Observations of new particle production in the atmosphere of a moderately polluted site in eastern England

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D14, PAGES 17,819-17,832, JULY 27, 2000

Observations of new particle production in the atmosphere of a moderately polluted site in eastern England

Roy M. Harrison, J. Lee Grenfell, Nick Savage, and Andrew Allen Division of Environmental Health and Risk Management, University of Birmingham, Birmingham, England

Kevin C. Clemitshaw • and Stuart Penkett School of Environmental Sciences, University of East Anglia, Norwich, England

C. Nicholas Hewitt and Brian Davison Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, England

Abstract. Measurements of particle number density and Fuchs surface area, together with a range of gaseous pollutant concentrations, have been made in June 1995 at a coastal site in eastern England which receives air from a range of polluted and less polluted origins. Periods of enhanced local particle production were identified and found to be associated predominantly with relatively polluted air sectors. An examination of the factors contributing to homogeneous nucleation and hence new particle production suggests that those most important at this location are probably the production of hydroxyl radicals and the availability of ammonia. A numerical modeling study calculating characteristic timescales for new particle production and for condensation onto existing aerosol surfaces is able to predict periods of new particle production. The model suggests that oxidation of dimethyl sulphide and sulphur dioxide and homogeneous nucleation of sulphuric acid and water, probably in combination with ammonia, are the source of new particles at this site.

1. Introduction

Atmospheric particles play a crucial role in influencing the Earth's climate. They exert their influence in several ways, most importantly through direct scattering and absorption of solar radiation and indirectly through their ability to act as cloud condensation nuclei [Houghton et al., 1996]. In recent years much research has focused on the role of dimethyl sulphide (DMS) emissions from the oceans as a precursor of particle formation subsequent to DMS oxidation [Charlson et al., 1987]. It appears that new particle formation from DMS oxidation may have a significant influence upon the number density, both of total particles and of cloud condensation nuclei in remote marine regions, and the evidence supporting the role of oceanic DMS emissions as a key climate regulator is steadily accumulating [Hobbs, 1993]. A related phenomenon which has received less attention in recent years is that of particle production via the oxidation of biogenic hydrocarbons released from forests. This is reported to lead to increases in total particle number sufficient to cause the blue haze observable over forests in hot summer weather [Davies, 1974].

Both the formation of sulphate particles in the marine atmosphere and of hydrocarbon-based particles in the forest atmosphere are primarily clean air phenomena. There are good reasons why this should be so, as the process of homogeneous

•Now at Centre for Environmental Technology, Imperial College of Science, Technology, and Medicine, Silwood Park, Ascot, England.

Copyright 2000 by the American Geophysical Union.

Paper number 2000JD900086. 0148-0227/00/2000JD900086509.00

nucleation to form new particles is competitive with heterogeneous condensation of vapor onto existing particles. Therefore clean air in which the available surface of existing particles is limited offers 'a favorable environment for new particle production [Seinfeld, 1986]. There has consequently been little research on the formation of particles in more polluted air [Harrison, 1997], and in this paper we describe studies of particle formation at a coastal site in eastern England which receives air with a highly variable pollutant content ranging from relatively clean maritime air arriving from the Arctic to heavily polluted air advected from southeast England. This site therefore provides an ideal environment in which to examine factors influencing new particle formation. The measurements were made in June 1995.

Coastal sites appear to show interesting and probably exceptional properties in relation to particle formation. Mihalopoulos et al. [1992] reported particle number concentrations as high as 4.5 x 105 particles cm '3 during a campaign of measurements on the Atlantic coast of Brittany; at this site particle production events appear related to periods of low tide. McGovern et al. [1996] also report bursts of particle formation at Mace Head on the Atlantic coast of Ireland. They suggested a particle-sunlight relationship in the period April 12- 17, 1991, although new data from this site suggest that the phenomenon may be more closely related to the onset of low tide rather than to solar intensity [Allen et al., 1999]. A third Atlantic coastal site at which major particle burst phenomena have been observed is South Uist in the Outer Hebrides

[Harrison, 1997]. While probably the most spectacular bursts of particle

formation have been observed at coastal sites, they have also been observed in the tropical marine boundary layer [Clarke et al., 1998] and at a remote continental site [Marti et al., 1997].

17,819

17,820 HARRISON ET AL.: OBSERVATIONS OF NEW PARTICLE PRODUCTION

Weber et al. [1996] reviewed some 15 sets of recent field observations of new particle formation. The mechanisms are as yet uncertain, and indeed may vary from site to site. Clarke et al. [1998] reported that aerosol nucleation and growth were linked to the natural marine sulphur cycle, but that classic binary nucleation theory predicted no nucleation under the observed marine boundary layer conditions. Similarly, Weber et al. [1996, 1998] report that the functional dependence of observed particle formation rates on sulphuric acid vapor concentrations is much weaker than predicted by binary theory. They suggest that participation of a third species, possibly ammonia, might stabilize subcritical clusters thereby enhancing nucleation rates. In their report on new particle formation at a remote continental site, Marti et al. [ 1997] suggest that sulphuric acid was probably responsible for most of the observed new ultrafine particle formation, although their evidence is highly indirect. They also report that low volatility organic compounds may have caused particle formation under the right condition, but were more likely to condense upon preexisting particles. Detailed analysis of data from Mace Head, Ireland, indicates that at that site, oxidation of DMS alone cannot account for the observed new

particle production [Grenfell et al., 1999]. In this paper we describe observations of new particle

production at a site more polluted than most of those studied in earlier research. While this is a coastal site, the observed new

particle production phenomena show marked differences to observations at some other coastal locations.

2. Experimental Methods

All measurements were made at the Weyboume Atmospheric Observatory (WAO) located at 16 m above sea level at Weybourne on the North Norfolk Coast at 52ø57'23"N and 1 ø 7'40"E. The observatory is located in a predominantly open area of mainly fields and is approximately 100 m from the sea. The nearest village to the site is Weybourne itself (population 500) which is approximately 1 km distant. The area is surrounded by agricultural land with the closest city being Norwich at about 60 km to the southeast. Air samples and meteorological data were collected at a height of 10 m from the top of a tower.

The instrumentation used to obtain the measurements

included in this paper are as follows:

2.1. Particle Measurements

Particle number density was measured with a Thermo Systems Inc. (TSI) model 3025 ultrafine condensation nuclei counter (TSI Inc., St Paul, Minnesota, United States) based on the design of Stolzenburg and McMurry [ 1991 ], which is able to measure particles in a range of diameters from 3 nm to 10 gm in diameter at particle concentrations up to 1.0 x 10 s particles cm -3. The lower size limit of the instrument has been determined

experimentally to lie within the range 2-3 nm [Wiedensohler et al., 1997] Particle Fuchs surface area was measured with an epiphaniometer [Gaggeler et al., 1989]. The operational principle of the epiphaniometer involves the production of short-lived gaseous 211pb atoms delivered by a 227Ac source within the instrument. The 2•pb atoms attach onto aerosol

particles which are subsequently collected on a filter and determined by an alpha detector. The measured signal is

proportional to the exposed Fuchs surface of the aerosol particles [Fuchs, 1964]. For smaller particles (less than 100 nm) the epiphaniometer signal is proportional to the geometric surface area of the particles and thus to d 2. At larger diameters (greater than 3 gm), access for gaseous atoms to the particles is diffusion-limited, and the signal is proportional to d. Between these regions there is a transition regime. For a polydisperse aerosol the total signal is the integral of the differential products dn.ds with n = particle number concentration and s = Fuchs surface - rc.d '• where x varies between 1 and 2 depending on particle diameter. The signal produced by the instrument is in arbitrary units, and the epiphaniometer requires calibration to provide real data. A calibration curve was developed by generating monodisperse aerosol of 50 nm diameter at different number densities thus varying the total particle surface area with particles in the molecular bombardment regime. An advantage of the instrument is that because surface area is measured in

terms of the access of gas phase atoms, it provides an excellent description of the availability of surface for heterogeneous chemical processes, including condensation of supersaturated vapors. A numerical routine based on calculations described by Pandis et al. [ 1991 ] and Rogak et al. [ 1991 ] was then employed to invert the data, that is, to allow for radioactive decay occurring over the measurement interval. Short lengths of metal tubing (- 1 m) were used for both instruments, and inlet streams were unheated. The only size range for which significant particle losses may occur is that <10 nm [Hinds, 1982]; these are calculated to be 3, 19, and 8% at 3, 5, and 10 nm diameter, respectively.

2.2. Trace Gases

2.2.1. NO, NO2, and 03. Measurements of NO and NO2 were made with an Ecophysics Cranox system which comprises a Tecan CLD 770 chemiluminescent NO analyzer coupled to a Tecan PLC 760 photolytic converter. The CLD 770 unit was calibrated daily with 16.0 ppb NO using an Environics S100 gas calibrator unit to blend together a BOC alpha standard gas mixture of 5.20 ppm NO in N2, certified accurate to 1%, with air dry supplied from an Ecophysics PAG 002 pure air generator. The efficiency of the photolytic conversion of NO2 to NO was measured daily via a back titration procedure involving the reaction of 12.0 ppb 03 with 16.0 ppb NO. Daily checks were also made of the NO2 content of the NO calibration gas and the NOr content of the pure air supply of the PAG 002 unit. The uncertainty of the measurements is estimated at +5%. Ozone was determined photometrically using a an Environics S300 ozone analyzer which was incorporated into the Cranox system described above.

2.2.2. NOy. Measurements of NOy were made with a gold- catalyzed NOy converter coupled to a Thermo-Electron model 42S chemiluminescent NOr analyzer maintained in the NO measurement mode. For an integration time of 60 s and an air sample intake of 1L/min, the NOr analyzer had a detection limit of-•30 ppt. The NOy converter comprised a 6 mm OD gold tube of volume 6.0 cm 3 (Goodfellow Metals, 99.9%) fitted at each end with a modified Swagelok 6 mm OD stainless steel bulkhead union. The gold tube, two 220 W rod heaters, and a K-type thermocouple probe were sandwiched between two copper plates which were insulated with Triton B in a stainless steel housing. With the converter heated to 350 +/- IøC in the presence of 0.15% vol/vol CO (BOC research grade), both the efficiency of the passage of NO through the system and the

HARRISON ET AL.: OBSERVATIONS OF NEW PARTICLE PRODUCTION 17,821

conversion of NO2, HNO3, and peroxyacetyl nitrate (PAN) to NO were at least 99%. Note that the NOy converter will tend to measure the smaller (submicron) particulate nitrates with good efficiency but displays a lowered efficiency for the larger, sea- salt (e.g., NaNO3) particles [Colvile et al., 1996]. No tests on nitriles or amines, nor for artefact NH3, HCN, and CH3CN were performed. More detail of the procedures and tests adopted is given by Solberg et al. [1997] and Kliner et al. [1997]. A correction for cross-sensitivity to NH3/NH4 + is described by Harrison et al. [ 1999b].

2.2.3. Dirnethylsulphide (DMS). Air samples were pressurized over a few minutes into 6 L stainless steel (Summa) sampling bottles using a Teflon headed pump. DMS and other reduced sulfur gases were then cryogenically preconcentrated from a known quantity of air onto a Teflon loop containing Tenax held at -150øC. This temperature prevented cotrapping of liquid 02. Oxidants such as NO2 and 03 were removed from the sampled air prior to preconcentration using Mg(C104)2 and KI. The gases were thermally desorbed to a HP 5890 gas chromatograph (GC) fitted with a Chromasil 300 analytical column and a flame photometric detector (FPD) for analysis of sulfur compounds. Further details are given by Davison and Hewitt [1992] and Davison et al. [1996]. Calibration was carried out on a daily basis using a DMS permeation tube.

2.2.4. Sulphur Dioxide. Continuous sulphur dioxide measurements were made (1 minute averages) using a model 43 pulsed fluorescence SO2 analyzer (Thermo Environmental Instruments) which has a nominal detection limit of 0.1 ppb. The instrument was calibrated on a weekly basis throughout the campaign using an SO2 permeation tube, and cross-checked with dilution from a cylinder of 1 ppm SO2 in air (BOC Ltd).

2.3. Photolysis Rate Coefficients

The O3-->O(1D) photodissociation coefficient, do•D measurements were made with a Meteorologie Consult radiometer mounted on top of the 10m tower at WAO. dolD was determined via continuous measurements of the solar actinic

flux in the 290-330 nm spectral region of a 2to steradian hemisphere, using an optical interference filter centered on 303 nm and an upward facing, solar blind Hamamatsu, R431S photomultiplier tube. The radiometer is also based on the design of dunkermann et al. [1989], modified with a quartz optical input system such that the maximum deviation from a uniform angular response is ñ5%. The temperature of the radiometer was maintained at 35øC, and the actual value of do• was calculated using a proprietary software package.

The NO2 photodissociation rate coefficient dNo2 was measured with a Meteorologie Consult radiometer mounted vertically in a north to south orientation at the top of the WAO tower. dNo2 was determined via continuous measurements of the solar actinic flux in the 300-380 nm spectral region of a 4rr steradian hemisphere using upward and downward facing photodiodes (Hamamatsu, R488-2) each with a Schott UG 11 optical prefilter. The instrument is based on the device reported by dunkermann et al. [ 1989], modified by using a set of quartz diffusers to minimize the dependence of the signal on the angle of incident light. The accuracy of the measurements was estimated at ñ5%. Calibration of the radiometer is carried out

on an annual basis by the manufacturers using a chemical actinometric technique. Full details of the calibration method can be found elsewhere [Volz-Thomas et al., 1996].

2.4. Meteorological Data and Back Trajectories

Information on local wind direction, wind speed, humidity, and irradiance were all obtained from the automatic weather

station (AWS) located at the top of the tower at WAO. The AWS was linked to a computer inside the main building, and all meteorological parameters were logged at 1-min intervals. Five- day backward trajectories produced by the trajectory Atmospheric Environment Service-Long-Range Transport Model of Air Pollution (AES-LRTAP model) at the Canadian Meteorological Centre (CMC) [Voldner et al., 1981] were calculated every 6 hours four times a day at 0000, 0600, 1200, and 1800 UTC. The model, based on a constant acceleration

formulation of the trajectory equations, employed three- dimensional objectively analyzed wind fields for the four pressure levels: 1000, 850, 700, and 500 hPa. Cubic interpolation was used to obtain winds at intermediate levels in the vertical direction and between horizontal grid points. Calculations were performed on the standard CMC gridscale which gives a polar stereographic projection of the Earth, equal to 381 x381 km at 60øN.

At the 1000 mbar level, where the observed wind data are sparse, the analyzed winds become approximately geostrophic. Thus, to account partially for the ageostrophic component or cross-isobaric flow, a frictional turning term has been introduced. It is based on the Cressman drag coefficient [Haltinet, 1971] and is described in detail elsewhere [Olson et al., 1978]. The vertical wind speed is computed from the "omega" equation using the Halfiner technique [Haitinet et al., 1963] at 850, 700, and 500 mbar levels and from the terrain pressure fields and Cressman's drag coefficient [Haitinet, 1971] at the surface. Input windfields, available every 6 hours on the grid, are interpolated in time and space to coincide with the time step used in the trajectory computation. Terminal air mass trajectories were compared with 6-hourly average local wind directions and found to agree very closely (R 2 = 0.92; N- 105).

Information on the state of the tide in the study area was obtained from the British Oceanographic Data Centre and from a commercially available computer prediction program (Tidecalc). The data obtained were for Cromer (located approximately 8 miles to the east of Weybourne), but further information suggested that the times of the high and low tides at Cromer and Weybourne would differ only by some 10-15 min.

3. Results and Discussion

The location of the Weybourne measurement site (Figure 1) is such that it receives air from a wide variety of origins. From analyses of 5-day back trajectories we were able to define four categories of back trajectory illustrated in Figure 1. These were as follows:

i. In type 1 the air mass arriving at WAO had passed across populated areas of southern England and had been in contact with the land immediately prior to arrival in the study area.

ii. In type 2 the air mass had passed over the shallow estuarine area known as The Wash and/or adjacent areas of the North Sea before arriving at WAO. This is different to type 1 trajectories in that the airm mass had passed over 40-50 km of sea before arriving at Weybourne.

iii. In type 3 the air mass arriving at WAO had originated from a northerly direction but had crossed land as it moved southward towards WAO. In general, it had traveled a distance


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of at least 100 km over the sea without making landfall prior to arrival at WAO.

iv. In type 4 the back trajectory showed arrival from a northerly direction without contact with land for several days before arrival at WAO. These were either Arctic air masses

transported for several days over the sea, or air which had originated in mid Atlantic and traversed around the north of Scotland before approaching Weybourne from the north.

Particle measurements were analysed in relation to the air mass back trajectory type as well as the local wind direction and the NOy content of the air. Earlier analyses of data collected at WAO have shown a very high degree of correlation between carbon monoxide and NOy (R 2 = 0.87) [Cc•rdenas et al., 1998] indicating that both species are an excellent indicator of the degree of pollution of the air mass. The use of this categorization of air mass types has also proved useful in identi•ing different air mass characteristics in relation to oxidized and reduced nitrogen compounds [Harrison et al., 1999b].

In this work it was important to distinguish between particles arising from pollution sources and those formed recently within the atmosphere. In more recent studies we and others have deployed tandem particle counters with, for example, lower size cutoffs of 3 and 7 nm, respectively [e.g., Harrison et al., 1999a]. In this case the difference between the counts from the two

instruments represents particles in the 3-7 nm region indicative of particles newly formed from homogeneous nucleation, although such processes can occur in the exhaust from combustion processes [Shi and Harrison, 1999] as well as through homogeneous nucleation within the atmosphere itself. In the study reported here, only one particle counter was available with a lower cutoff at 3 nm diameter. It is, however, worthy of note that event at roadside locations mean particle counts are generally below l0 s cm -3 despite massive particle emissions from heavy traffic [Harrison et al., 1999a]. Therefore the particle concentrations observed within this work, which on a number of days came close to l0 s cm '3, are far above those explicable by the influence of local pollutant emissions at rural sites where particle counts lie more typically within the range of 103-104 cm '3. The nearest road of any significance is several hundred meters from the measurement site and is not a heavily trafficked highway, and there are no major point sources in the locality. Ship traffic was observable off the coast, although this is not a heavy shipping lane. Ship stack emissions were perceptible from tiny peaks in the NOx trace, but the impact upon sulphur dioxide and particle concentrations was negligible. We are therefore wholly confident that while pollutant emissions are largely responsible for the baseline in particle count, they are in no way able to explain the massive excursion in particle number concentration observable on several days of the

HARRISON ET AL.' OBSERVATIONS OF NEW PARTICLE PRODUCTION 17,823

campaign. A further possibility which has been examined is that subsidence of particles from free tropospheric air might give increases in particle number density. The synoptic situation for the month of June started with high pressure centered over the Atlantic Ocean to the southwest of the United Kingdom bringing a week of westerly winds followed by a week of northerly winds. The first half of the month was exceptionally cool and dull. On June 7 the anticyclone which had been centred to the southwest of the United Kingdom transferred northward and a cool cloudy northerly airflow became established. At Norwich to the south of the measurement site, the Sun shone for less than 4 hours during the 8-day period June 8-15. A brief spell of westerly weather ensued from the Jun 16- 20 followed by establishment of an anticyclone over the British Isles from June 20 (J171) onward which brought much fairer weather. It is therefore only latterly that anticyclones brought

subsiding air, but this is not thought likely to have had a major influence on particle concentrations, since during the Atlantic Stratocumulous Transition Experiment-Marine Aerosol and Gas Exchange (ASTEX-MAGE) campaign in the Azores we measured particle number concentrations on the Island of Santa Maria, rarely seeing concentrations above 103 cm '3 despite persistent anticyclonic conditions [Harrison et al., 1996a]. The highest number concentration was 4000 cm '3 corresponding to a local pollution event. While particle number concentrations were not measured in the free troposphere during that campaign, measurements of cloud condensation nuclei (CN) [Harrison et al., 1996b] showed number concentrations comparable with those in the boundary layer.

An overview of the data appears in Figure 2. Despite the unavailability of data from tandem condensation nucleus counters, there are ample data upon which to establish the

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Figure 2. Overview of campaign data: (a) particle count, dO1D, surface area, wind direction; (b) sulphur dioxide, ammonia, hydroxyl, DMS, and relative humidity; (c) NO, NO2, and ozone from J 159 (June 8) to J 182 (June 31).

17,824 HARRISON ET AL.' OBSERVATIONS OF NEW PARTICLE PRODUCTION

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presence of particles formed from homogeneous nucleation processes and to identify the impact, where it occurs, of air pollution phenomena. This was achieved by examination of particle number to surface area ratio and the fact that Fuchs surface area is well related to NOy concentration (R 2 = 0.49), as shown in Figure 3. In contrast, a plot of CN number density against NOy concentration (Figure 4) shows a much weaker

correlation (R 2 = 0.08) with a predominance of points to the left of the graph with CN concentration below about 18,000 cm '3, with a second population of points with higher CN concentrations up to 105 cm -3, but NOy concentrations remaining mostly of the order of 5 ppb. This is shown more clearly in Figure 5 in which condensation nucleus counter:surface area ratio data greater than about 50 I. tm -2 are indicative of

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Figure 3. Scatterplot of NOy (ppb) versus surface area (epiphaniometer) !lm-2/cm '3) during WAOSE 1995 (hourly averaged data).


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Figure 4. Scatterplot of NOy (ppb) versus condensation nuclei (CN) (particles cm '3) during WAOSE 1995 (hourly averaged data).

exceptional particle number densities in relation to the degree of pollution of the air mass and are therefore probably reflective of new particle formation. The diurnal pattern of particle numbers (see below) is also indicative of new particle formation, rather than an advection phenomenon.

The particle number to surface area ratio (CN/Epi) has been evaluated in relation both to back trajectory type and to local wind direction sector. This shows perhaps surprisingly that the highest mean ratio is associated with trajectory type 2, that is, those approaching Weybourne from a westerly or noahwesterly direction across The Wash and/or North Sea and having had rather recent contact with the land. In comparison, the type 4 "clean" air trajectories from the north showed a mean CN/Epi ratio (25 •m '2) twofold lower than the type 2 trajectories (mean equal to 50 •m '2) suggesting that new particle formation is not as favorable in the cleaner air. This was confirmed by analysis of CN/Epi ratio in relation to local wind direction sector (Figure 6) which shows peaks centered on 225 ø and 315 ø . These

directions are respectively south-westerly and northwesterly; the former would be expected to carry air having traveled across polluted southern England, while the latter air would probably have traveled across the land of central or northern England before crossing the sea to Weyboume. It is not immediately obvious why these local wind directions should be most associated with the particle production events. It is, however, clear from Figure 6 that trajectories between 0 ø and 180 ø (i.e., those likely to have originated from the Arctic or having a substantial continental European input) appear to have no tendency whatsoever to enhanced new particle production.

Further support for the idea that the massive excursions in particle count and CN/Epi ratio are associated with new particle formation rather than local pollutant emissions comes from plots of particle number concentration and of Fuchs surface area versus wind speed (not shown). In the case of Fuchs surface area there is a tendency for concentrations to reduce exponentially with increasing wind speed as would be expected

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Figure 5. Scatterplot of NOy (ppb) versus CN (particles cm '3)/surface area (gm2/cm 3) ratio during WAOSE 1995 (hourly averaged data).


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Figure 6. Scatterplot of CN (particles cm '3) /surface area (•tm2/cm 3) ratio versus wind direction sector (hourly averaged data).

for a dilution phenomenon. This is therefore clearly associated with primary emissions. On the other hand, CN concentrations show no obvious relationship to wind speed with the highest particle number counts occurring within the wind speed range 4- 9 m s '•. The two measures of particles thus show a quite different behavior giving a further strong indication that the higher particle number densities are associated with homogeneous nucleation within the atmosphere.

3.1. Specific Factors Which Might Influence New Particle Formation

High data recovery was achieved for measured parameters for some 26 days during the Weybourne Summer 1995 campaign. Of these, there was evidence of strong production of new particles on a total of 5 days, on all of which the particle count climbed to in excess of 35,000 cm -3. On a further 4 days there was quite good evidence of particle production leading to peak CN counts between 8000 and 25,000 cm '3. The entire data set (summarized in Figure 2) was scrutinized with a view to evaluating which potential chemical and physical influences upon particle formation might be having the greatest impact. The factors considered were as follows:

3.1.1. Background particle number concentration. Interestingly, the days indicated by CN/Epi ratio as giving the highest number of new particles (J159 and J169-J172) were those starting from a high background number of existing particles, on these days within the range 8000-15,000 cm '3. Other days showing evidence of new particle production had typically around 2000 cm '3, significantly above a clean air background concentration of condensation nuclei for this site of 500 cm -3.

3.1.2. Particle surface area. This was expected to be an influential factor, but examination of the data sets revealed that the days showing the greatest new particle production (J159 and J169-J172) had amongst the highest preexistent particle surface area at the onset of new particle production. On a number of days, for example J 172 shown in Figure 7, the first burst of new particle production corresponded to a minimum in the surface area count, but this may be purely coincidental, as the drop in

aerosol surface area is concurrent with the dawn rise in ozone

photolysis (Jo•o) and the morning rise in the top of the surface boundary layer. Thus new particle formation on J 172 took place in the presence of a Fuchs surface area of about 200 gm2/cm 3 on the epiphaniometer, whereas on other days without obvious formation of new particles, epiphaniometer counts could be more than tenfold lower. Existing surface area does therefore not appear to be a very important determinant of the new particle formation potential of the air, contrary to the predictions of theory [Pirjola and Kulmala, 1998].

3.1.3. Humidity. High airborne water vapor concentrations are expected to favor homogeneous binary nucleation of sulphuric acid and water; although ternary nucleation involving ammonia is predicted to be independent of relative humidity [Korhonen et al., 1999]. The ambient relative humidity at 0900 LT which ranged from 60-105% was therefore considered as a surrogate measure of water vapor at the time of day when particle bursts most commonly occurred. No obvious relationship of particle production to relative humidity was discernible as revealed by correlation analysis. On most days with a major particle burst the burst commenced at a time in the early morning when sunshine was beginning to heat the lower atmosphere, with temperture consequently increasing and relative humidity decreasing. This was observed also by Weber et al. [1997], who suggested that the drop in relative humidity might be associated with a reduction in particle surface area due to the sampling of cleaner air or a reduction in surface area of hygroscopic particles, either of which would favor particle nucleation. It appears that the influence on surface area by either of these mechanisms may outweigh any dependence on water vapor concentration.

3.1.4. Local wind direction. This point has been discussed above in terms of analysis of the entire hourly dataset, but examination of specific occasions when new particle bursts were observed showed wind directions in the sector between 210 ø and

315 ø . This includes direction sectors associated with air

movement both over the land and the sea, and no obviously common feature was recognizable. Thus no local pollution source appears to be responsible. This conclusion is reinforced by the very different relationships between CN and Fuch surface area with wind speed referred to above.

HARRISON ET AL.: OBSERVATIONS OF NEW PARTICLE PRODUCTION 17,827

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Figure 7. CN (particles cm'3), surface area (epiphaniometer)/100 gm2/cm 3, and JO•D (s'•), June 21 1995.

3.1,5. Dimethyl sulfide concentration. DMS is an obvious potential precursor of particle production, and although the data were a little sparse, one or more measurements of DMS was available for almost all of the days in the campaign. Days of high particle production were associated with a wide range of DMS concentrations from the lowest in the campaign (75 ng S m -3) to concentrations around the median (400 ngS m -3 on J 159). Thus no clear link to DMS was discernible. The fact that particle production is so clearly related to J(O•D) (see below) suggests very local formation of new particles, and it is therefore local, rather than more distant DMS concentrations which would be relevant.

3.1.6. Sulfur dioxide concentration. Rather than involving DMS as a precursor, direct oxidation of sulphur dioxide leads to formation of sulphuric acid which can undergo binary nucleation with water vapor to form new particles. Concentrations of sulphur dioxide on the days of enhanced particle production were generally relatively high being in the range 2-20 ppb. This provides a possible link with particle production, although on J157, having 20 ppb SO2 and peaks to 50 ppb, and a wind direction in the favored sector, new particle production was not observable. The fact that on several of the days of new particle production the air had come off the land effectively rules out ship emissions as being the source of both SO2 and particles.

3.1.7. Hydroxyl radical concentration. Homogeneous oxidation of sulfur dioxide is predominantly by reaction with the hydroxyl radical, and relationships between OH concentration measured at the site and new particle production were sought. However, the OH radical data set was rather sparse with data available for only 12 days of the campaign, and no clear conclusion on this relationship is possible, although relatively high concentrations (4-6 x 106 cm -3) were observable on 5 days within the campaign when high or moderate new particle production was observable.

3.1.8. Ozone photolysis. The rate constant for production of O•D oxygen atoms from ozone photolysis was evaluated using a photometer. This photolysis frequency relates to the rate of production of hydroxyl radical by reaction of O•D with water vapour m•d hence this variable may be taken as a crude surrogate of hydroxyl radical production as ozone and water vapor concentrations did not vary greatly through the campaign. There were days such as that shown in Figure 7 on which the early morning rise in do•z• was closely paralleled by the rise in condensation nuclei count. Thus it seems probable that OH production was one of the factors connected with new particle formation. The days on which highly enhanced particle formation was observed generally showed quite high values of donzi, but there were other days with equally high values of dolz• with sulphur dioxide measurably present which showed no formation of new particles. It appears that this is an important variable, as would be expected if photochemical processes were responsible for formation of the new particles, but clearly other factors are also at play. This point is returned to later in the case study of June 18-21, 1995 (J 169-J 172).

3.1.9. Ammonia concentration. Recent research has

suggested that a ternary nucleation process involving sulphuric acid, water vapor and ammonia may be a more effective producer of new particles than the binary nucleation of sulphuric acid and water vapor alone [Coffman and Hegg, 1995]. Ammonia concentrations measured with a denuder between

0600 and 1400 hours ranged from 0.02 to 0.85 gg m '3. Lowest concentrations, as anticipated, were generally associated with wind trajectories off the sea, and as noted above, the type 4 "clean" air trajectory were those not associated with new particle production. Some of the most enhanced production of new particles was seen on days J169-J172 (June 18-21) with local wind directions from the southwest or west and relatively high ammonia concentrations (0.10-0.85 [tg m-3). It is therefore possible that this is an important parameter influencing new


particle production potential. Unfortunately, the 8-hourly resolution of the ammonia data does not allow us to draw a

firmer conclusion.

3.1.10. State of the tide. Recent work conducted at Mace

Head on the western coast of Ireland has shown a distinct

association between bursts in new particle production and low tide [Allen et al., 1999; O 'Dowd et al., 1998]. An examination of the relationship between particle production and the tide at Weyboume during this campaign showed little obvious relationship, with a far clearer association between particle bursts and Jo]o than for low tide. There were days upon which particle production appeared to coincide with the low-tide period, but the overall impression was that at this site the state of the tide is probably not an important factor. However, it should be noted that this is a shingle coastline and that the march of the tide leads to little exposure of biota and little variation in the proximity of seawater to the site. This is unlike some other locations, including coastal areas to the west of Weyboume where low tide is associated with exposure of substantial areas of sand and saltmarsh to the atmosphere. Given the sensitivity of the tidal cycle to location and the generally westerly nature of the winds during the particle burst situations, it is possible that an influence of low tide in the upwind regions may be important when winds were in the westerly (but not south-westerly) sectors. The phenomenon reported by O'Dowd et al. [1998] and Allen et al. [1999] at Mace Head is, however, a very local phenomenon and apparently quite different to that at Weybourne.

3.2. Case Study of June 18-21, 1995 (J169-J172)

Over the period June 8-29, 1995 at least 5 days were present when J(O]D) reached its maximum value for this time of year (- 2 x 10 '5 s'•), and several other days were present when only a small drop in the maximum occurred due to cloud attenuation. Midday air temperatures during this period varied from 11 ø-

20øC, and relative humidities varied from 55 to 80%. Wind direction all had a westerly component, although they varied from day to day mainly between southwesterly and northwesterly. The period as a whole, however, was compromised by the presence of a sea mist which undoubtedly also had some influence on the CN record. In spite of this, it is possible to draw some conclusions regarding the mechanism of CN production in clean and polluted air.

The 4-day period between June 18 and 21 (J169-J172) is of particular interest since the air was significantly polluted with plumes reaching the observatory from major urban areas: Birmingham (J169), London (J170) and Scottish Central Lowlands (J 172). The CN count reached 100,000 particles cm '3 on 2 of these days (J 169 and J170) and reached 70,000 cm '3 on 2 others (J171 and J172). The sequencing between the rapid rise in CN count in the morning and the move away from the baseline of J(O•D) is quite exact for most of these days (see Figure 8). Large CN counts were observed well before the maximum in J(O•D), and the CN count then decayed even though J(O•D) was still increasing. The relationship between growth of CN and growth of NO from NO2 photolysis is shown in Figure 9. There is a distinct lag of at least 2 hours between the rise of NO and the rise of CN emphasizing that initiation of particle production requires light of the quality necessary to produce hydroxyl radicals from ozone photolysis (i.e.,)• < 315 nm) rather than that required to photolyze NO2 (• < 420 nm).

The influence of SO2 on potential CN production is examined in Figure 10. There is no doubt that the period of maximum CN production is associated with the presence of significant quantities of SO2 in the region of 1 to l0 ppb. However, the presence of SO2 does not seem to be the only critical factor in producing large numbers of new condensation nuclei. On June 29 (J180), for example, an increase was observed in the CN count in association with an increase in SO2 from near zero to about 4 ppb. On this day, however, the CN count maximized before J(OID) had increased above the

14

'7, 12

0 10

a, 8

0

• 4

18 19 20 21

NO .... jN02 ................ j(O1D) •CN Count

120000

100000

80000

60000

40000

20000

Figure 8. Time series plot of CN count, J(o]z>), NO, and OINO2 from June 18-21, 1995.


o z o

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10000O

80000

60000

40000

20000

18 19 20 21

[ .................. CN Count -- NO I

4.5

3.5

2.5

2

1.5

0.5

Figure 9. Time series plot of CN count in relation to NO formation from NO2 photolysis, June 18-21, 1995.

baseline, and the most likely explanation for this is not in situ production but rather long-range transport of an aged plume containing SO2 along with other gases and particulate matter. This behavior was clearly different. however, to the days with greatest particle number concentrations with the CN/Epi ratio not deviating appreciably form background. Small increases in CN through the day were also observed on June 14 and 15 (J 165 and J 166) that may have been associated with in situ production, but the growth rate was much smaller than observed in the polluted air present after J167. On other days such as J176, however, no increase in CN was observed at all. On these days little SO2 was present, and the lack of increase in CN may have been associated with the copresence of coastal fog as indicated by low J(O•D) values. It is perhaps surprising that no significant CN production occurred in the period June 27 to 29 (J178 - J180) even though the J(O•D) record suggests sunny days uninterrupted by cloud or sea mist and small amounts of SO2 in the range 1-2 ppb. Overall, the observations made at Weybourne suggest that CN production is associated with hydroxyl radical chemistry and the presence of SO2. The observations also suggest though that the copresence of some other species is necessary to account for the differences between days such as J180 and J173 and for other sunny days with significant SO2 concentration, plenty of daylight, but no significant CN production. It is possible, but by no means conclusive, that ammonia (0600-1400 concentrations of 0.85 •tg m '3 and 0.15 •tg m '3 on J 170 and J 173, respectively) played that role.

3.3. Model Study of Homogeneous Nucleation

The above data analysis points to oxidation of sulphur compounds and formation of sulphuric acid aerosol being a likely local source of particle production. In order to analyze

this possibility further, a model study was conducted which followed the approach suggested by Seinfeld [1986] to allow evaluation of conditions when homogeneous nucleation is favored. The approach depends upon estimation of characteristic timescales for the two competing processes of formation of new particles on the one hand and condensation of volatile vapors on existing particle surfaces on the other hand. This approach is facilitated by the availability of epiphaniometer measurements of aerosol Fuchs surface area which are directly applicable to the estimation of timescales for the condensation process. Calculations were conducted to estimate timescales for these processes on Julian days 159, 170, and 172 for which data capture was high for all of the variables of interest.

Two characteristic timescales relating to the aerosol are calculated, that is (1) x,,, the timescale for new particles to form via homogeneous binary nucleation (the gaseous precursors are assumed to be H2804 and H20 ) and (2) xp, the characteristic timescale for loss of new particles on pre-existing aerosol. The value of x, is calculated from the following expression [Seinfeld, 1986]:

'r.= H2SO4max/RH2SO4 (1)

where H2SO4max denotes the H2SO 4 maximum gas-phase concentration at the point where the Gibbs free energy surface to homogeneous particle formation has just been surmounted. This quantity is obtained by first estimating the nucleation rate o r, hence the activity a, as a function of %RH and T. This nucleation rate was calculated with a binary nucleation model [Shi and Harrison, 1999] based on the nucleation theory of Kulmala et al. [ 1998]. Then,

NH2SO4max = a (10 lø's) RH2SO 4 (2)

RH2SO4 is the rate of production of H2SO4 via gas-phase oxidation from DMS and SO2, obtained from a photochemical


120000 - 50

100000

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soooo !l 20000 l• • i

0 ......... •"'•' •/, •-•, & 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 O1

I .......... CN Count .---[S021 ]

Figure 10. Time series plot of CN count in relation to SO2 concentrations, June 18-21, 1995.

45

40

- 35

- 30

-25 • O

- 20

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10

box model, constrained to a suite of hourly averaged observed values. The model has been described by Grenfell et al. [ 1998]. In the calculations described here, a DMS-oxidation scheme [denkin, 1996] has been incorporated. The model was constrained to observed CH4, CO, JOlD, 03, NO, NO2, NO3 (where available), HCHO, %RH, T, SO2, NMHCs (including DMS and isoprene, where available), OH, and aerosol Fuchs surface area. For some periods, measurements of OH were available; for others, model-generated OH concentrations were used.

The value of z v was calculated from the expression due to Seinfeld [ 1986]:

'c t, = NH2SO4/13A = 2(rcm/kT)V'/A (3) where

NH2804 [H2804] molecules cm'3; A surface area (from epiphaniometer); 13 rate of impingement; k Boltzmann's constant; m mass H2SO 4 molecule; T absolute temperature.

Note: x v and x,are analogous in that they both measure the change of ambient H2SO4 (numerator term) relative to a particular rate of change for this molecule (denominator term) which measures the gas-phase formation, and hence on aerosol for 'c v and % respectively. Note that 'c v (and z, do not directly depend on NH2SO4 (this term cancels in equation (2)) for their evaluation which is fortuitous since this is difficult to estimate

using a numerical model. Figure 11 shows hourly averaged values of CN

(particles/cm 3) and x,,/xp coplotted for J159, J170, and J172, 1995. Vertical, dotted lines delineate the point at which particle production begins to take place. The figure indicates that on

J 170 and J 172, the onset of particle production corresponds to similar values of x,/xp. namely, 270 and 383, respectively. On J159, however, this value is reduced to 95. A plot of (CN/surface area) ratio versus the z,,/'cp ratio, for periods in which particle formation was taking place, namely 0900- midday, 0700-1100, and 0700-1000 LT on J159, J170, and J172, respectively, showed the expected negative relationship and a correlation coefficient R = -0.54.

The plot in Figure 10 shows that the ratio 'cJ'c•, is quite a good predictor of the periods when rapid bursts of new particle formation take place. If the theory were perfect, these bursts might be expected to take place when 'cJ'c•, reaches values of less than one. However, there are two very plausible explanations of why nucleation is occurring at higher values of the ratio. The first is that homogeneous nucleation theory is known to be highly imperfect and that weaknesses in the theory may account for this divergence. Equally probable is that our treatment assumes binary nucleation of sulphuric acid and water, whereas ternary nucleation involving ammonia also, as suggested by Coffman and Hegg [ 1995] and, more recently, by Korhonen et al. [1999] may be taking place. Both papers show that the presence of quite modest concentrations of ammonia can increase homogeneous nucleation rates considerably, and it is notable that as commented upon above, nucleation phenomena were associated with wind sectors in which ammonia

concentrations were highly significant. It is interesting that based upon formation reaction rate data

alone, Marti et al. [1997] similarly conclude that the SO2-OH reaction is probably responsible for ultrafine aerosol formation at a remote continental site.

3.4. Particle Growth Rates

Weber et al. [1997] have estimated ultrafine particle growth rates during observed atmospheric nucleation events in clean air


10000

ß •' 1000 "

10 ,,, t • t I • • i 1000 00000000000000000000

00000000000000000000000000000000000 ........................................................................

0••0• •0•0••0••0•0••0••0• 00000 •• • •00000• ••• 00000•• ••

Time (GMT)

J159 J170 J172

>'• • • • • 100000

,••;•(n)/tau(p)

10000 •

Figure 11. CN (particles cm '3) and % (nucleation timescale) / x t, (heterogeneous loss timescale) ratio for J 159, J 170, and J172 During WAOSE 1995.

at a continental site at Idaho Hill, Colorado. They estimated the growth rate of new particles from the time taken from an increase in gas phase sulphuric acid concentrations to manifest itself as measured 3 nm diameter particles. The observed growth rates of 0.5-2.0 nm/h were faster than those calculated for

growth of sulphuric acid/water particles by a factor of about 10. In our work, gaseous H2SO4 was not measured, and nor were measurements of OH radical concentrations made at dawn. We

are therefore not able to make a directly comparable calculation, but have estimated particle growth rates by assuming that the lag between the rise in JO•D and in ultrafine particle count corresponds to the time taken to grow particles from H2SO4/I-I20 nuclei of 1 nm diameter to 3 nm, the smallest size detectable by the ultrafine particle counter. The results for JD169, 170, 172, and 173 are remarkably consistent with dO•D moving above the baseline at 0500, beginning a significant rise at 0600 followed by the rise in particle number count at 0730. There appears therefore to be a delay of around 1.5 hours in the appearance of the ultrafine particles suggesting a growth rate from 1 to 3 nm diameter of the order of 1.3 nm/h, highly consistent with the rates estimated by Weber et al. [1997]. We are unable to compare this measured rate with that calculated for growth of H2SO4/I-I20 particles because of the lack of measurement data for gas phase H2SO 4.

4. Conclusions

The abruptness of the rise in particle number count and number density to surface area ratio are clearly indicative of a rather local homogeneous nucleation phenomenon. It is difficult to envisage any other explanation. The fact that the onset of particle production so often correlated with measurements of JO•D gives a strong hint that the hydroxyl radical is involved in initiating the production of particles. This is in contrast to measurements at some other coastal locations where, although the presence of sunlight is important, the timing of particle bursts is related more closely to the low tide period and seems therefore to be connected with the production of precursors or of reactive intermediates other than OH. In the case of Weyboume the nature of the coastline is such that enhanced precursor

production due to exposure of marine macrophytes at low tide is likely to be only a minor effect.

The model study goes further in showing that particle burst phenomena occur at times when the timescale for nucleation is heading for a minimum with respect to the timescale of the competing processes of condensation of volatiles on existing particle surfaces. The fact that nucleation is occurring at times when the calculated timescale for nucleation still exceeds that

for condensation may be connected to imperfections in nucleation theory, or more probably to the involvement of ammonia in a ternary nucleation process. The enhanced particle production events occur for air masses in the two most polluted sectors. This is probably due to the relative abundance of sulphur dioxide and of ammonia in the air associated with these sectors suggesting that at this relatively polluted site the oxidation of sulphur dioxide is a more important contributor to formation of new sulphate particles than oxidation of DMS derived from the sea.

Acknowledgments. The authors are grateful to the U.K. Natural Environment Research Council for funding under the TIGER (GST/02/624)) and LOIS (GST/02/0780) Community Research Programmes. They thank Susan Askey and John Peak for assistance in sample collection, and Jiping Shi for advice upon the modeling of homogeneous nucleation processes.

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A.Allen, J. L. Grenfell, R. M. Harrison (corresponding author), and N. Savage, Division of Environmental Health and Risk Management, University of Birmingham, Birmingham B15 2TT, England, U.K. (r'm'harrisøn'ipe@bham'ac'uk)

K. C. Clemitshaw, Centre for Environmental Technology, Imperial College of Science, Technology, and Medicine, Silwood Park, Ascot, SL5 7PY, England, U.K.

B. Davison and C. N. Hewitt, Instittue of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, England, U.K.

S. Penkett, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, England, U.K.

(Received October 25, 1999; revised February 1, 2000; accepted February 4, 2000.)

Documents

Observations of new particle production in the atmosphere of a moderately polluted site in eastern England