUREMENT GUIDELINES FOR N2O

With the aim of achieving a uniformly high data quality within the GAW network, the development of Measurement Guidelines (MG) and, when appropriate Standard Operating

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Procedures (SOP), is foreseen in the 'Strategy for the Implementation of the Global Atmosphere Watch Programme (2001 - 2007). In line with the goals defined in the WMO/GAW Reports No. 142 and 156, this chapter contains guidelines for the measurement of atmospheric N2O.

7.1 Introduction

Basic information about N2O and the scope of its global monitoring has been summarised in the Global Atmosphere Watch Measurements Guide (WMO/GAW Report No. 143) from which part of the following paragraphs have been taken as introduction for the subsequent Measurement Guidelines. Importance

Nitrous oxide increased from about 270 ppb in the pre-industrial era to about 320 ppb in 2005. Its average rate of increase in the past few decades, 0.8 ppb yr-1, indicates that emissions are greater than sinks by 30%. The major natural sources of N2O to the atmosphere are the ocean and soil processes; anthropogenic sources are agriculture, through use of nitrogen fertilizers, biomass burning, industrial processes (nylon and nitric acid production), and cattle feedlots. Two major sink processes are responsible for N2O removal from the atmosphere, both of them in the stratosphere: photolysis and reaction with electronically excited oxygen atoms (O(1D)). The contribution of nitrous oxide to radiative forcing is about 0.15 W m-2. Actions to mitigate N2O impacts on the environment require better understanding of the global N2O budget and the effects of changing land use and climate. Measurements of N2O from GAW stations provide the basis for this improved understanding (GAW Report No. 172, 50-51). Siting requirements

Measurements of N2O concentrations can be performed at GAW Global, Regional or Contributing stations, with the location being chosen such that it is regionally representative and is normally free of the influence of significant local pollution sources. In particular, background levels of N2O can be representatively determined at GAW global stations. Moreover, measurements at regional stations may give useful new insights into the global N2O budget. Methods of measurement and sampling frequency

A gas chromatograph equipped with electron capture detector (ECD) is used to separate and detect N2O in ambient air. The ECD signal is recorded by an integrator or computer which determines peak heights and areas. Collecting discrete samples of air in flasks is an alternative method of monitoring N2O. Flasks would be returned to a central laboratory for analysis. Typical sampling frequencies are weekly or bi-weekly sampling. Mole fraction calculation

N2O in ambient air samples is determined relative to one or more standards. The quantity measured is “dry-air mole fraction”, expressed in nmol mol-1 (or ppb). The sequence of working standards (W) and ambient samples (A) may vary between one standard per hour and sequences such as "W W / A A / W W …" or "W / AA / W …". Optimal working conditions should be determined individually at each site. Quality control and selection of data

The data are screened for periods when the instrument was not operating optimally and for periods when the levels do not reflect background conditions. The check of instrumental performance should involve the parameters baseline noise, stability of the retention time as well as peak start and peak end. Moreover, the quality of the target gas results should be evaluated. Ancillary measurements

These might include meteorological data, such as wind speed and direction, temperature, dew point and pressure. Moreover, tracers for defining background conditions (e.g. carbon monoxide) would be desirable.

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Archiving procedures N2O data are archived at the WMO World Data Center for Greenhouse Gases (WDCGG),

operated by the Japan Meteorological Agency in Tokyo (http://gaw.kishou.go.jp/wdcgg/). The World Data Center for Greenhouse Gases Data Submission and Dissemination Guide (GAW Report No. 174) should be referred to for details. Application of results

N2O data are used in climate models and for describing the nitrogen cycle in the atmosphere. 7.2 Procedural

7.2.1 Scope and application The method that is subject of these MGs is intended for quasi-continuous monitoring of

nitrous oxide (N2O) in ambient air and is also applicable to grab sample analyses. These instructions are written for a separate analytical system for N2O, although it is recognized that air samples are analyzed for N2O along with other gases. The principles of operation remain the same in either case.

7.2.2 Summary of method For N2O measurements at GAW stations, the method of choice is gas chromatographic

(GC) analysis with electron capture detector (ECD). The analytical set-up can vary within a wide range, depending on details such as:

• Type (manufacturer) of the GC • Columns used for the separation • Type of carrier gas (e.g. Ar/CH4 (95/5) or N2 with dopant) • Valve switching configuration (e.g. use of pre-column and backflush) • Type of make-up gas for the ECD, if any (e.g. Ar/CH4 (95/5) or doped N2) • Peak integration system Consequently the operating procedures for the individual systems will vary.

The following subsystems are integral parts of the analytical system:

(a) Air intake line according to specifications for trace gas measurements at global GAW stations.

(b) Drying unit and suitable pump. (c) Valve unit for control of sample and standards. (d) Gas chromatograph equipped with electron capture detector (ECD). (e) Cylinders with compressed gases for carrier gas, make-up gas for ECD (optional), and

standards. (f) Data acquisition and control hardware and software. (g) Peak integrating system.

7.2.3 Interferences Possible interferences can result in the N2O chromatogram from O2, H2O, CO2, SF6, and

some contaminants present in the sample. Depending on the analytical conditions used, the following techniques are applied in practice to address this problem:

(a) Removal of water cryogenically, with Nafion or with chemical adsorbent. (b) Proper selection of column packing material and GC parameters, so that the peaks are fully

resolved. (c) Use of a carrier gas such as 5% methane in argon or CO2 in N2 to minimize the effect of

CO2 on the chromatogram.

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(d) Removal of CO2 with a sodium-hydroxide-based trap (e.g., Ascarite or Ascarite II). Note: This approach is not recommended for the GAW global network.

(e) Correction of quantitatively determined interference during data processing. With this method problems can arise if gas mixtures are compared that do not have the same matrix (i.e. N2O plus CO2 and SF6). Note: This approach is not recommended for the GAW global network.

7.2.4 Personnel requirements Personnel operating this instrumentation should be familiar with general principles of trace

gas sampling and with details of gas chromatographic analyses.

7.2.5 Facility requirements Depending on national regulations for the handling of radioactive material (in this case

within the ECD), special laboratory facilities and/or labelling of the dedicated room may be required. In addition, a clean temperature-controlled (± 2 °C or better) environment should be maintained. Insure that laboratory temperature changes slowly; abrupt changes in room temperature, e.g., when a window-mounted air conditioner turns on, will affect the quality of the measurements. As the ECD is sensitive to temperature changes of the environment, attention has to be paid to the way the temperature control system is operating. As known from experience, a periodic cycling of ± 2 °C can cause artefacts.

7.2.6 Safety requirements

7.2.6.1 Compressed gases General safety rules for the handling of compressed gases must be respected. For details

there are various sources of information: National regulations, gas company information bulletins, or web search (keywords: compressed gas safety, general safety guidelines)

7.2.6.2 Electron capture detector Special instruction of the personnel and careful handling of the ECD are necessary. Details

are according to national regulations for the handling of radioactive material. Since these regulations vary from country to country, no general reference is given here (web search: regulations for the handling of radioactive material)

7.2.7 Apparatus The basic components of the apparatus consist of a gas chromatographic system equipped

with electron capture detector (ECD) and components for carrier gas control. N2O typically is separated isothermally with columns packed with porous polymers (such as Porapak Q or HayeSep Q) or zeolites (such as Molecular Sieve). Optionally employed, advanced techniques make use of a pre-column and back-flushing. Injected sample volumes typically vary between 3 and 15 mL, depending on the individual system.

7.2.7.1 Installation Minimum equipment shall consist of:

• Air intake line with pump (using contamination-free material, such as glass, PFA and/or stainless steel for the tubing)

• Unit for drying the air sample (e.g. cryogenic cooler or Nafion tube setup) • A series of valves and flow controllers for sample and standards as well as for sampling

and injection on to the separation column • Gas chromatograph • High-pressure cylinders for carrier gas • High-pressure cylinders or pressurised tanks for a suite of standards of different N2O mole

fraction (five levels recommended)

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• Contamination-free pressure regulators (ECD quality) for the cylinders • Data acquisition and integration system (hardware and software), optionally supplemented

with a chart recorder for additional visual inspection of the ECD signal

7.2.7.2 Configuration Gas chromatograph and peripheral devices: As per manufacturers' operation manuals.

Integrating system (computer based integrator preferred): As per manufacturer's operation manual. Strip chart recorder: Optional, run in parallel with integrator.

7.2.8 Handling of high-pressure cylinders General instructions given in GAW publications on trace gas measurements are applicable. Read and follow all safety procedures for handling high-pressure gas cylinders. Details on

standard gas cylinder handling have been compiled by Lang (1998). Read these instructions carefully. In the following some of the major points are listed.

Aluminium is the most commonly used material for cylinders containing gas mixtures (standards) for N2O measurements at ambient levels, although electropolished stainless steel cylinders also are used.

Pressure regulators: Cylinders with gas mixtures used for N2O measurements by GC-ECD require special high-purity (ECD type) regulators.

Dedicate a regulator for each standard gas cylinder. For the regulator installation follow basic cylinder/regulator installation procedures as indicated in the instructions sent with each regulator.

After the regulator is attached to the cylinder, check for leaks.

Both the high and low sides of a two stage regulator should be flushed (pressurised and vented) after initial installation and before each use. Pressurise and vent the regulator at least four times.

The regulator should be conditioned with standard air for at least one week before use. Failure to condition the regulator may result in incorrect measurements.

Following installation or, if the instrument is run intermittently, the regulator should be flushed (vented and pressurised) at least four times before use. Allow the regulator to fully depressurise before opening the cylinder valve. At the end of the day or when a series of measurements is completed the cylinder valve should be closed, leaving the regulator pressurised (high side).

7.2.9 Calibration

7.2.9.1 Calibration scale The world N2O standard is maintained by NOAA (WMO/GAW Report No. 142). For details

on the NOAA standards see http://www.esrl.noaa.gov/gmd/hats/standard/N2O_scale.html as well as Butler et al., 1998; Thompson et al., 2004; Hall et al., 2007. An updated description of the N2O scale as of 2006 is given in the above chapter on Data Quality Objectives.

7.2.9.2 Laboratory standards A set of standards with five different N2O mole fractions calibrated by NOAA ESRL should

be obtained by each station and should serve as the station's highest-level standards. These are to be safeguarded, used only for infrequent calibrations of working standards or reference gas, and they should be recalibrated by NOAA every 3 years.

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7.2.9.3 Working standards Working standards can be either appropriately prepared synthetic gas mixtures or dried

ambient air compressed into high-pressure aluminium cylinders. Besides N2O, synthetic mixtures should contain atmospheric levels of N2, O2, and CO2 as a minimum. In order to check the proper separation of N2O from SF6 in the chromatogram, the addition of this component is recommended. For the use at a GAW station, the working standards have to be calibrated by comparison with the station's set of laboratory standards or an equivalent set of standards traceable to the NOAA scale.

7.2.9.4 Instrument calibrations It is recommended that injections of standards and unknown samples should alternate.

Depending on the individual systems, running a standard after several (maximum 4) ambient samples can still be acceptable.

Calibration curves for non-linearity of the detector response should be performed regularly by way of five standards covering the nominal range 290 to 350 ppb. Preferably these checks should be made monthly, however, at least twice a year, or upon need if any analytical conditions were altered.

7.2.10 Analysis procedures When employed at stations, the method is assumed as a real-time procedure, but can also

be used for grab sample analyses. At stations, air is drawn with a pump from either a dedicated air intake or from a manifold of the main sample air intake. The tubing will consist of PFA, stainless steel or equivalent, optionally supplemented with a dust filter.

7.2.11 Calculations The ambient N2O mole fraction is calculated by comparing the integrated detector signal

obtained with ambient air to the signal obtained with the working standard(s) bracketing the ambient runs. For the determination of ambient mole fractions both peak area and peak height can be used. Which of them will yield the better results is depending on the individual GC system (shape of the N2O peak, precision achieved when using area or height).

7.2.12 Quality control and selection of data Data obtained during periods when the analytical system was not operating optimally are

identified by placing a “flag” (code) in the database. Variations in values derived from the chromatography (e.g., peak response, retention time, peak width) or engineering data may be used to assess the validity of data. It is also typical for programme scientists to use their knowledge of local conditions to select the data for conditions that reflect sampling of well-mixed volumes of the atmosphere. Potential information to base these judgements on are wind speed and direction or another tracer, such as CO. A different flag is used here than for editing to alert users that the measurement is valid, but it may not be useful if they are seeking data representative of background conditions.

7.2.13 Data management A data management strategy is necessary from the start of a measurement programme to

handle the large volumes of data that will be produced. An example strategy is outlined in GAW Reports No. 129 and 150. As part of this strategy, researchers as well as station staff should have tools in place to look at their measurement results and engineering data.

7.2.14 Data reporting The ambient N2O mole fractions from quasi-continuous measurements as calculated from

the sequence of analyses of ambient air and working standards are typically reported to the WDCGG as 1-hour (or 1/2-hour) averages in dry-air mole fraction, unit nmol mol-1 (often abbreviated ppb for "parts per billion" (109) by mole). The position of the time stamp relative to the averaging interval (begin, middle or end) should be indicated in the meta data.

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7.2.15 Corrective action Troubleshooting and/or repair will be undertaken whenever the instrument performance

exceeds the data quality objectives (DQO) during "calibration" or one of the instrumental system parameters falls outside normal operating limits or the limits recommended by the manufacturer. The nature of the problem will dictate the type of corrective action which will be taken.

7.2.16 Maintenance Because of the complexity of the gas chromatographic system, no general maintenance

schedule will be given as part of these MGs, except that all traps (sample drying and gas purification) should be exchanged on a regular basis. The instrument and system should be maintained according to the instrument manufacturer's instructions and according to experience gained with the particular system at a site or upon need. It is recommended that specific limits for the system parameters are defined for each system. The parameters and any exceedance of the limits should be monitored. Any work performed should be noted on a maintenance log sheet or in a dedicated notebook. 7.3 Supplementary information for the initiation of N2O measurements

In addition to the general information given in the preceding paragraphs, some details are discussed here that might be useful for those intending to initiate their first N2O measurement system. Generally, it is recommended that prospective N2O analysts make contact with well-established laboratories and GAW stations in order to learn from their experience. Moreover, the reader is referred to publications by authors from existing major networks and to respective web sites. Those who are less familiar with gas chromatography might also consider reading an introductory monograph about chromatography (e.g. Miller, 2005) and another one about chromatographic integration methods (e.g. Dyson, 1998).

7.3.1 Gas chromatographic instrumentation Although GC instrumentation is available from a number of manufacturers, care should be

taken to select an instrument that will fulfil the requirements of high-quality N2O measurements at ambient levels. Experience gained with GC instruments at GAW stations and associated laboratories should be considered. When discussing a possible experimental set-up with GC manufacturers, it should be kept in mind that the requirements of sensitivity, reproducibility, etc. within the GAW network might exceed those from usual applications.

7.3.2 Selection of peripheral devices All equipment foreseen for the new system should be of sufficient and proven quality.

Pressure regulators, tubings, fittings, gas sampling valves, etc. must be suited for high-purity gases.

7.3.3 Quality of the carrier gas High-quality carrier gas is a prerequisite for N2O analysis. Some gas companies term the

recommended degree of purity "ECD quality". Constant quality delivered by the gas supplier of choice is of importance. It is recommended to install traps for oxygen and moisture (e.g. commercially available cartridges) between carrier gas cylinder and GC instrument.

7.3.4 Columns Packed columns are still the columns of choice for N2O separation (status as of 2007). This

is an aspect with which GC manufacturers might be less familiar. It is noted that research efforts with the aim of introducing modern capillary columns for N2O separation would be appreciated within GAW.

For packed columns a widely used material is Porapak Q or HayeSep Q (80/100 mesh).

Columns are made using stainless steel tubes with typical lengths of about 3 m (main column) and 2 m (precolumn), and outer diameters between 1/8 and 3/16 inch.

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7.3.5 Valve configurations Although N2O can already be detected when using a simple 6-port gas sampling valve

system, it is recommended to use a somewhat more complex valve configuration with the aim of achieving better data quality. Aspects to consider in this context are (i) backflush and (ii) cut-off of the unwanted oxygen peak before it reaches the ECD. For details of possible valve configurations, the reader is referred to the literature and to web sites of GAW stations and experienced laboratories.

7.3.6 Software for GC control and peak processing Modern gas chromatographs are usually equipped with a software package delivered by

the GC manufacturer. The features available for instrument control and peak processing vary widely among these products, as may do their user-friendliness. In most cases special adaptations, depending on station requirements, will be necessary. Again, expertise gained at other GAW stations should be considered.

Individual parameters for peak integration, such as sensitivity for peak detection, data

sampling rate, etc. have to be carefully optimized at the individual instrument. With packed columns the data sampling rate can be considerably lower than usually chosen for capillary columns. Both peak area and peak height should be recorded and stored for data processing. It is recommended to store also the digitized raw chromatograms, because in the future it could be necessary to reintegrate the chromatograms by changing some of the integration parameters. Depending on peak shape, tailing, and other parameters either area or height might be favoured for obtaining the best data quality.

7.3.7 Conditioning of a new N2O system For a new system the optimal shape of the N2O peak and its separation from both CO2 and

SF6 have to be carefully determined. Basic parameters to vary are oven temperature, carrier gas pressure and flow as well as times for events, such as valve switching.

After commissioning of a new N2O system, the ECD performance generally improves with

time during continuous operation. Frequent analysis runs of air samples are recommended. The reproducibility should be checked regularly to document the process of stabilization.

7.4 Quality control

7.4.1 Internal quality control (QC) checks and frequency The frequency of internal comparisons of working standards with laboratory standards

should be once per year. Furthermore, comparisons are necessary whenever working standards are exchanged.

It is recommended to run analyses of samples of assigned N2O mole fraction from a "target cylinder" (sometimes denoted as surveillance cylinder) regularly, at least once per day, but preferably not at the same time every day (e.g., every 23 hours). This will enable an early detection of minor malfunctions of the analytical system. The target cylinder should preferably have a mole fraction different from the working standard(s) bracketing the sample analysis.

For additional quality control measures, a number of system parameters, such as pressures, temperatures, and flow rates should be checked regularly, preferably daily or weekly, but at least on a monthly basis. Supplementary chromatographic data delivered by peak integration systems, such as peak retention time, peak width, the ratio of area to height, also should be checked as indicators of consistent flow rate and temperature.

Check for any changes in chromatographic behaviour after connecting the system to a new

cylinder of carrier gas.

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Check for possible interference of CO2, SF6, and water. If correction techniques are applied, make sure that the parameters used are up-to-date.

If discrete samples are collected at the site, compare measurements of N2O in those with the in-situ measurements from the hour in which the discrete sample was collected.

7.4.2 Intercomparison experiments Intercomparisons between different laboratories and stations (notably twinning partners) are

strongly recommended as a quality assurance measure. It is vital that each laboratory have an on-going intercomparison experiment with another laboratory or station making the same measurements. It can occur by both laboratories making an N2O measurement in the same weekly discrete sample, or comparison of measurement of a discrete sample from one lab with an hourly average at the time the discrete sample was collected at another site.

Moreover, GAW stations should participate in round-robin experiments organized by the CCL or WCC. N2O intercomparisons will generally be based on five gas mixtures of different mole fraction ranging between about 290 and 350 ppb. Small round-robins will only involve GAW participants, and the results will be evaluated by the WCC-N2O. Extended experiments, which aim at including a large part of the atmospheric N2O community, will need an external referee.

7.4.3 Reporting of intercomparison results For audits, the results of the intercomparisons performed by the station operator will be

reported to the WCC-N2O, where a summary report will be prepared. In the case of round-robin experiments, the procedure will be determined depending on the overall organisational structure of the experiment.

8. CONCEPTS FOR AUDITS AT WMO/GAW SITES 8.1 Introductory remarks

Audits at WMO/GAW stations are part of the quality assurance (QA) measures requested by the Strategic Plan 2001 – 2007 (WMO/GAW Report No. 142). In that report distinctions are made between 'performance audits' and 'system audits' (for definitions see the glossary in chapter 2). Also in the new GAW Strategic Plan 2008 – 2015 (WMO/GAW Report No. 172) audits form an important part of the updated quality assurance concept.

Performance audits are primarily a check of the measurement and data quality of one or more parameters measured at a site. System audits have a wider focus, which are aimed at a general check of the station's facilities. Both types of audits are voluntary checks and therefore only conducted with the consent of the respective station. According to WMO/GAW Report No.142 the responsibility for auditing lies with the World Calibration Centres (WCC). In the case of CH4 and N2O these are WCC-Empa and WCC-N2O, respectively. In practice a station will be visited by one or more staff members of the WCC that performs the audit. An essential part of the quality control for CH4 and N2O consists of comparisons of WCC travelling standards with standards available at the station under consideration. As a final step of the audit, a report is prepared by the WCC, where the audit results are summarized and recommendations given. 8.2 General concepts

Conducting an audit involves different activities. First, the management of the audit has to be defined. In Figure 1 the process flow for management of the GAW audit programme is shown. Based on this management plan, audits are initiated and performed at GAW stations with the activities summarised in Figure 2.

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Figure 1 - Process flow for the management of the GAW audit programme (schematic diagram adopted from ISO19004)

8.3 Documents for system and performance audits of atmospheric trace gas

measurements at WMO/GAW sites Two documents have emerged from the work of two WCCs involved in audits for O3, CO,

CH4 and N2O and were jointly prepared by QA/SAC Switzerland, WCC-Empa and WCC-N2O. These are (i) "Standard Operating Procedure (SOP) for system and performance audits of atmospheric trace gas measurements at WMO/GAW sites" and (ii) "Audit questionnaire for system and performance audits of atmospheric trace gas measurements at WMO/GAW sites". They can be downloaded from the WMO/GAW homepage and from the web site of WCC-Empa (http://www.empa.ch/gaw).

Although tailored for the requirements of trace gas measurements, the documents are

structured in such a way that the overall lay-out will be applicable to most of the parameters under scrutiny during WMO/GAW initiated audits.

8.3.1 SOP for audits The SOP for system and performance audits of atmospheric trace gas measurements at

WMO/GAW sites outlines the overall framework of an audit and provides check lists for the entire procedure, ranging from pre-audit to post-audit actions. It consists of the following major parts:

• Preparation of audit at home • Audit procedures on site • Completion of audit • Summary rating for audited parameter

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Figure 2 - Overview of audit activities (schematic diagram adopted from ISO19004)

8.3.2 Questionnaire for audits The audit questionnaire for system and performance audits of atmospheric trace gas

measurements at WMO/GAW sites is intended for use during the audit. It serves for compiling information on the station in general and on details required for the parameters under consideration. The questionnaire consists of the following major parts:

• General audit information • Site and laboratory characteristics • Documentation of station • Organisation and personnel • Air inlet system • Instrumentation • Operation and maintenance • Standards • Data acquisition and processing • Data management and submission

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• Documentation • Actions to be taken after audit 9. REFERENCES Butler, J.H. et al., Trace gases in and over the West Pacific, East Indian Ocean during the El Nino-Southern Oscillation event of 1987, NOAA Data Report ERL ARL-16, Silver Spring, MD, 104 pp. (1988). Butler, J.H., et al., Tropospheric and dissolved N2O of the West Pacific and East Indian Oceans during the El Nino-Southern Oscillation events of 1987, J. Geophys. Res. 94, 14865-14877 (1989). Butler, J.H., et al., Chapter 5, Nitrous Oxide and Halocarbons, in: Climate Monitoring and Diagnostics Laboratory Summary Report No. 24, 1996-1997, D.J. Hoffman, J.T. Petersen, R.M. Rosson, editors, U.S. Department of Commerce, 166 pp. (1998). Crill, P.M., Butler, J.H., Cooper, D.J., Novelli, P.C., Standard Analytical Methods for Measuring Trace Gases in the Environment, in: Biogenic Trace Gases: Measuring Emissions from Soil and Water, Methods in Ecology, edited by P.A. Matson and R.C. Harris, University Press, Cambridge, U.K., 164-205 (1995). Cunnold, D.M. et al., In Situ Measurements of Atmospheric Methane at GAGE/AGAGE Sites During 1985-2000 and Resulting Source Inferences. J. Geophys. Res., 107(D14), doi: 10.1029/2001JD 001226 (2002). Dlugokencky, E.J., Myers, R.C., Lang, P.M., Masarie, K.A., Crotwell, A.M., Thoning, K.W., Hall, B.D., Elkins, J.W. and Steele, L.P.,. Conversion of NOAA Atmospheric Dry Air CH4 Mole Fractions to a Gravimetrically Prepared Standard Scale. J. Geophys. Res. 110, D18306, doi: 1029/2005JD006035 (2005). Dyson, N., Chromatographic Integration Methods. Second edition. The Royal Society of Chemistry (1998). Hall, B.D., Dutton, G.S., Elkins, J.W., The NOAA Nitrous Oxide Standard Scale for Atmospheric Observations, J. Geophys. Res., 112, D09305, doi: 10.1029/2006JD007954 (2007). Lang, P.M., Guidelines for Standard Gas Cylinder and Pressure Regulator Use, NOAA CMDL Carbon Cycle-Greenhouse Gases (1998) http://www. esrl.noaa.gov/gmd/ccgg/refgases/reg.guide.html (accessed June 2007). Miller, J.M., Chromatography: Concepts and Contrasts. Second edition. Wiley-Interscience, New Jersey (2005) Prinn, R.G. et al., A History of Chemically and Radiatively Important Gases in Air Deduced from ALE/GAGE/AGAGE, J. Geophys. Res., 105, 17751-17792 (2000). Thompson T.M., et al., Chapter 5, Halocarbons and Other Atmospheric Trace Species, in: Climate Monitoring and Diagnostics Laboratory Summary Report No. 27, 2002-2003, R.C. Schnell, A.M.Buggle, R.M. Rosson, editors, U.S. Department of Commerce, 174 pp. (2004). World Meteorological Organization, Global Atmosphere Watch, Report No. 80, "Report of the WMO Meeting of Experts on the Quality Assurance Plan for the Global Atmosphere Watch, Garmisch-Partenkirchen, Germany, 26-30 March 1992", WMO TD No. 513. World Meteorological Organization, Global Atmosphere Watch, Report No. 86, "Global Atmosphere Watch Guide", WMO TD No. 553. World Meteorological Organization, Global Atmosphere Watch, Report No. 97, "Quality Assurance Project Plan (QAPjP) for Continuous Ground-based Ozone Measurements", WMO TD No. 634. World Meteorological Organization, Global Atmosphere Watch, Report No. 129, "Guidelines for Atmospheric Trace Gas Data Management" (Ken Masarie and Pieter Tans), 1998, WMO: TD No. 907. World Meteorological Organization, Global Atmosphere Watch, Report No. 142, "Strategy for the Implementation of the Global Atmosphere Watch Programme (2001-2007)", WMO TD No. 1077.

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World Meteorological Organization, Global Atmosphere Watch, Report No. 143, "Global Atmosphere Watch Measurements Guide", WMO TD No. 1073. World Meteorological Organization, Global Atmosphere Watch, Report No. 150, "Updated Guidelines for Atmospheric Trace Gas Data Management" (Prepared by Ken Masarie and Pieter Tans), WMO TD No. 1149. World Meteorological Organization, Global Atmosphere Watch, Report No. 156, "Addendum for the Period 2005-2007 to the Strategy for the Implementation of the Global Atmosphere Watch Programme (2001-2007), GAW Report No. 142", WMO TD No. 1209. World Meteorological Organization, Global Atmosphere Watch, Report No 161, "12th WMO/IAEA Meeting of Expert on Carbon Dioxide Concentration and Related Tracers Measurements Techniques", Toronto, Canada, 15-18 September 2003, WMO TD No.1275. World Meteorological Organization, Global Atmosphere Watch, Report No.168, "13th WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracers Measurement Techniques", Boulder, Colorado, USA, 19-22 September 2005, WMO TD No. 1359. World Meteorological Organization, Global Atmosphere Watch, Report No.172, "WMO Global Atmosphere Watch (GAW) Strategic Plan: 2008 – 2015", WMO TD No. 1384. World Meteorological Organization, Global Atmosphere Watch, Report No.174, "World Data Centre for Greenhouse Gases Data Submission and Dissemination Guide", WMO TD No. 1416.

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ANNEX A

Abbreviations and Acronyms CCL Central Calibration Laboratory

CMDL Climate Monitoring and Diagnostics Laboratory, NOAA (now Global

Monitoring Division of the Earth System Research Laboratory, NOAA ESRL)

CSIRO Commonwealth Scientific & Industrial Research Organisation

DBMS Data Base Management Strategy

DQO Data Quality Objectives

ECD Electron Capture Detector

EMPA Swiss Federal Laboratories for Materials Testing and Research

ESRL Earth System Research Laboratory, NOAA

FID Flame Ionisation Detector

GAW Global Atmosphere Watch (WMO Programme)

GG or GHG Greenhouse Gases

GMD Global Monitoring Division (as part of NOAA ESRL)

ICP InterComParison experiment

ISO International Organization for Standardization

MG Measurement Guidelines

NIST National Institute of Standards and Technology

NMI National Metrology Institute, National Measurement Institute

NOAA National Oceanic and Atmospheric Administration (USA)

QA Quality Assurance

QC Quality Control

QA/SAC Quality Assurance/Science Activity Centre

SAG Scientific Advisory Group

SIO Scripps Institution of Oceanography

SOP Standard Operating Procedure

SRM Standard Reference Material

WCC World Calibration Centre

WDCGG World Data Center for Greenhouse Gases

WMO World Meteorological Organization

36

ANNEX B

List of Contributors and Reviewers Dr James H. Butler NOAA ESRL 325 Broadway Boulder, CO 80305 USA Dr Ed Dlugokencky NOAA ESRL 325 Broadway Boulder, CO 80305 USA Mr Angel J. Gomez-Pelaez Agencia Estatal de Meteorología (AEMET) La Marina, 20, Planta 6 38071 Santa Cruz de Tenerife Spain Dr Brad Hall NOAA ESRL 325 Broadway Boulder, CO 80305 USA Dr Armin Jordan Max-Planck-Institute for Biogeochemistry Hans Knöll Str. 10 07745 Jena Germany Dr Jörg Klausen Empa Laboratory for Air Pollution/Environmental Technology GAW QA/SAC Switzerland Überlandstrasse 129 8600 Dübendorf Switzerland

Dr Ed de Leer NMi VSL Thijsseweg 11 2629 JA Delft The Netherlands Dr Hans-Eckhart Scheel Forschungszentrum Karlsruhe, IMK-IFU Kreuzeckbahnstr. 19 82467 Garmisch-Partenkirchen Germany Dr Rainer Steinbrecher Forschungszentrum Karlsruhe, IMK-IFU Kreuzeckbahnstr. 19 82467 Garmisch-Partenkirchen Germany Dr Oksana A. Tarasova Scientific Officer, AER/RES World Meteorological Organization 7bis, avenue de la Paix CH-1211 Geneva 2 Switzerland Dr Robert Wielgosz Bureau International des Poids et Mesures Pavillon de Breteuil F-92312 Sevres Cedex France

37

GLOBAL ATMOSPHERE WATCH REPORT SERIES 1. Final Report of the Expert Meeting on the Operation of Integrated Monitoring Programmes, Geneva, 2 -5 September 1980. 2. Report of the Third Session of the GESAMP Working Group on the Interchange of Pollutants Between the Atmosphere and

the Oceans (INTERPOLL-III), Miami, USA, 27-31 October 1980. 3. Report of the Expert Meeting on the Assessment of the Meteorological Aspects of the First Phase of EMEP, Shinfield Park,

U.K., 30 March - 2 April 1981. 4. Summary Report on the Status of the WMO Background Air Pollution Monitoring Network as at April 1981. 5. Report of the WMO/UNEP/ICSU Meeting on Instruments, Standardization and Measurements Techniques for Atmospheric

CO2, Geneva, 8-11; September 1981. 6. Report of the Meeting of Experts on BAPMoN Station Operation, Geneva, 23–26 November 1981. 7. Fourth Analysis on Reference Precipitation Samples by the Participating World Meteorological Organization Laboratories

by Robert L. Lampe and John C. Puzak, December 1981. 8. Review of the Chemical Composition of Precipitation as Measured by the WMO BAPMoN by Prof. Dr. Hans-Walter

Georgii, February 1982. 9. An Assessment of BAPMoN Data Currently Available on the Concentration of CO2 in the Atmosphere by M.R. Manning,

February 1982. 10. Report of the Meeting of Experts on Meteorological Aspects of Long-range Transport of Pollutants, Toronto, Canada, 30

November - 4 December 1981. 11. Summary Report on the Status of the WMO Background Air Pollution Monitoring Network as at May 1982. 12. Report on the Mount Kenya Baseline Station Feasibility Study edited by Dr. Russell C. Schnell. 13. Report of the Executive Committee Panel of Experts on Environmental Pollution, Fourth Session, Geneva, 27 September -

1 October 1982. 14. Effects of Sulphur Compounds and Other Pollutants on Visibility by Dr. R.F. Pueschel, April 1983. 15. Provisional Daily Atmospheric Carbon Dioxide Concentrations as Measured at BAPMoN Sites for the Year 1981, May

1983. 16. Report of the Expert Meeting on Quality Assurance in BAPMoN, Research Triangle Park, North Carolina, USA, 17-21

January 1983. 17. General Consideration and Examples of Data Evaluation and Quality Assurance Procedures Applicable to BAPMoN

Precipitation Chemistry Observations by Dr. Charles Hakkarinen, July 1983. 18. Summary Report on the Status of the WMO Background Air Pollution Monitoring Network as at May 1983. 19. Forecasting of Air Pollution with Emphasis on Research in the USSR by M.E. Berlyand, August 1983. 20. Extended Abstracts of Papers to be Presented at the WMO Technical Conference on Observation and Measurement of

Atmospheric Contaminants (TECOMAC), Vienna, 17-21 October 1983. 21. Fifth Analysis on Reference Precipitation Samples by the Participating World Meteorological Organization Laboratories by

Robert L. Lampe and William J. Mitchell, November 1983. 22. Report of the Fifth Session of the WMO Executive Council Panel of Experts on Environmental Pollution, Garmisch-

Partenkirchen, Federal Republic of Germany, 30 April - 4 May 1984 (WMO TD No. 10). 23. Provisional Daily Atmospheric Carbon Dioxide Concentrations as Measured at BAPMoN Sites for the Year 1982.

November 1984 (WMO TD No. 12).

38

24. Final Report of the Expert Meeting on the Assessment of the Meteorological Aspects of the Second Phase of EMEP, Friedrichshafen, Federal Republic of Germany, 7-10 December 1983. October 1984 (WMO TD No. 11).

25. Summary Report on the Status of the WMO Background Air Pollution Monitoring Network as at May 1984. November

1984 (WMO TD No. 13). 26. Sulphur and Nitrogen in Precipitation: An Attempt to Use BAPMoN and Other Data to Show Regional and Global

Distribution by Dr. C.C. Wallén. April 1986 (WMO TD No. 103). 27. Report on a Study of the Transport of Sahelian Particulate Matter Using Sunphotometer Observations by Dr. Guillaume A.

d'Almeida. July 1985 (WMO TD No. 45). 28. Report of the Meeting of Experts on the Eastern Atlantic and Mediterranean Transport Experiment ("EAMTEX"), Madrid

and Salamanca, Spain, 6-8 November 1984. 29. Recommendations on Sunphotometer Measurements in BAPMoN Based on the Experience of a Dust Transport Study in

Africa by Dr. Guillaume A. d'Almeida. September 1985 (WMO TD No. 67). 30. Report of the Ad-hoc Consultation on Quality Assurance Procedures for Inclusion in the BAPMoN Manual, Geneva, 29-31

May 1985. 31. Implications of Visibility Reduction by Man-Made Aerosols (Annex to No. 14) by R.M. Hoff and L.A. Barrie. October 1985

(WMO TD No. 59). 32. Manual for BAPMoN Station Operators by E. Meszaros and D.M. Whelpdale. October 1985 (WMO TD No. 66). 33. Man and the Composition of the Atmosphere: BAPMoN - An international programme of national needs, responsibility and

benefits by R.F. Pueschel, 1986. 34. Practical Guide for Estimating Atmospheric Pollution Potential by Dr. L.E. Niemeyer. August 1986 (WMO TD No. 134). 35. Provisional Daily Atmospheric CO2 Concentrations as Measured at BAPMoN Sites for the Year 1983. December 1985

(WMO TD No. 77). 36. Global Atmospheric Background Monitoring for Selected Environmental Parameters. BAPMoN Data for 1984. Volume I:

Atmospheric Aerosol Optical Depth. October 1985 (WMO TD No. 96). 37. Air-Sea Interchange of Pollutants by R.A. Duce. September 1986 (WMO TD No. 126). 38. Summary Report on the Status of the WMO Background Air Pollution Monitoring Network as at 31 December 1985.

September 1986 (WMO TD No. 136). 39. Report of the Third WMO Expert Meeting on Atmospheric Carbon Dioxide Measurement Techniques, Lake Arrowhead,

California, USA, 4-8 November 1985. October 1986. 40. Report of the Fourth Session of the CAS Working Group on Atmospheric Chemistry and Air Pollution, Helsinki, Finland, 18-

22 November 1985. January 1987. 41. Global Atmospheric Background Monitoring for Selected Environmental Parameters. BAPMoN Data for 1982, Volume II:

Precipitation chemistry, continuous atmospheric carbon dioxide and suspended particulate matter. June 1986 (WMO TD No. 116).

42. Scripps reference gas calibration system for carbon dioxide-in-air standards: revision of 1985 by C.D. Keeling, P.R.

Guenther and D.J. Moss. September 1986 (WMO TD No. 125). 43. Recent progress in sunphotometry (determination of the aerosol optical depth). November 1986. 44. Report of the Sixth Session of the WMO Executive Council Panel of Experts on Environmental Pollution, Geneva, 5-9 May

1986. March 1987. 45. Proceedings of the International Symposium on Integrated Global Monitoring of the State of the Biosphere (Volumes I-IV),

Tashkent, USSR, 14-19 October 1985. December 1986 (WMO TD No. 151).

39

46. Provisional Daily Atmospheric Carbon Dioxide Concentrations as Measured at BAPMoN Sites for the Year 1984. December 1986 (WMO TD No. 158).

47. Procedures and Methods for Integrated Global Background Monitoring of Environmental Pollution by F.Ya. Rovinsky,

USSR and G.B. Wiersma, USA. August 1987 (WMO TD No. 178). 48. Meeting on the Assessment of the Meteorological Aspects of the Third Phase of EMEP IIASA, Laxenburg, Austria, 30

March - 2 April 1987. February 1988. 49. Proceedings of the WMO Conference on Air Pollution Modelling and its Application (Volumes I-III), Leningrad, USSR, 19-

24 May 1986. November 1987 (WMO TD No. 187). 50. Provisional Daily Atmospheric Carbon Dioxide Concentrations as Measured at BAPMoN Sites for the Year 1985.

December 1987 (WMO TD No. 198). 51. Report of the NBS/WMO Expert Meeting on Atmospheric CO2 Measurement Techniques, Gaithersburg, USA, 15-17 June

1987. December 1987. 52. Global Atmospheric Background Monitoring for Selected Environmental Parameters. BAPMoN Data for 1985. Volume I:

Atmospheric Aerosol Optical Depth. September 1987. 53. WMO Meeting of Experts on Strategy for the Monitoring of Suspended Particulate Matter in BAPMoN - Reports and papers

presented at the meeting, Xiamen, China, 13-17 October 1986. October 1988. 54. Global Atmospheric Background Monitoring for Selected Environmental Parameters. BAPMoN Data for 1983, Volume II:

Precipitation chemistry, continuous atmospheric carbon dioxide and suspended particulate matter (WMO TD No. 283). 55. Summary Report on the Status of the WMO Background Air Pollution Monitoring Network as at 31 December 1987 (WMO

TD No. 284). 56. Report of the First Session of the Executive Council Panel of Experts/CAS Working Group on Environmental Pollution and

Atmospheric Chemistry, Hilo, Hawaii, 27-31 March 1988. June 1988. 57. Global Atmospheric Background Monitoring for Selected Environmental Parameters. BAPMoN Data for 1986, Volume I:

Atmospheric Aerosol Optical Depth. July 1988. 58. Provisional Daily Atmospheric Carbon Dioxide Concentrations as measured at BAPMoN sites for the years 1986 and 1987

(WMO TD No. 306). 59. Extended Abstracts of Papers Presented at the Third International Conference on Analysis and Evaluation of Atmospheric

CO2 Data - Present and Past, Hinterzarten, Federal Republic of Germany, 16-20 October 1989 (WMO TD No. 340). 60. Global Atmospheric Background Monitoring for Selected Environmental Parameters. BAPMoN Data for 1984 and 1985,

Volume II: Precipitation chemistry, continuous atmospheric carbon dioxide and suspended particulate matter. 61. Global Atmospheric Background Monitoring for Selected Environmental Parameters. BAPMoN Data for 1987 and 1988,

Volume I: Atmospheric Aerosol Optical Depth. 62. Provisional Daily Atmospheric Carbon Dioxide Concentrations as measured at BAPMoN sites for the year 1988 (WMO TD

No. 355). 63. Report of the Informal Session of the Executive Council Panel of Experts/CAS Working Group on Environmental Pollution

and Atmospheric Chemistry, Sofia, Bulgaria, 26 and 28 October 1989. 64. Report of the consultation to consider desirable locations and observational practices for BAPMoN stations of global

importance, Bermuda Research Station, 27-30 November 1989. 65. Report of the Meeting on the Assessment of the Meteorological Aspects of the Fourth Phase of EMEP, Sofia, Bulgaria, 27

and 31 October 1989. 66. Summary Report on the Status of the WMO Global Atmosphere Watch Stations as at 31 December 1990 (WMO TD No.

419).

40

67. Report of the Meeting of Experts on Modelling of Continental, Hemispheric and Global Range Transport, Transformation and Exchange Processes, Geneva, 5-7 November 1990.

68. Global Atmospheric Background Monitoring for Selected Environmental Parameters. BAPMoN Data For 1989, Volume I:

Atmospheric Aerosol Optical Depth. 69. Provisional Daily Atmospheric Carbon Dioxide Concentrations as measured at Global Atmosphere Watch (GAW)-BAPMoN

sites for the year 1989 (WMO TD No. 400). 70. Report of the Second Session of EC Panel of Experts/CAS Working Group on Environmental Pollution and Atmospheric

Chemistry, Santiago, Chile, 9-15 January 1991 (WMO TD No. 633). 71. Report of the Consultation of Experts to Consider Desirable Observational Practices and Distribution of GAW Regional

Stations, Halkidiki, Greece, 9-13 April 1991 (WMO TD No. 433). 72. Integrated Background Monitoring of Environmental Pollution in Mid-Latitude Eurasia by Yu.A. Izrael and F.Ya. Rovinsky,

USSR (WMO TD No. 434). 73. Report of the Experts Meeting on Global Aerosol Data System (GADS), Hampton, Virginia, 11 to 12 September 1990

(WMO TD No. 438). 74. Report of the Experts Meeting on Aerosol Physics and Chemistry, Hampton, Virginia, 30 to 31 May 1991 (WMO TD No.

439). 75. Provisional Daily Atmospheric Carbon Dioxide Concentrations as measured at Global Atmosphere Watch (GAW)-BAPMoN

sites for the year 1990 (WMO TD No. 447). 76. The International Global Aerosol Programme (IGAP) Plan: Overview (WMO TD No. 445). 77. Report of the WMO Meeting of Experts on Carbon Dioxide Concentration and Isotopic Measurement Techniques, Lake

Arrowhead, California, 14-19 October 1990. 78. Global Atmospheric Background Monitoring for Selected Environmental Parameters BAPMoN Data for 1990, Volume I:

Atmospheric Aerosol Optical Depth (WMO TD No. 446). 79. Report of the Meeting of Experts to Consider the Aerosol Component of GAW, Boulder, 16 to 19 December 1991 (WMO

TD No. 485). 80. Report of the WMO Meeting of Experts on the Quality Assurance Plan for the GAW, Garmisch-Partenkirchen, Germany,

26-30 March 1992 (WMO TD No. 513). 81. Report of the Second Meeting of Experts to Assess the Response to and Atmospheric Effects of the Kuwait Oil Fires,

Geneva, Switzerland, 25-29 May 1992 (WMO TD No. 512). 82. Global Atmospheric Background Monitoring for Selected Environmental Parameters BAPMoN Data for 1991, Volume I:

Atmospheric Aerosol Optical Depth (WMO TD No. 518). 83. Report on the Global Precipitation Chemistry Programme of BAPMoN (WMO TD No. 526). 84. Provisional Daily Atmospheric Carbon Dioxide Concentrations as measured at GAW-BAPMoN sites for the year 1991

(WMO TD No. 543). 85. Chemical Analysis of Precipitation for GAW: Laboratory Analytical Methods and Sample Collection Standards by Dr

Jaroslav Santroch (WMO TD No. 550). 86. The Global Atmosphere Watch Guide, 1993 (WMO TD No. 553). 87. Report of the Third Session of EC Panel/CAS Working Group on Environmental Pollution and Atmospheric Chemistry,

Geneva, 8-11 March 1993 (WMO TD No. 555). 88. Report of the Seventh WMO Meeting of Experts on Carbon Dioxide Concentration and Isotopic Measurement Techniques,

Rome, Italy, 7-10 September 1993, (edited by Graeme I. Pearman and James T. Peterson) (WMO TD No. 669).

41

89. 4th International Conference on CO2 (Carqueiranne, France, 13-17 September 1993) (WMO TD No. 561). 90. Global Atmospheric Background Monitoring for Selected Environmental Parameters GAW Data for 1992, Volume I:

Atmospheric Aerosol Optical Depth (WMO TD No. 562). 91. Extended Abstracts of Papers Presented at the WMO Region VI Conference on the Measurement and Modelling of

Atmospheric Composition Changes Including Pollution Transport, Sofia, 4 to 8 October 1993 (WMO TD No. 563). 92. Report of the Second WMO Meeting of Experts on the Quality Assurance/Science Activity Centres of the Global

Atmosphere Watch, Garmisch-Partenkirchen, 7-11 December 1992 (WMO TD No. 580). 93. Report of the Third WMO Meeting of Experts on the Quality Assurance/Science Activity Centres of the Global Atmosphere

Watch, Garmisch-Partenkirchen, 5-9 July 1993 (WMO TD No. 581). 94. Report on the Measurements of Atmospheric Turbidity in BAPMoN (WMO TD No. 603). 95. Report of the WMO Meeting of Experts on UV-B Measurements, Data Quality and Standardization of UV Indices, Les

Diablerets, Switzerland, 25-28 July 1994 (WMO TD No. 625). 96. Global Atmospheric Background Monitoring for Selected Environmental Parameters WMO GAW Data for 1993, Volume I:

Atmospheric Aerosol Optical Depth. 97. Quality Assurance Project Plan (QAPjP) for Continuous Ground Based Ozone Measurements (WMO TD No. 634). 98. Report of the WMO Meeting of Experts on Global Carbon Monoxide Measurements, Boulder, USA, 7-11 February 1994

(WMO TD No. 645). 99. Status of the WMO Global Atmosphere Watch Programme as at 31 December 1993 (WMO TD No. 636). 100. Report of the Workshop on UV-B for the Americas, Buenos Aires, Argentina, 22-26 August 1994. 101. Report of the WMO Workshop on the Measurement of Atmospheric Optical Depth and Turbidity, Silver Spring, USA, 6-10

December 1993, (edited by Bruce Hicks) (WMO TD No. 659). 102. Report of the Workshop on Precipitation Chemistry Laboratory Techniques, Hradec Kralove, Czech Republic, 17-21

October 1994 (WMO TD No. 658). 103. Report of the Meeting of Experts on the WMO World Data Centers, Toronto, Canada, 17 - 18 February 1995, (prepared by

Edward Hare) (WMO TD No. 679). 104. Report of the Fourth WMO Meeting of Experts on the Quality Assurance/Science Activity Centres (QA/SACs) of the Global

Atmosphere Watch, jointly held with the First Meeting of the Coordinating Committees of IGAC-GLONET and IGAC-ACE, Garmisch-Partenkirchen, Germany, 13 to 17 March 1995 (WMO TD No. 689).

105. Report of the Fourth Session of the EC Panel of Experts/CAS Working Group on Environmental Pollution and Atmospheric

Chemistry (Garmisch, Germany, 6-11 March 1995) (WMO TD No. 718). 106. Report of the Global Acid Deposition Assessment (edited by D.M. Whelpdale and M-S. Kaiser) (WMO TD No. 777). 107. Extended Abstracts of Papers Presented at the WMO-IGAC Conference on the Measurement and Assessment of

Atmospheric Composition Change (Beijing, China, 9-14 October 1995) (WMO TD No. 710). 108. Report of the Tenth WMO International Comparison of Dobson Spectrophotometers (Arosa, Switzerland, 24 July - 4

August 1995). 109. Report of an Expert Consultation on 85Kr and 222Rn: Measurements, Effects and Applications (Freiburg, Germany, 28-31

March 1995) (WMO TD No. 733). 110. Report of the WMO-NOAA Expert Meeting on GAW Data Acquisition and Archiving (Asheville, NC, USA, 4-8 November

1995) (WMO TD No. 755).

42

111. Report of the WMO-BMBF Workshop on VOC Establishment of a “World Calibration/Instrument Intercomparison Facility for VOC” to Serve the WMO Global Atmosphere Watch (GAW) Programme (Garmisch-Partenkirchen, Germany, 17-21 December 1995) (WMO TD No. 756).

112. Report of the WMO/STUK Intercomparison of Erythemally-Weighted Solar UV Radiometers, Spring/Summer 1995,

Helsinki, Finland (WMO TD No. 781). 112A. Report of the WMO/STUK ’95 Intercomparison of broadband UV radiometers: a small-scale follow-up study in 1999,

Helsinki, 2001, Addendum to GAW Report No. 112. 113. The Strategic Plan of the Global Atmosphere Watch (GAW) (WMO TD No. 802). 114. Report of the Fifth WMO Meeting of Experts on the Quality Assurance/Science Activity Centres (QA/SACs) of the Global

Atmosphere Watch, jointly held with the Second Meeting of the Coordinating Committees of IGAC-GLONET and IGAC-ACEEd, Garmisch-Partenkirchen, Germany, 15-19 July 1996 (WMO TD No. 787).

115. Report of the Meeting of Experts on Atmospheric Urban Pollution and the Role of NMSs (Geneva, 7-11 October 1996)

(WMO TD No. 801). 116. Expert Meeting on Chemistry of Aerosols, Clouds and Atmospheric Precipitation in the Former USSR (Saint Petersburg,

Russian Federation, 13-15 November 1995). 117. Report and Proceedings of the Workshop on the Assessment of EMEP Activities Concerning Heavy Metals and Persistent

Organic Pollutants and their Further Development (Moscow, Russian Federation, 24-26 September 1996) (Volumes I and II) (WMO TD No. 806).

118. Report of the International Workshops on Ozone Observation in Asia and the Pacific Region (IWOAP, IWOAP-II), (IWOAP,

27 February-26 March 1996 and IWOAP-II, 20 August-18 September 1996) (WMO TD No. 827). 119. Report on BoM/NOAA/WMO International Comparison of the Dobson Spectrophotometers (Perth Airport, Perth, Australia,

3-14 February 1997), (prepared by Robert Evans and James Easson) (WMO TD No. 828). 120. WMO-UMAP Workshop on Broad-Band UV Radiometers (Garmisch-Partenkirchen, Germany, 22 to 23 April 1996) (WMO

TD No. 894). 121. Report of the Eighth WMO Meeting of Experts on Carbon Dioxide Concentration and Isotopic Measurement Techniques

(prepared by Thomas Conway) (Boulder, CO, 6-11 July 1995) (WMO TD No. 821). 122. Report of Passive Samplers for Atmospheric Chemistry Measurements and their Role in GAW (prepared by Greg

Carmichael) (WMO TD No. 829). 123. Report of WMO Meeting of Experts on GAW Regional Network in RA VI, Budapest, Hungary, 5 to 9 May 1997. 124. Fifth Session of the EC Panel of Experts/CAS Working Group on Environmental Pollution and Atmospheric Chemistry,

(Geneva, Switzerland, 7-10 April 1997) (WMO TD No. 898). 125. Instruments to Measure Solar Ultraviolet Radiation, Part 1: Spectral Instruments (lead author G. Seckmeyer) (WMO TD No.

1066) 126. Guidelines for Site Quality Control of UV Monitoring (lead author A.R. Webb) (WMO TD No. 884). 127. Report of the WMO-WHO Meeting of Experts on Standardization of UV Indices and their Dissemination to the Public (Les

Diablerets, Switzerland, 21-25 July 1997) (WMO TD No. 921). 128. The Fourth Biennial WMO Consultation on Brewer Ozone and UV Spectrophotometer Operation, Calibration and Data

Reporting, (Rome, Italy, 22-25 September 1996) (WMO TD No. 918). 129. Guidelines for Atmospheric Trace Gas Data Management (Ken Masarie and Pieter Tans), 1998 (WMO TD No. 907). 130. Jülich Ozone Sonde Intercomparison Experiment (JOSIE, 5 February to 8 March 1996), (H.G.J. Smit and D. Kley) (WMO

TD No. 926).

43

131. WMO Workshop on Regional Transboundary Smoke and Haze in Southeast Asia (Singapore, 2 to 5 June 1998) (Gregory R. Carmichael). Two volumes.

132. Report of the Ninth WMO Meeting of Experts on Carbon Dioxide Concentration and Related Tracer Measurement

Techniques (Edited by Roger Francey), (Aspendale, Vic., Australia). 133. Workshop on Advanced Statistical Methods and their Application to Air Quality Data Sets (Helsinki, 14-18 September

1998) (WMO TD No. 956). 134. Guide on Sampling and Analysis Techniques for Chemical Constituents and Physical Properties in Air and Precipitation as

Applied at Stations of the Global Atmosphere Watch. Carbon Dioxide (WMO TD No. 980). 135. Sixth Session of the EC Panel of Experts/CAS Working Group on Environmental Pollution and Atmospheric Chemistry

(Zurich, Switzerland, 8-11 March 1999) (WMO TD No.1002). 136. WMO/EMEP/UNEP Workshop on Modelling of Atmospheric Transport and Deposition of Persistent Organic Pollutants and

Heavy Metals (Geneva, Switzerland, 16-19 November 1999) (Volumes I and II) (WMO TD No. 1008). 137. Report and Proceedings of the WMO RA II/RA V GAW Workshop on Urban Environment (Beijing, China, 1-4 November

1999) (WMO-TD. 1014) (Prepared by Greg Carmichael). 138. Reports on WMO International Comparisons of Dobson Spectrophotometers, Parts I – Arosa, Switzerland, 19-31 July 1999,

Part II – Buenos Aires, Argentina (29 Nov. – 12 Dec. 1999 and Part III – Pretoria, South Africa (18 March – 10 April 2000) (WMO TD No. 1016).

139. The Fifth Biennial WMO Consultation on Brewer Ozone and UV Spectrophotometer Operation, Calibration and Data

Reporting (Halkidiki, Greece, September 1998)(WMO TD No. 1019). 140. WMO/CEOS Report on a Strategy for Integrating Satellite and Ground-based Observations of Ozone (WMO TD No. 1046). 141. Report of the LAP/COST/WMO Intercomparison of Erythemal Radiometers Thessaloniki, Greece, 13-23 September 1999)

(WMO TD No. 1051). 142. Strategy for the Implementation of the Global Atmosphere Watch Programme (2001-2007), A Contribution to the

Implementation of the Long-Term Plan (WMO TD No.1077). 143. Global Atmosphere Watch Measurements Guide (WMO TD No. 1073). 144. Report of the Seventh Session of the EC Panel of Experts/CAS Working Group on Environmental Pollution and

Atmospheric Chemistry and the GAW 2001 Workshop (Geneva, Switzerland, 2 to 5 April 2001) (WMO TD No. 1104). 145. WMO GAW International Comparisons of Dobson Spectrophotometers at the Meteorological Observatory

Hohenpeissenberg, Germany (21 May – 10 June 2000, MOHp2000-1), 23 July – 5 August 2000, MOHp2000-2), (10 – 23 June 2001, MOHp2001-1) and (8 to 21 July 2001, MOHp2001-2). Prepared by Ulf Köhler (WMO TD No. 1114).

146. Quality Assurance in monitoring solar ultraviolet radiation: the state of the art. (WMO TD No. 1180). 147. Workshop on GAW in RA VI (Europe), Riga, Latvia, 27-30 May 2002. (WMO TD No. 1206). 148. Report of the Eleventh WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracer Measurement

Techniques (Tokyo, Japan, 25-28 September 2001) (WMO TD No 1138). 149. Comparison of Total Ozone Measurements of Dobson and Brewer Spectrophotometers and Recommended Transfer

Functions (prepared by J. Staehelin, J. Kerr, R. Evans and K. Vanicek) (WMO TD No. 1147). 150. Updated Guidelines for Atmospheric Trace Gas Data Management (Prepared by Ken Maserie and Pieter Tans (WMO TD

No. 1149). 151. Report of the First CAS Working Group on Environmental Pollution and Atmospheric Chemistry (Geneva, Switzerland, 18-

19 March 2003) (WMO TD No. 1181). 152. Current Activities of the Global Atmosphere Watch Programme (as presented at the 14th World Meteorological Congress,

May 2003). (WMO TD No. 1168).

44

153. WMO/GAW Aerosol Measurement Procedures: Guidelines and Recommendations. (WMO TD No. 1178). 154. WMO/IMEP-15 Trace Elements in Water Laboratory Intercomparison. (WMO TD No. 1195). 155. 1st International Expert Meeting on Sources and Measurements of Natural Radionuclides Applied to Climate and Air Quality

Studies (Gif sur Yvette, France, 3-5 June 2003) (WMO TD No. 1201). 156. Addendum for the Period 2005-2007 to the Strategy for the Implementation of the Global Atmosphere Watch Programme

(2001-2007), GAW Report No. 142 (WMO TD No. 1209). 157. JOSIE-1998 Performance of EEC Ozone Sondes of SPC-6A and ENSCI-Z Type (Prepared by Herman G.J. Smit and

Wolfgang Straeter) (WMO TD No. 1218). 158. JOSIE-2000 Jülich Ozone Sonde Intercomparison Experiment 2000. The 2000 WMO international intercomparison of

operating procedures for ECC-ozone sondes at the environmental simulation facility at Jülich (Prepared by Herman G.J. Smit and Wolfgang Straeter) (WMO TD No. 1225).

159. IGOS-IGACO Report - September 2004 (WMO TD No. 1235). 160. Manual for the GAW Precipitation Chemistry Programme (Guidelines, Data Quality Objectives and Standard Operating

Procedures) (WMO TD No. 1251). 161 12th WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracers Measurement Techniques

(Toronto, Canada, 15-18 September 2003). 162. WMO/GAW Experts Workshop on a Global Surface-Based Network for Long Term Observations of Column Aerosol

Optical Properties, Davos, Switzerland, 8-10 March 2004 (edited by U. Baltensperger, L. Barrie and C. Wehrli) (WMO TD No. 1287).

163. World Meteorological Organization Activities in Support of the Vienna Convention on Protection of the Ozone Layer (WMO

No. 974). 164. Instruments to Measure Solar Ultraviolet Radiation: Part 2: Broadband Instruments Measuring Erythemally Weighted Solar

Irradiance (WMO TD No. 1289). 165. Report of the CAS Working Group on Environmental Pollution and Atmospheric Chemistry and the GAW 2005 Workshop,

14-18 March 2005, Geneva, Switzerland (WMO TD No. 1302). 166. Joint WMO-GAW/ACCENT Workshop on The Global Tropospheric Carbon Monoxide Observations System, Quality

Assurance and Applications (EMPA, Dübendorf, Switzerland, 24 – 26 October 2005) (edited by J. Klausen) (WMO TD No. 1335).

167. The German Contribution to the WMO Global Atmosphere Watch Programme upon the 225th Anniversary of GAW

Hohenpeissenberg Observatory (edited by L.A. Barrie, W. Fricke and R. Schleyer (WMO TD No. 1336). 168. 13th WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracers Measurement Techniques

(Boulder, Colorado, USA, 19-22 September 2005) (edited by J.B. Miller) (WMO TD No. 1359). 169. Chemical Data Assimilation for the Observation of the Earth’s Atmosphere – ACCENT/WMO Expert Workshop in support

of IGACO (edited by L.A. Barrie, J.P. Burrows, P. Monks and P. Borrell) (WMO TD No. 1360). 170. WMO/GAW Expert Workshop on the Quality and Applications of European GAW Measurements (Tutzing, Germany, 2-5

November 2004) (WMO TD No. 1367). 171. A WMO/GAW Expert Workshop on Global Long-Term Measurements of Volatile Organic Compounds (VOCs) (Geneva,

Switzerland, 30 January – 1 February 2006) (WMO TD No. 1373). 172. WMO Global Atmosphere Watch (GAW) Strategic Plan: 2008 – 2015 (WMO TD No. 1384). 173. Report of the CAS Joint Scientific Steering Committee on Environmental Pollution and Atmospheric Chemistry (Geneva,

Switzerland, 11-12 April 2007) (WMO TD No.1410). 174. World Data Center for Greenhouse Gases Data Submission and Dissemination Guide (WMO TD No. 1416).

45

175. The Ninth Biennial WMO Consultation on Brewer Ozone and UV Spectrophotometer Operation, Calibration and Data Reporting (Delft, Netherlands, 31-May – 3 June 2005) (WMO TD No. 1419).

176. The Tenth Biennial WMO Consultation on Brewer Ozone and UV Spectrophotometer Operation, Calibration and Data

Reporting (Northwich, United Kingdom, 4-8 June 2007) (WMO TD No. 1420). 177. Joint Report of COST Action 728 and GURME – Overview of Existing Integrated (off-line and on-line) Mesoscale

Meteorological and Chemical Transport Modelling in Europe (ISBN 978-1-905313-56-3) (WMO TD No. 1427). 178. Plan for the implementation of the GAW Aerosol Lidar Observation Network GALION, (Hamburg, Germany, 27 - 29 March

2007) (WMO TD No. 1443). 179. Intercomparison of Global UV Index from Multiband Radiometers: Harmonization of Global UVI and Spectral Irradiance

(WMO TD No. 1454). 180. Towards a Better Knowledge of Umkehr Measurements: A Detailed Study of Data from Thirteen Dobson Intercomparisons

(WMO TD No. 1456). 181. Joint Report of COST Action 728 and GURME – Overview of Tools and Methods for Meteorological and Air Pollution

Mesoscale Model Evaluation and User Training (WMO TD No. 1457). 182. IGACO-Ozone and UV Radiation Implementation Plan (WMO TD No. 1465). 183. Operations Handbook – Ozone Observations with a Dobson Spectrophotometer (WMO TD No. 1469). 184. Technical Report of Global Analysis Method for Major Greenhouse Gases by the World Data Center for Greenhouse Gases

(WMO TD No. 1473).