EEA/IDM/15/026/LOT1

Report on European greenhouse gascolumn cal/val network sustainability

Issue: 1.0Date: 19/01/2020

Framework Service Contract EEA/IDM/15/026/LOT 1 for Servicessupporting the European Environment Agency’s (EEA) implementation

of cross-cutting activities for coordination of the in-situ component ofthe Copernicus Programme Services

Report on European greenhouse gas columncal/val network sustainability

Dietrich G. Feist,Mahesh Kumar Sha,

and Thorsten Warneke

Issue: 1.0Date: 19/01/2020

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Report on European greenhouse gascolumn cal/val network sustainability

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[Document Release]Name(s) Affiliation

Coordinated by: Dietrich Feist TCCON / DLR-IPA / LMU

Contributions: Mahesh Kumar ShaThorsten WarnekeRüdiger LangRichard EngelenMartine De MazièreClaus Zehner

BIRA IASBUniversity of BremenEUMETSATECMWFBIRA IASBESA ESRIN

Approval: Henrik Steen Andersen European Environment Agency

Prepared for: European Environment Agency (EEA)

Represented by: (Project Manager)

Henrik Steen Andersen

Contract No. EEA/IDM/15/026/LOT1

[Change Record]Version Date Changes

0.9 20/11/2019 First draft release presented at Nov. 2019 team meeting

1.0 19/01/2020 Second draft release – incorporating comments from team meeting Nov. 2019 and other contributors.

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Table of Contents

1 Executive summary..............................................................................................................4

2 Introduction.........................................................................................................................5

2.1 Column-averaged vs. ambient-air greenhouse gas (GHG) measurements..................5

2.2 Satellite column-averaged GHG observations..............................................................6

2.3 Ground based column-averaged GHG observation networks......................................7

3 Importance of ground-based column GHG networks for the European Union's Earth observation programme Copernicus and beyond..................................................................9

3.1 Importance to the Copernicus space component........................................................9

3.2 Importance to Copernicus Services............................................................................10

4 Requirements for operational data users..........................................................................11

4.1 Data quality.................................................................................................................11

4.2 Delivery time requirements........................................................................................11

4.3 Network coverage.......................................................................................................12

5 Current situation of the European TCCON network..........................................................14

5.1 Funding sources of European TCCON stations...........................................................14

5.2 Operational cost of a TCCON station..........................................................................14

5.3 Long-term operational sustainability and risks..........................................................15

6 Pathways to sustainable operation for European GHG cal/val networks.........................17

6.1 Fiducial Reference Measurement (FRM) networks for European GHG satellite missions............................................................................................................................17

6.2 Long-term integration of European TCCON stations into ICOS..................................17

7 Recommendations.............................................................................................................19

8 References.........................................................................................................................21

9 Contributors.......................................................................................................................23

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1 Executive summaryFuture European greenhouse gas (GHG) satellite missions will critically depend on the availability of high quality reference data for calibration and validation. For greenhouse gas observations, very small signals have to be distinguished from a rather large background concentration. Therefore, the precision requirements for satellite GHG observations are on the order of 0.25%. This precision requirement is about an order of magnitude stricter than what is required for many other atmospheric observations. Futureapplications for GHG emission monitoring will require even higher precision and minimal systematic bias.As satellites observe the full atmospheric column from the ground to ~100 km altitude, these reference cal/val data cannot be provided by direct in-air concentration measurements (e.g from ICOS) as these cannot be performed over the full vertical range. Instead, the best reference are high-precision column measurements from ground based remote sensing instruments. The Total Column Carbon Observing Network (TCCON) has been providing the most precise column GHG measurements since 2004. This network is the key cal/val reference for all current and future GHG satellite missions. In the recent past, the ground based network has been complemented by smaller instruments from theCOCCON network which have lower resolution but are more mobile.12 out of the current 26 TCCON stations are run by European groups. Besides the European mainland, some of them also cover critical regions in the Arctic or around Africa. However, there is a striking discrepancy between the importance of these stations for the upcoming European GHG missions and their funding situation. Currently, none of the European TCCON stations has the sustainable funding that would be required for supporting the upcoming European GHG mission CO2M in an operational way. Many cannot even guarantee to keep operating until the launch of the mission without additional support. At the same time, the CO2M mission requirements will likely call for additional stations in strategic locations to better cover the whole parameter space.There are two potential pathways for improving this situation:

• The European TCCON network could be supported as a Fiducial Reference Measurement (FRM) network and thus become an integral part of the European CO2M mission. This would be very similar to the way that many non-European TCCON stations are supported through the US and Japanese GHG satellite missions(OCO-2 & OCO-3, GOSAT & GOSAT-2).

• The other option would be an integration of the European TCCON network into theexisting ICOS research infrastructure. So far, ICOS covers near-surface GHG measurements but not column measurements.

In both cases, the Copernicus , ESA, EUMETSAT and ICOS member states would have to make the decision and provide the necessary long-term funding for the actual operation.

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2 Introduction

2.1 Column-averaged vs. ambient-air greenhouse gas (GHG) measurementsEvery observation method for greenhouse gases has specific advantages and disadvantages. While ambient-air in situ measurements from ground-based stations or aircraft are very precise and accurate, they only cover a part of the total atmospheric column. Ground-based Fourier transform infrared (FTIR) or satellite instruments cover the total atmospheric column but are less precise and the accuracy is dependent on the in situcalibration. Figure 1 illustrates what information different observation techniques can provide about the true vertical CO₂ distribution in the atmosphere.

The well-established ground-based networks for ambient-air GHG measurements (NOAA, ICOS) provide observations near the surface and up to ~300 m in the case of tall-tower sites. These observations are representative for the Planetary Boundary Layer (PBL), roughly the lower 10% of the total column.

Aircraft measurements provide profiles that cover about 60-80% of the total column. The actual coverage depends on the minimum and maximum flight altitude. Even jet aircraft cannot reach the top 20% of the column in the stratosphere. Air traffic control regulations may further limit the coverage of the lower 10-20% of the column – especially above settled areas.

Aircore measurements from a balloon platform can cover about 95-98% of the total column. However, these observations are limited to sites with only minor air traffic where the probe can be easily recovered after launch (no densely populatedareas, no islands)

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Figure 1:The same CO₂ profile (green line) observed by different methods: at the surface (black star), by aircraft (blue plus signs), column-averaged XCO₂ from satellite (orange), column-averaged XCO₂ from TCCON. The width of the column bars represents precision.

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Satellite observations cover the full 100% of the column. The quantity they provideis the total number of molecules of the target species in the optical path. With further information about the path length and some assumptions about the vertical distribution, this is converted into the column-averaged mole fraction of the target species. In the case of CO₂, this quantity is denoted as XCO₂. Satelllite observations provide the best geographical coverage.

The ground-based total column measurement networks TCCON (the Total Carbon Column Observing Network) and COCCON (the Collaborative Carbon Column Observing Network) provide the same quantity as satellite observations (XCO₂, XCH₄, etc.) at higher precision but only at selected locations.

In summary, the column-averaged observations from satellites are not directly comparable to near-surface ambient air measurements and only partially comparable to aircraft/Aircore profile observations. Calibration and validation of satellite column observations requires ground-based column observations. The current reference network for satellite calibration and validation of greenhouse gas observations is TCCON.

2.2 Satellite column-averaged GHG observationsGreenhouse gas observations from space will be a crucial element for reporting and monitoring under the United Nations Framework Convention on Climate Change (UNFCCC). However, currently, the satellite missions observing greenhouse gases like CO₂ or CH₄, face four main challenges:(1) Satellite observations give us the average concentration of the respective greenhouse

gases from the top of the atmosphere to the surface, called the column average. Ground-based in situ stations measuring ambient air give us the concentrations near the surface. Both types of observations are extremely valuable but cannot be compared directly or used for cross-calibration/validation.

(2) The precision requirements for satellite greenhouse gas observations are significantly higher than for many other atmospheric species. Far from the greenhouse gas sources and sinks, satellites have to be able to observe very small greenhouse gas concentration changes of less than 0.5%. To measure such small signals, very precise calibration and validation are crucial.

(3) The satellite retrievals can be subject to a spatial bias, e.g. due to changing surface albedo and/or a temporal drift of the satellite sensor. Both, spatial bias and temporal drift, must be corrected.

(4) The satellite retrievals use spectroscopic data and the retrieved column-averaged mole fractions are not on the same reference scale as the in situ measurements. For flux inversions, it is highly important that the satellite retrievals are on the same reference scale as the in situ measurements.

(5) For the generation of long term climate data sets (e.g. Essential Climate Variables) themeasurements of different satellites have to combined by a careful assessment of the different data sets based on instrument stability, different spatial sampling/coverage, and different wavelength range and algorithms used.

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2.3 Ground based column-averaged GHG observation networks2.3.1 The Total Carbon Column Observing Network (TCCON)The calibration and validation reference for all satellite greenhouse gas observations is theTotal Carbon Column Observing Network (TCCON): a global network of ground-based stations that use remote sensing techniques to observe column-averaged greenhouse gas mole fractions. The network is calibrated against the WMO greenhouse gas standards and provides a link between satellite observations and the ground-based ambient air measurement network.TCCON was set up in 2004 with three stations supporting NASA's Orbiting Carbon Observatory (OCO) mission. Since then, the TCCON network has grown to 26 stations in 14countries (see Fig. 2). TCCON stations use the same instruments (Fourier-Transform Infrared spectrometer, Bruker Model IFS125HR) and a common retrieval code with a stringent quality control to give the most precise column-average measurements of CO₂, CH₄, N₂O, HF, CO, H₂O, and HDO. All current (GOSAT, GOSAT-2, OCO-2, OCO-3, TanSat, Sentinel-5P) and planned (MicroCarb, MethaneSAT, Sentinel5, GEOCARB, MERLIN, TanSat-2, GOSAT-3, CO2M) satellite missions observing greenhouse gases rely on TCCON for their calibration and validation.The current European TCCON network consists of 12 stations located in 4 European countries on the continent or in the Mediterranean (Finland, France, Germany, Cyprus), 3 islands that are part of a European country (Reunion, Svalbard, Tenerife) and 1 UK overseaterritory (Ascension). The respective host institutions are from 4 different countries (Belgium, Finland, France, Germany). Figure 3 illustrates this further.

2.3.2 The COllaborative Carbon Column Observing Network (COCCON)COCCON is an emerging network of small portable FTIR instruments of the type Bruker EM27/SUN which provide column-averaged abundances of CO₂, CH₄, CO and H₂O from

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Figure 2: Map of the Total Carbon Column Network TCCON (2018). Notes: the Garmisch and Zugsitze stations are indistinguishable on this scale and the Bialystok station has been moved to Cyprus in 2019.

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the recorded solar absorption spectra [17]. The network has been initiated by Karlsruhe Institute of Technology (KIT) which provides support for instrument calibration, characterisation, and retrieval software for analysis of the measured data.The EM27/SUN records spectra at 0.5 cm-1 which is a factor of 25 lower compared to TCCON instruments. Several groups have demonstrated the performance and stability achieved by the EM27/SUN spectrometer [4] [5] [8]. It can be used for applications and locations that are not suitable for TCCON instruments. It is light-weight, portable and easy to use even by non-specialists. Therefore, it is ideal as a campaign instrument for the observation of localised sources of GHGs with an array of several spectrometers. It can also be used as a travelling standard for checking site-to-site biases between TCCON instruments. The EM27/SUN instruments in the COCCON network are linked to TCCON by a calibration with respect to a reference TCCON instrument.Currently, users do the full data analysis themselves with open-source code provided by KIT. However, central processing would be preferred and required facilities for that are being set up at KIT. The long-term planning is that only the generation of calibrated spectra including Level-1 QC would be done on-site while the trace gas analysis would be performed at the COCCON central facility. Most stations do not run measurement fully automatic and need an operator on site. A commercial builder for an automatic shelter would be needed to resolve this. At the moment, COCCON would not be ready for integration into an operational network like ICOS.

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Figure 3: Distribution of countries in the European TCCON network by site location and country of the home institution. The site in Cyprus was moved from Poland in 2019.

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3 Importance of ground-based column GHG networks for the European Union's Earth observation programme Copernicus and beyondCopernicus builds upon three components: the space component, the in situ component and the services. While the space component and the services are directly financed via Copernicus, the in situ component is not. However, the in situ component is vital for the Copernicus space component and the services. In the following it is outlined why ground-based column measurements of GHGs are needed for the implementation of the Copernicus space and service components.

3.1 Importance to the Copernicus space componentThe Copernicus Sentinel and contributing missions need total column measurements for validation and calibration of their GHG retrievals. Profile measurements from aircraft or balloon instruments require dedicated expensive campaigns. Despite that, these profile measurements can only provide part of the the total column that the satellites observe (see Fig. 1). Therefore, true total column measurements of GHGs, as done by ground based TCCON instruments are vital for the

calibration of GHG satellite retrievals against WMO standard: Comparison of TCCON with vertically resolved calibrated in situ measurements (AirCore, Aircraft), then comparison of TCCON with satellite.

validation of GHG satellite retrievals.The validation is essential for the detection and correction of biases in the satellite retrievals. Sources for biases are manifold. Below, the most common biases are listed.

3.1.1 Bias due to surface albedoSatellite retrievals need to take the surface albedo into account and might show a dependence on surface albedo. TCCON measures direct sunlight and therefore the albedo at the site has no impact. While TCCON is well suited for this validation, additional sites covering a representative range of surface albedo would be desired.

3.1.2 Bias due to aerosols and cloudsAerosols and clouds reflect light back to the satellite in higher layers of the atmosphere. Hence, the light detected by the satellite is not only coming from the Earth’s surface, but from different altitudes. The satellite retrievals have to be corrected and filtered for aerosol and cloud impact. Some key regions for GHGs are strongly impacted by clouds (e.g. tropics) and aerosols (e.g. downwind of a powerplant/city). In the presence of a thickcloud layer neither the satellite nor TCCON can provide useful retrievals. However, TCCON can measure during partially cloudy conditions and in the presence of thin clouds and aerosols. These are the situations when also satellite retrievals could be used. TCCON demonstrated that its direct sunlight measurements are not biased by thin clouds or aerosols. Therefore, TCCON stations need to be set up at both polluted and unpolluted sites to monitor the effect of aerosols on GHG satellite data.

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3.1.3 Bias due to interfering gasesThe main interfering gas in the spectral region of the GHG retrievals is water vapour. A dependence of the GHG retrieval on water vapour could lead to a spatial and latitudinal bias. An example is the first global first CH₄ retrievals from space, showing strongly enhanced CH₄ over the tropical forests [2], which could not be verified by ship-borne ground-based column measurements on the Atlantic Ocean [10]. The satellite data was revised using updated spectroscopic data for water vapour [3]. TCCON measurements have a higher spectral resolution than the satellites. Since TCCON resolves the individual absorption lines, spectroscopic problems can be detected more easily. Therefore, TCCON sites need to cover different latitudinal regions.

3.1.4 Bias due to viewing geometryAn example is the validation of an across track pixel bias. This is possible with TCCON, but additional stations in suitable locations would be desired.

3.1.5 Bias due to sensor drifOnce in space the satellite sensor could be subject to a temporal drift. This could be detected with the current TCCON. However, this crucial role of TCCON in a future operational GHG satellite constellation requires continuous and robust data provision withsignificant statistics from various sites covering different satellite observations, terrain andalbedo conditions.

3.2 Importance to Copernicus ServicesThe Copernicus Atmosphere Monitoring Service (CAMS) assimilates greenhouse gas data from satellites to model and forecast the atmospheric concentration of CO₂, CH₄ and CO. TCCON data is used to validate the modelled concentrations.The Copernicus Climate Change Service (C3S) maintains the Climate Data Store, which should provide a single access point to a variety of climate datasets, including TCCON data.For all Copernicus activities, the financial security of the network is crucial for sustaining the measurements and ensuring timely data availability. Pinty et al. [7] outlined the vital importance of TCCON for an operational anthropogenic CO₂ Emissions Monitoring & Verification Support (MVS) capacity in Europe. They outline that important networks, among them TCCON, do not meet all operational requirements for the European CO₂ support capacity and state that: “Given the importance of this network [TCCON] to any future constellation, CEOS should strongly encourage its member agencies to identify a more coordinated and sustainable method for supporting and expanding the TCCON network and the distribution of its products.”

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4 Requirements for operational data usersThe requirements of operational data users for in situ cal/val networks depend on the application. The general aspects are data quality and its continuous monitoring, logistical issues like data delivery, and coverage of the parameter space.

• For the direct validation of satellite observations, the requirements are defined for each mission by the respective space agencies.

• It is expected that the requirements for the future European CO₂ Sentinel mission will be similar to the currently active CO₂ missions [7].

• For the CAMS GHG assimilation system, atmosphere measurements covering the entire vertical extent are desired for the validation activities. Profiles would be ideal but total column values are already very helpful.

4.1 Data qualityTo be used as a cal/val reference, the ground-based networks have to provide better precision and systematic uncertainty than the satellite mission requirements:

GOSAT observation systematic uncertainty: 4 ppm for XCO₂, 34 ppb for XCH₄ at a 1000 km mesh over land [6] [13].

GOSAT-2 observation systematic uncertainty: 0.5 ppm for XCO₂, 5 ppb for XCH₄ at a 500 km mesh over land and 2000 km over the ocean [6] [13].

OCO-2/3: <0.3% (1 ppm) for XCO₂ expected precision for a collection of 100 cloud-free soundings [14] [15].

Sentinel 5P ( systematic uncertainty / precision): 15% / < 10% for XCO, 1.5% / 1% for XCH₄ [16].

The TCCON and COCCON networks fulfil these criteria for current satellite missions with <0.25% (~1 ppm) for XCO₂ for solar zenith angles smaller than ~82 degrees and 0.2% (~3.6 ppb) / 0.5% (~9 ppb) systematic uncertainty / precision for XCH₄ for solar zenith angle smaller than ~85 degrees, error values are 2 sigma estimates [11] [12]. However, it is expected that future GHG satellite missions will have more stringent requirements, so both networks will have to improve their precision and systematic uncertainty in the future.

4.2 Delivery time requirementsFor operational users, timely and reliable data delivery is critical. The actual time scale depends on the application:

real time: <3 h near real time: <2-3 days a posteriori: <1 month final product: <1 year with a data availability of ideally >95%

The current TCCON data processing and quality assurance scheme cannot provide fully-quality-checked data on time scales of less than one month as the final product QC

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requires human intervention. Successful operational delivery of the final quality checked product within 3 months has been demonstrated but cannot be kept up in the current funding situation. Most TCCON stations deliver their data to the public archive within 4-6 months, even though TCCON regulations would allow up to 12 months. CAMS currently uses public TCCON data for their 3-monthly validation. Typically, only a subset of the actual data will already be available for this.Some stations are able to provide an optional rapid delivery (RD) data product which has not undergone full QC within days. However, only few stations have these capabilities. In many cases (e.g. limited data transmission capabilities) this requires additional facilities for the complete unattended data processing at the site.

4.3 Network coverageThe ideal ground based cal/val network would be evenly spaced across the globe or at least the land masses. In addition, it would be optimised for the need of the validation of a space-based operational GHG monitoring system: covering a representative range of albedo, terrain conditions (homogeneity and terrain-height variation), as well as polluted and non-polluted conditions .However, the current TCCON network has a very uneven geographical coverage (s. Fig. 2): 80% of the stations are on the Northern Hemisphere, so the coverage of the Southern Hemisphere is very sparse. Also, most of the Northern Hemisphere stations are in Europe and North America. Whole continents like Africa and South America are not covered at all.The current TCCON station locations are mainly a compromise between scientific interest and logistical constraints. It is obviously easier to operate a TCCON station near its home institution than in a remote location.

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Figure 4: Footprint analysis of the TCCON network. The coloured areas represent the origin of airmasses passing over the TCCON stations on the time scale of one week. The unit of the colour scale is the sensitivity of CO2 concentrations with respect to the concentrations in adjacent cells. Check [1] for details.

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During the last 7-8 years, there have been practically no funding opportunities for European partners to set up additional remote TCCON stations. An ambitious proposal (ACABON), that would have set up three TCCON stations in Africa for 2 MEUR with support from several African states was simply rejected.The current geographic coverage has a number of consequences. Figure 4 illustrates the footprint – a qualitative measure for the origin of air passing over the station – for the TCCON network. Air from the white areas is not observed by the network, so satellite observations cannot be validated there. In summary:

Europe, Japan and parts of North America and Australia/New Zealand are well covered.

South America (Amazon), Africa, India, and Central Asia are completely missing.Another important aspect of coverage is the range of surface albedo that the network covers. This also limited as most of the current reference sites are located in a rather narrow albedo range of 0.1 to 0.3 (see Fig. 5). With the exception of the Edwards station, there are no TCCON sites in high-albedo regions like deserts. There is also a strong need for ground-based stations covering very low albedo (<0.05) regions. The experience from the validation of the S5P CH4 product shows that having more stations in varying albedo conditions would significantly improve the satellite data product.The current geographical coverage also limits the range of solar zenith angles for validation to 0-70°.

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Figure 5: World map of the median cloud-free ground albedo at 2300 nm in 2003–2008, with the positions of TCCON stations shown as dots (black if used for validation, grey if not) or circles (unusable) [9].

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5 Current situation of the European TCCON networkSince 2004, TCCON has grown to 26 stations of which 12 are operated by European groups(see Figs. 2 & 3). The history and the overall setup of the network is scientific, not operational. However, more and more users have started using the data in an operational way.The global TCCON is a decentralized network of independent groups which share knowledge and best practices. There is neither central international organization nor a central funding source behind the network.

5.1 Funding sources of European TCCON stationsThere is an important difference between the European TCCON and many other stations in the international TCCON: at least in North America (and partly Australia) and Japan, the TCCON stations have been integral parts of the respective OCO/OCO-2/OCO-3 and GOSAT/GOSAT-2 missions from the beginning. Together, NASA and JAXA/NIES have fundedthe initial setup and continued operation of 5 stations in North America, 1 in Australia, 3 in Japan, and 1 in the Philippines – roughly 2/3 of the non-European stations.This is not the case for the European TCCON stations. Their funding is diverse, typically a mixture of institutional and competitive 3rd party funding. About 2/3 of the European stations require a continuous supply of substantial 3rd party funding to keep up their basicoperations and key personnel. And even the ones with tenured key personnel often rely on PhD students for data acquisition and analysis and would only be able to offer reduced service without them.At this time the German and Belgian groups receive funding from their national space agencies and ESA for Sentinel 5P validation. This funding prioritised combined NDACC/TCCON sites and important TCCON sites. Orleans and Cyprus (formerly Bialystok), were excluded from this Sentinel 5P validation funding. Also in the case of Belgium, this funding only covers the validation itself and not the actual operational cost.Unlike many of their non-European counterparts with their respective GHG missions, the European TCCON sites are not integrated into the upcoming European satellite GHG missions. Also, the last substantial funding for European TCCON stations from the EU Research Framework Programme ended more than 10 years ago.

5.2 Operational cost of a TCCON stationThe operational costs of TCCON stations are also diverse. The actual numbers depend on the home country and the location of the respective station. The typical range for European stations at the current non-operational level is:

About 1 to 1.5 full-time equivalent scientist/technician positions per station. Thereis some synergy in running more than one station but there is also a lower limit of at least one full-time position (PI) for a single station. Salary costs vary strongly between countries, so this cannot be expressed in monetary terms easily.

Operational cost from 10'000 to 50'000 Euro per year. This includes rental cost, electric power, communications, site visits, maintenance and spare parts. Sites

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close to the home institution tend to be at the lower end, very remote sites at the upper.

More detailed information about the cost structure of each European TCCON station has been collected for the TCCON cost book that will be submitted to ICOS.

For long-term operation, the total cost distribution is dominated by personnel (Fig. 6). Dueto its long lifetime of ~20 years, the initial cost for the FTIR instrument becomes less important over time. The container may also be a substantial initial investment. However, in many locations, putting the instrument into a building may be a better option.

5.3 Long-term operational sustainability and risks5.3.1 Short and mid-term time scalesWith the current diverse funding structure, long-term projections for the European TCCONnetwork are difficult. At least on short and intermediate time scales the following assumptions are reasonable:

On a 2-year time scale (2020-21), all European TCCON stations should be funded and operational.

On a 3-5 year time scale (2022-24), 50-65% of the stations will require successful applications for national or European funding to continue their operation.

5.3.2 Long-term time scalesOn a time scale of 5-10 years, there are several risks for operational data users that shouldbe addressed:

Retirement of key personnel could lead to a change of research goals and a loss ofsupport for the stations from their home institutions. This happened to the Ascension Island station in 2018/19 when the head of the department retired. A

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Figure 6: Cost breakdown for long-term operation of a remote TCCON station. The numbers were calculated from the actual cost of the Ascension Island station for a 10-year period.

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new home institution had to be found to avoid a complete shutdown of the only equatorial TCCON station. The station was completely unfunded for 5 months, lost all personnel except its PI and will not be back to normal operations before early 2020.For the future, this is not just an abstract risk: Two key institutions (KIT Karlsruhe and University of Bremen), which run 6 of the 12 European TCCON stations, will beaffected by retirements of the group leader/department head during this time frame. Continuation of current activities including station operation will require that staffing levels are maintained (minimum requirement) and that the future research strategies of the respective group/department continue in similar directions. Both cannot be guaranteed at this time.

Stations may have to be moved to a different location to reduce operational cost. This happened to the Bialystok station in 2018/19. The new location will not necessarily be chosen by the needs of the data users.

Loss of key personnel, which is very hard to replace, may result from short fundinggaps or even from an uncertain funding perspective. In the relatively small TCCON community, qualified personnel are the most critical resource.

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6 Pathways to sustainable operation for European GHG cal/val networksThere are two general options for putting the European GHG cal/val networks on a sustainable operational level. They could become an integral part of the upcoming European GHG missions or they could be integrated into the existing ICOS research infrastructure. Both options have specific strengths and weaknessed..

6.1 Fiducial Reference Measurement (FRM) networks for European GHG satellite missionsTCCON could be set up as a Fiducial Reference(FRM) network within the upcoming framework of Sentinel GHG missions. FRM activities for Sentinel-5P CO and CH4 product validation are already ongoing that can be considered as a preparation activity for future Sentinel GHG missions. This solution would be similar to the setup of the TCCON stations funded by NASA and JAXA/NIES. The stations would be an integral part of the European GHG missions and would receive funding in order to produce what is needed according to the mission requirements.

• This setup would be compatible with the existing TCCON organization. There would be no need to set up the organizational structures which are required by European Research Infrastructures like ICOS.

• Already a 2-3% share of the typical cost of a GHG mission would likely cover the operational cost of the whole European TCCON network during the expected mission lifetime.

Setup, governance, operational mode, data product delivery, and future expansionof the network could all be tailored to the mission requirements.

This solution would be less dependent on the commitment of individual national funding agencies. However, it is not clear whether the Copernicus member states and the ESA council would support this pathway.

If the funding is on a per-mission basis, solutions for gaps between missions (if any) wouldhave to be found. A close collaboration between Copernicus and the national space agencies would be desirable for such cases.

6.2 Long-term integration of European TCCON stations into ICOSA concept for a potential integration of TCCON into ICOS has been developed within the EU project RINGO. A concept document has been written and a 4th thematic centre besides the current three thematic centres within ICOS has been proposed. This document has to be approved by ICOS. Following that, the national funding agencies have to be approached for funding.

• An integration into ICOS would include a centralised retrieval and enhanced QA/QC improving the consistency within the European network. Both the European TCCON and the ICOS infrastructure would benefit: It would link ICOS to the remote sensing measurements, which are becoming increasingly important for

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monitoring the global GHG cycle. TCCON could make use of the existing governance structure within ICOS. Both infrastructures together would become a stronger player in addressing the challenge of the first global stocktake planned for2023.

• For the European TCCON the integration, accompanied with national funding, would secure the operation of the sites. TCCON data are increasingly used by their users (satellite agencies, modellers) for operational monitoring. The developed concept guarantees that TCCON data from the participating sites would be available within 3-month after the measurement and a rapid delivery data product(similar to the in situ NRT data) would become available 3-5 days after the measurements.

• The improved QA/QC suggested would include a travelling EM27/SUN instrument for inter-calibration of the TCCON sites and a gas cell intercomparison for monitoring the line shape of the FTIR spectrometers employed at the TCCON sites.Both activities aim to remove site-to-site biases and thereby improve the consistency among the European TCCON sites.

• TCCON would be integrated into an existing research infrastructure. Hence, the governance structure needed for an operational anthropogenic CO₂ emission Monitoring & Verification Support (MVS) [7] already exists. Integration of TCCON into ICOS would also facilitate the user experience in terms of centralized access tothe data via a coordinated platform. Users would benefit from the latest FAIR (Findable, Accessible, Interoperable, Reusable) implementation principles notably in terms of interoperability.

However, a prerequisite for joining ICOS is the provision of national funding, which would have to be secured first. Of the twelve European TCCON stations, Germany would have to fund the largest share of five. The four KIT stations (Karlsruhe, Izana, Garmisch, Zugspitze) would not formally join the ICOS infrastructure but would be open to some kind of associated status.

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7 Recommendations(1) Ground based remote sensing observations of column GHG from TCCON are essential

for cal/val of current and future GHG observations from space. For operational use, the European part of TCCON would also need operational funding. The operational users cannot rely on the assumption that the network will provide what they need from research funding. It is of highest priority to provide sustainable funding for the European part of TCCON. In their own interest, Copernicus and the space agencies need to play an active role in the discussions with their respective member states to find a suitable solution.

(2) COCCON has demonstrated its value for campaign-based observations but also has potential for complementing the ground based cal/val network. COCCON should be used to fill gaps in the current ground-based observing system on a campaign basis, especially for emission monitoring of hotspots, such as cities, and for satellite validation on a small spatial scale around these hotspots.

(3) Minimum requirements for GHG cal/val need to be defined by the space agencies andthe Copernicus services in a more detailed report. The requirements should cover thewhole observational parameter space but will not necessarily cover all scientifically interesting cases. The recommendations should be consistent with the CO₂ task force report [7] and would provide the basis for defining the structure of a future operational European in situ cal/val network.

(4) A decision on the preferred pathway(s) for achieving long-term sustainable GHG cal/val observations needs to be made: (a) FRM network as part of the European GHGmission, (b) integration of column observations into ICOS or (c) a combination of both. In all cases, the Copernicus, ESA, EUMETSAT and ICOS member states (which overlap in many cases) need to make this decision.

(5) In the case of the integration of the European TCCON stations into ICOS, the national funding agencies of all involved ICOS member states have to support this step. A concept for this integration has already been developed. After such an integration theparticipating sites would automatically meet the operational requirements for data users such as Copernicus. However, the fundamental requirement would be that the ICOS member states that run TCCON stations also provide the required national funding on a long-term sustainable basis.

(6) An integration of the European TCCON stations into ICOS, which would likely take at least 5 years, should be flanked by supporting projects from the Horizon Europe framework programme or dedicated funding from Copernicus' Fiducial Reference Measurement (FRM) programme. Such support should start as soon as possible to secure the operation of the existing TCCON stations.

(7) A gap analysis for cases that cannot be covered by ICOS has to be provided. Delta funding for these cases as well as special activities beyond the scope of ICOS should be provided by Copernicus and the European space agencies through dedicated service level agreements (SLA).

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(8) For the case that an integration of the European TCCON stations into ICOS cannot be achieved, contingency plans have to be set up by the key players Copernicus, ESA, EUMETSAT, the national space agencies and TCCON.

(9) Pinty et al. [7] recommend that both the TCCON and COCCON networks should be expanded for the future anthropogenic CO₂ emission Monitoring & Verification Support. This expansion needs to be planned and funded in a further step.

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8 References[1] Belikov, D. A., Maksyutov, S., Ganshin, A., Zhuravlev, R., Deutscher, N. M., Wunch, D.,

Feist, D. G., Morino, I., Parker, R. J., Strong, K., Yoshida, Y., Bril, A., Oshchepkov, S., Boesch, H., Dubey, M. K., Griffith, D., Hewson, W., Kivi, R., Mendonca, J., Notholt, J., Schneider, M., Sussmann, R., Velazco, V. A., and Aoki, S.: Study of the footprints of short-term variation in XCO₂ observed by TCCON sites using NIES and FLEXPART atmospheric transport models, Atmos. Chem. Phys., 17, 143–157, https://doi.org/10.5194/acp-17-143-2017, 2017.

[2] Frankenberg, C., J. F. Meirink, M. van Weele, U. Platt, and T. Wagner: Assessing methane emissions from global space-borne observations, Science, 308, 1010–1014, 2005.

[3] Frankenberg C, P Bergamaschi, A Butz, S Houweling, JF Meirink, J Notholt, AK Petersen, H Schrijver, T Warneke, I Aben: Tropical methane emissions: A revised view from SCIAMACHY onboard ENVISAT. Geophysical Research Letters 35. https:// doi. org/ 10.1029/2008gl034300, 2008.

[4] Frey, M., Hase, F., Blumenstock, T., Groß, J., Kiel, M., Mengistu Tsidu, G., Schäfer, K., Sha, M. K., and Orphal, J.: Calibration and instrumental line shape characterization of a set of portable FTIR spectrometers for detecting greenhouse gas emissions, Atmos. Meas. Tech., 8, 3047–3057, https://doi.org/10.5194/amt-8-3047-2015, 2015.

[5] Frey, M., Sha, M. K., Hase, F., Kiel, M., Blumenstock, T., Harig, R., Surawicz, G., Deutscher, N. M., Shiomi, K., Franklin, J. E., Bösch, H., Chen, J., Grutter, M., Ohyama, H., Sun, Y., Butz, A., Mengistu Tsidu, G., Ene, D., Wunch, D., Cao, Z., Garcia, O., Ramonet, M., Vogel, F., and Orphal, J.: Building the COllaborative Carbon Column Observing Network (COCCON): long-term stability and ensemble performance of the EM27/SUN Fourier transform spectrometer, Atmos. Meas. Tech., 12, 1513–1530, https://doi.org/10.5194/amt-12-1513-2019, 2019.

[6] Akihiko Kuze, Hiroshi Suto, Masakatsu Nakajima, and Takashi Hamazaki: Thermal and near infrared sensor for carbon observation Fourier-transform spectrometer on the Greenhouse Gases Observing Satellite for greenhouse gases monitoring, Appl. Opt. 48, 6716-6733, 2009.

[7] Pinty B., P. Ciais, D. Dee, H. Dolman, M. Dowell, R. Engelen, K. Holmlund, G. Janssens-Maenhout, Y. Meijer, P. Palmer, M. Scholze, H. Denier van der Gon, M. Heimann, O. Juvyns, A. Kentarchos and H. Zunker: An Operational Anthropogenic CO₂ Emissions Monitoring & Verification Support Capacity – Needs and high level requirements for in situ measurements, European Commission Joint Research Centre, EUR 29817 EN, doi:10.2760/182790, https://www.copernicus.eu/sites/default/files/2019-09/CO2_Green_Report_2019.pdf, 2019.

[8] Sha, M. K., De Mazière, M., Notholt, J., Blumenstock, T., Chen, H., Dehn, A., Griffith, D.W. T., Hase, F., Heikkinen, P., Hermans, C., Hoffmann, A., Huebner, M., Jones, N., Kivi, R., Langerock, B., Petri, C., Scolas, F., Tu, Q., and Weidmann, D.: Intercomparison of low and high resolution infrared spectrometers for ground-based solar remote sensing measurements of total column concentrations of CO₂, CH₄ and CO, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2019-371, in review, 2019.

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[9] Tol, Paul: Local Height and Albedo Maps of TCCON Stations, Technical Report, SRON-TROPSC-TN-2013-004, 2013.

[10] Warneke T, JF Meirink, P Bergamaschi, JU Grooss, J Notholt, GC Toon, V Velazco, APH Goede, O Schrems: Seasonal and latitudinal variation of atmospheric methane: A ground-based and ship-borne solar IR spectroscopic study. Geophysical Research Letters 33, https:// doi. org/ 10.1029/2006gl025874, 2006.

[11] Wunch, D., Toon, G. C., Wennberg, P. O., Wofsy, S. C., Stephens, B. B., Fischer, M. L., Uchino, O., Abshire, J. B., Bernath, P., Biraud, S. C., Blavier, J.-F. L., Boone, C., Bowman, K. P., Browell, E. V., Campos, T., Connor, B. J., Daube, B. C., Deutscher, N. M., Diao, M., Elkins, J. W., Gerbig, C., Gottlieb, E., Griffith, D. W. T., Hurst, D. F., Jiménez, R., Keppel-Aleks, G., Kort, E. A., Macatangay, R., Machida, T., Matsueda, H., Moore, F., Morino, I., Park, S., Robinson, J., Roehl, C. M., Sawa, Y., Sherlock, V., Sweeney, C., Tanaka, T., and Zondlo, M. A.: Calibration of the Total Carbon Column Observing Network using aircraft profile data, Atmos. Meas. Tech., 3, 1351–1362, https://doi.org/10.5194/amt-3-1351-2010, 2010.

[12] Wunch, D., Toon, G. C., Sherlock, V., Deutscher, N. M., Liu, X.,Feist, D. G., and Wennberg, P. O.: The Total Carbon Column Observing Network’s GGG2014 Data Version, https://doi.org/ 10.14291/tccon.ggg2014.documentation.r0, 2015.

[13] https://global.jaxa.jp/activity/pr/brochure/files/sat38.pdf[14] https://directory.eoportal.org/web/eoportal/satellite-missions/o/oco-2[15] https://directory.eoportal.org/web/eoportal/satellite-missions/i/iss-oco-3[16] https://sentinel.esa.int/documents/247904/2474724/Sentinel-5P-Science-Validation-

Implementation-Plan[17] http://www.imk-asf.kit.edu/english/COCCON.php

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9 ContributorsThe individuals and organizations listed in Table 1 have contributed to this report. Most were also present at the first Expert Meeting on GHG column cal/val network sustainability in Darmstadt, Germany, on April 15, 2019.

Name Organization

Andersen, Henrik Steen (HSA) EEA

Bojkov, Bojan (BB) EUMETSAT

De Mazière, Martine (MDM) BIRA IASB

Dehn, Angelika (AD) ESA ESRIN

Deniel, Carole (CD) CNES

Engelen, Richard (RE) ECMWF

Fehr, Thorsten (TF) ESA ESTEC

Feist, Dietrich (DF) TCCON / DLR IPA / LMU

Fix, Andreas (AF) DLR IPA

Fjæraa, Ann Mari (AMF) NILU

Hase, Frank (FH) KIT

Holmlund, Ken (KH) EUMETSAT

Kutsch, Werner (WK) ICOS ERIC

Lang, Rüdiger (RL) EUMETSAT

Machin, Simon Metoffice

Meijer, Yasjka (YM) ESA ESTEC

Pinty, Bernard (BP) EC

Rattenborg, Mikael (MR) Consultant for EEA

Sha, Mahesh Kumar (MKS) BIRA IASB

Vermeulen, Alex (AV) ICOS ERIC

von Bargen, Albrecht (AvB) DLR RFM

Warneke, Thorsten (TW) University of Bremen

Zehner, Claus (CZ) ESA ESRIN

Table 1: Workgroup experts

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