The Tropospheric Monitoring Instrument (TROPOMI) on the Sentinel-5 Precursor (S5P) satellite is a unique instrument, combining daily global coverage, very high signal-to-noise, a broad spectral range and very small pixels up to 3.5 x 5.5 km². Retrievals are available for a large number of species, including NO₂. Due to the very small pixels and daily revisit, TROPOMI provides detailed information on individual sources and source sectors like individual power plants, industrial complexes, cities and suburbs, highways, and even individual ships. The TROPOMI Level-2 NO₂ product is available from 30 April 2018 onwards.

Validation exercises of TROPOMI v1.2 & v1.3 data (2018-2020) with OMI and ground-based remote sensing observations have shown that TROPOMI's tropospheric NO₂ column are low by up to 50% over highly polluted areas compared to independent data. In contrast, the underlying slant columns of TROPOMI agree well with OMI and independent SAOZ observations. Differences between OMI and TROPOMI have been mainly attributed to the different cloud height retrieval, using the O₂-O₂ versus O₂-A bands respectively.

In our presentation we discuss recent improvements in the TROPOMI NO₂ retrieval and the impact these have on the tropospheric columns and on the comparisons with OMI and ground-based remote-sensing data.

Version v1.4, which became operational on 2 December 2020, entails a major improvement in the cloud height retrieval, based on a modification of the FRESCO-S cloud retrieval using the O₂-A band observations. In particular the cloud height over scenes with a small cloud coverage have increased, resulting in larger tropospheric columns in the retrievals over polluted areas.

Version v2.2, to become operational in April/May 2021, includes similar cloud retrieval modifications. Furthermore, it provides a better treatment of saturation issues and transients, is using improved (ir)radiance measurements (level-1b v2 spectra) including degradation corrections, and includes a new albedo treatment.

The TROPOMI NO₂ retrievals are compared with OMI retrievals (from the QA4ECV product) and to ground-based observations with MAXDOAS and PANDORA instruments.

How to cite: van Geffen, J., Eskes, H., Sneep, M., Pinardi, G., Verhoelst, T., Compernolle, S., ter Linden, M., Boersma, F., and Veefkind, P.: TROPOMI NO2 retrieval: December 2020 (v1.4) and April 2021 (v2.2) upgrades, and comparisons with OMI and ground-based remote sensing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9668, https://doi.org/10.5194/egusphere-egu21-9668, 2021.

For more than three years now, the first atmospheric satellite of the Copernicus EO programme, Sentinel-5p (S5P) TROPOMI, has acquired spectral measurements of the Earth radiance in the visible range, from which near-real-time (NRTI) and offline (OFFL) processors retrieve the total, tropospheric and stratospheric  column abundance of  NO₂.   The S5P Mission Performance Centre  performs continuous QA/QC of these data products enabling users to verify the fitness-for-purpose of the S5P data. Quality Indicators are derived from comparisons to ground-based reference data, both station-by-station in the S5P Automated Validation Server (AVS), and globally in more in-depth analyses.  Complementary quality information is obtained from product intercomparisons (NRTI vs. OFFL) and from satellite-to-satellite comparisons.  After three years of successful operation we present here a consolidated overview of the quality of the S5P TROPOMI NO₂ data products, with particular attention paid to the impact of the various processor improvements, especially in the latest version (v1.4), activated on 2 December 2020, which introduces an updated cloud retrieval resulting in higher NO₂ columns in polluted regions. Also the upcoming v2, due in April 2021 but already used to produce a Diagnostic Data Set, is discussed.

S5P NO₂ data are compared to ground-based measurements collected through either the ESA Validation Data Centre (EVDC) or network data archives (NDACC, PGN). Measurements from the Pandonia Global Network (PGN) serve as a reference for total NO₂ validation, Multi-Axis DOAS data for tropospheric  NO₂ validation, and NDACC zenith-scattered-light DOAS data for stratospheric NO₂ validation.  Comparison methods are optimized to limit spatial and temporal mismatch errors (co-location strategy, photochemical adjustment to account for local time difference). Comparison results are analyzed to derive Quality Indicators and to conclude on the compliance w.r.t. the mission requirements.  This include estimates of: (1) the bias, as proxy for systematic errors, (2) the dispersion of the differences, which combines random errors with seasonal and mismatch errors, and (3) the dependence of these on key influence quantities (surface albedo, cloud cover…)

Overall, the MPC quality assessment of S5P NO₂ data concludes to an excellent performance for the stratospheric data (bias<5%, dispersion<10%). The tropospheric data show a negative bias of -30% and a dispersion of 3Pmolec/cm² vs. ground-based data. This dispersion is larger than the mission requirement on data precision, but it can partly be attributed to comparison errors such as those due to differences in resolution. Total column data are found to be biased low by 20%, with a 30% station-to-station scatter. After gridding to monthly means on a 0.8°x0.4° grid, comparisons to OMI data yield a much smaller dispersion (within the requirement of 0.7Pmolec/cm²), and a minor relative bias. NRTI and OFFL perform similarly, even if they occasionally differ over specific scenes. Besides the impact of the processor upgrade to v1.4 on the bias in polluted scenes, we discuss the implications of the reported negative biases in S5P tropospheric (and total) columns on NO₂ reduction estimates, e.g. in the context of SARS-CoV-2 lockdown measures. Feedback from this work on the ground-based reference data is also briefly reported.         

How to cite: Verhoelst, T., Compernolle, S., Pinardi, G., Granville, J., Lambert, J.-C., Eichmann, K.-U., Eskes, H., Niemeijer, S., Fjæraa, A. M., Pazmino, A., Bazureau, A., Goutail, F., Pommereau, J.-P., Cede, A., and Tiefengraber, M.: Quality assessment of three years of Sentinel-5p TROPOMI NO2 data , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7499, https://doi.org/10.5194/egusphere-egu21-7499, 2021.

The TROPOMI/S5p instrument was launched in October 2017, aiming to measure from space the atmospheric composition for air quality and ozone monitoring. Since 30 April 2018, TROPOMI/S5p routinely delivers NO2 tropospheric VCDs in quasi-real-time. The first comparisons between this operational TROPOMI product and measurements from the ground and aircraft generally show good correlations but also a negative bias over polluted areas. Such a bias is expected from the low spatial resolution of the CTM used in the operational TROPOMI retrieval and several studies reported a better agreement with local measurements of NO2 VCDs when using a higher resolution model for the satellite AMFs, in practice, changing the original TM5-MP for the CAMS Ensemble. We compare mobile-DOAS measurements with the two aforementioned versions of the TROPOMI retrievals (TM5-MP and CAMS). Our Mobile-DOAS measurements were performed with the BIRA-IASB Mobile-DOAS during 19 clear sky days. We sampled polluted and clean areas during TROPOMI overpasses in Belgium and Germany between June 2018 and September 2020. Beside studying the effect of the CTM model on the comparisons, we investigate the general added-values of such mobile-DOAS measurements for the validation of TROPOMI/S5p and forthcoming missions.

How to cite: Merlaud, A., Tack, F., Van Roozendael, M., Eskes, H., and Douros, J.: On the use of Mobile-DOAS measurements for air quality satellite validation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15556, https://doi.org/10.5194/egusphere-egu21-15556, 2021.

Starting from January 2020, new IMO regulations limiting the Sulphur content of the fuel used by seagoing vessels came into force. As of 2021, new and stricter NOx emission standards are applied for newly built ships entering the North and Baltic Sea. There are various methods that are used to measure the pollution produced by ships in ports or off the coastal areas. Due to practical limitations, however, the conduction of such monitoring above the open sea has not been possible up to now.

The TROPOspheric Monitoring Instrument onboard the Copernicus Sentinel 5 Precursor satellite (TROPOMI/S5P) provides the atmosphere monitoring data with an unprecedented spatial resolution. With this instrument plumes produced by individual ships of substantial size can be detected. In our study we focus on application of the TROPOMI NO2 tropospheric column for tracking back the emission produced by individual ships at open sea.

On a global scale, individual ships are considered to be low-source pollution emitters. As a result, it is difficult to separate an emission plume from the background pollution, especially, in case of comparable background concentration. In order to improve the distinction between the plume and the background, we propose the use of the local spatial autocorrelation measure Moran’s I. This measure amplifies regular shaped high-concentration structures and suppresses random co-occurring concentration peaks. By means of the Automated Identification Signal (AIS) data that records historical ship locations, the detected structures can be associated with individual ships. We further propose heuristic algorithms using local weather conditions (wind speed/direction) for an efficient ship-plume matching and NO2 concentration estimation.

We evaluate the quality of a ship-plume assignment by comparing the estimated NO2 concentration with model-based emission estimations determined from speed and length of the ship. Notable linear correlation between our estimations and the model-based values supports the proposed method.

This work contributes to realising global scale verification/estimation of emission plumes with satellites by providing automated and enhanced processing of satellite retrievals for identifying and quantifying of NOx plumes produced by individual seagoing vessels.

This work is funded by the Netherlands Human Environment and Transport Inspectorate, the Dutch Ministry of Infrastructure and Water Management, and the SCIPPER project which receives funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement Nr.814893.

How to cite: Kurchaba, S., Veenman, C. J., van Vliet, J., and Verbeek, F. J.: Estimating Individual Sea Vessel NO2 Emissions using Spatial Autocorrelation on S5P-TROPOMI Satellite Data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3886, https://doi.org/10.5194/egusphere-egu21-3886, 2021.

We evaluate the satellite-based TROPOMI (TROPOspheric Monitoring Instrument) NO2 products against ground-based observations in Helsinki (Finland). TROPOMI NO2 total (summed) columns are compared with the measurements performed by the Pandora spectrometer during April–September 2018. The mean relative and absolute bias between the TROPOMI and Pandora NO2 total columns is about 10% and 0.12 × 10¹⁵ molec. cm^-2 respectively. 

We find high correlation (r = 0.68) between satellite- and ground-based data, but also that TROPOMI total columns underestimate ground-based observations for relatively large Pandora NO2 total columns, corresponding to episodes of relatively elevated pollution. This is expected because of the relatively large size of the TROPOMI ground pixel (3.5 × 7 km) and the a priori used in the retrieval compared to the relatively small field-of-view of the Pandora instrument. On the other hand, TROPOMI slightly overestimates relatively small NO2 total columns. Replacing the coarse a priori NO2 profiles with high-resolution profiles from the CAMS chemical transport model improves the agreement between TROPOMI and Pandora total columns for episodes of NO2 enhancement, but the overall bias remains the same (within the uncertainties).

In order to evaluate the capability of TROPOMI observations for monitoring urban air quality, we also analyse the consistency between satellite-based data and NO2 surface concentrations from the Kumpula air quality station in Helsinki. We find similar day-to-day variability between TROPOMI and in situ measurements, with NO2 enhancements observed during the same days. Both satellite- and ground-based data show a similar weekly cycle, with lower NO2 levels during the weekend compared to the weekdays as a result of reduced emissions from traffic and industrial activities (as expected in urban sites).

Several applications have been already carried on to support informed decision making and Finnish society in general. We developed a simple web platform to inform environmental authorities at municipal level about the use of satellite observations for air quality monitoring. We assisted the Finnish authorities during the first period of the COVID-19 pandemic in assessing the effect of the lockdown on air quality. We supported the Finnish Ministry of Environment in compiling the periodic national air pollution assessment report to the EU. We participated in several international cooperation projects for assessing the major air pollution sources and the available air quality monitoring systems over several developing countries and for providing recommendations on strengthening air quality monitoring. We collaborated with the department of Social Science at the Univ. of Helsinki for the assessment of the environmental impacts of the energy and extracting sector in Yakutia (Russia).

Reference: Ialongo, I., Virta, H., Eskes, H., Hovila, J., and Douros, J.: Comparison of TROPOMI/Sentinel-5 Precursor NO₂ observations with ground-based measurements in Helsinki, Atmos. Meas. Tech., 13, 205–218, https://doi.org/10.5194/amt-13-205-2020, 2020.

How to cite: Ialongo, I., Virta, H., Eskes, H., Hovila, J., Douros, J., and Sundström, A.-M.: Evaluation of TROPOMI/Sentinel-5 Precursor NO2 product against ground-based observations in Helsinki and first applications to Finnish society, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11081, https://doi.org/10.5194/egusphere-egu21-11081, 2021.

Lightning discharges are one of the main sources of atmospheric NO_x, contributing about 10% of NO_x emissions globally and playing an important role for the concentration of ozone and other chemical species in the upper troposphere. Lightning produces between 2-8 Tg N per year globally (100-400 mol NO_x per flash). Reducing the uncertainty of the NO_x production by lightning and understanding the factors that influence this production is still a challenge.

The TROPOspheric Monitoring Instrument (TROPOMI) is orbiting the Earth from a near-polar, sun-synchronous orbit since October 2017. TROPOMI is equipped with four spectrometers that provide information about the chemical composition of the troposphere with unprecedented horizontal spatial resolutions of 3.5 x 7 km before 6 August 2019 and 3.5 x 5.5 km after that date. In this work, we combine the DLR-NO₂ research product, the DLR cloud operational product and the TROPOMI v2.1_test NO₂ product to estimate the production of NO_x per flash (LNO_x). The v2.1_test NO₂ product contains more useful data pixels than the official offline v1.x data product, because of better treatment of saturation of the TROPOMI measurements (which occurs frequently over high bright clouds that are often linked with LNOx) and the use of an improved version of the FRESCO cloud algorithm.

We for the first time ever use these chemical measurements from TROPOMI combined with lightning radio measurements provided by the EUropean Cooperation for LIghtning Detection (EUCLID) and the Earth Network Total Lightning Network (ENTLN), together with lightning optical measurements provided by the space-based Lightning Imaging Sensor (LIS) to estimate the Detection Effiency (DE) of EUCLID and ENTLN. In addition, we use the ECHAM5/MESSy Atmospheric Chemistry (EMAC) simulations to calculate the air mass factor employed to convert tropospheric slant column of measured NO₂ to vertical column LNO_x and the winds provided by reanalysis data to eliminate the influence of upwind storms in the estimation of the background NO_x. Concentration.

We focus our analysis on different remote regions, where the background concentration of NO is relatively low. In particular, we focus our analysis on 11 thunderstorm cases taking place near the Pyrenees, where intense thunderstorms are frequent and the DE of EUCLID and ENTLN is relatively high and homogeneous. According to our preliminary results from a single case using the DLR-NO₂ research product, we get about 400 mol NO_x per flash when we estimate the background using NO_x from CARIBIC flights and about 200-600 mol per flash when we estimate the background using TROPOMI measurements from non-flashing pixels.

How to cite: Perez-Invernon, F. J., Huntrieser, H., Erbertseder, T., Loyola, D., Valks, P., Liu, S., Allen, D., Pickering, K., Bucsela, E., Jöckel, P., van Greffen, J., Eskes, H., Soler, S., and Gordillo-Vázquez, F. J.: Quantification of lightning-produced NOx over the Pyrenees by using different TROPOMI-NO2 research products, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10996, https://doi.org/10.5194/egusphere-egu21-10996, 2021.

Most countries around the world took actions to control COVID-19 spread that included social distancing, limiting air and ground travel, closing schools, suspending sports leagues, closing factories etc., leading to  economic shutdown. The reduced traffic and human movement compared to Business as Usual (BAU) scenario was tracked by Apple and Android cellphone use; the data showed substantial reductions in mobility in most metropolitan areas.  We analyzed reductions in on-road mobile NOx emissions from light and heavy duty vehicles in four major metropolitan and one rural areas in the United States that showed a reduction in NOx mobile emissions from 9% to 19% between February and March at the onset of lockdown in the middle of March; between March and April, the mobile NOx emissions dropped further by 8% to 31% when lockdown measures were the most stringiest.  These precipitous drops in NOx emissions correlated well with tropospheric NO₂ column amount observed by Sentinel 5 Precursor TROPospheric Ozone Monitoring Instrument (S5P TROPOMI).  Further, the changes in TROPOMI tropospheric NO₂ across the continental U.S. between 2020 and 2019 correlated well with changes in on-road NOx emissions (r=0.78) but correlated weakly with changes in emissions from the power plants (r=0.44). These findings confirm that power plants are no longer the major source of NO₂ in the United States. We also examined correlation between increase in unemployment rate between 2020 and 2019 to decrease in tropospheric NO₂ amount.  The negative correlation indicates that with increased unemployment rate combined with telework policies across the nation for non-essential workers, the NO₂ values decreased at the rate of 0.8 µmoles/m² decrease per unit percentage increase in unemployment rate.  There is a substantial amount of scatter in the data with some cities such as Atlanta, Dallas, and Houston showing no noticeable trend in tropospheric NO₂ changes during the time period when unemployment rate increased from 6% to 12%.   We examined the trends in on-road and power plant emissions for five different locations (four urban areas and one rural area) and show that the changes in NOx emissions during the lockdown are detectable in TROPOMI tropNO2 data, the economic indicators are consistent with emissions changes, and the trends reversing with the removal of lockdown measures in the major metro areas have not come back to pre-pandemic levels.  The COVID-19 pandemic experience has provided the scientific community an opportunity to identify emissions reductions scenarios that created a new normal for urban air quality and if the environmental protection agencies should look at this new normal as a guidance for instituting new policies. 

How to cite: Kondragunta, S.: Fingerprints of a New Normal Urban Air Quality in S5P TROPOMI Tropospheric NO2 Observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16363, https://doi.org/10.5194/egusphere-egu21-16363, 2021.

Smoke from wildfires are a significant source of air pollution, which can adversely impact ecosystems and the air quality in downwind populated areas. With increasing severity of wildfires over the years, these are a significant threat to air quality in densely populated areas. Emissions from wildfires are most commonly estimated by a bottom-up approach, using proxies such fuel type, burn area, and emission factors. Emissions are also commonly derived with a top-down approach, using satellite observed Fire Radiative Power. Furthermore, wildfire emissions can also be estimated directly from satellite-borne measurements.

Here, we present advancements and improvements of direct emission estimates of forest fire NO_x emissions by using TROPOMI (Tropospheric Monitoring Instrument) high-resolution satellite datasets, including NO₂ vertical column densities (VCDs) and information on plume height and aerosol scattering.  The effect of smoke aerosols on the sensitivity of TROPOMI to NO₂ (via air mass factors) is estimated with recalculated VCDs, and validated with aircraft observations. Different top-down emission estimation methods are tested on synthetic data to determine the accuracy, and the sensitivity to parameters, such as wind fields, satellite sampling, instrument noise, NO₂:NO_x conversion ratio, species atmosphere lifetime and plume spread. Lastly, the top-down, bottom-up and direct emission estimates of fire emissions are quantitatively compared.

How to cite: Griffin, D., Chan, J., Dammers, E., McLinden, C., Adams, C., Akingunola, A., Makar, P., Fehr, L., Bourassa, A., Degenstein, D., Hayden, K., Wren, S., and Liggio, J.: Towards an improved understanding of nitrogen dioxide emissions from forest fires , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10331, https://doi.org/10.5194/egusphere-egu21-10331, 2021.

We utilized IASI, OMI, TROPOMI, and GOME-2 data to quantify the effect of lockdown on the changes in ozone, CO, and NO₂ concentration over India, with a primary focus on the tropospheric profiles of ozone and CO as compared to the years 2018 and 2019. Twelve populated cities and India's largest thermal power plants (TPPs) were further selected to quantify lockdown effects. Changes in ozone and CO have not been uniform over the different regions in India, including their vertical distribution. An increase (up to ~20%) in vertical ozone distribution during lockdown was observed over central and western India compared to both 2019 and 2018. However, it decreased over the southern coastal regions. Further, a significant reduction (> 20%) is observed over northern and northeast regions when compared with 2018 while a dramatic increase (> 20%) compared to 2019 is observed over northern regions. The increased ozone over north India, particularly in contrast to 2019 further shows a successive increase at higher altitudes and exhibits the role of dynamics, while, for other places like western and central India, the enhanced ozone decreases with higher altitude, which shows the effect of photochemistry and surface emissions. For CO, the lockdown effect seems to have emerged more effectively in the boundary layer, where a reduction in the range of 2 - 18% is seen except in western regions. In-contrast, a consistent yearly increase (as high as 29%) was observed from 2018 to 2020 in the free troposphere. Similar to the profiles, the total CO shows an increase (~20%) over central and western India while a moderate decrease (5%) over northern India. Like CO, an increase of NO₂ (~ 15%) over the western region is also observed, particularly compared to 2019. The persistent increase of CO and NO₂ over western India suggests to have contributed more from the nearby coal-based thermal power plants, which have increased their production in 2020. Contrary to other surface-based studies during the lockdown, which has shown an apparent decrease in pollutant levels, the present study shows an increase in CO, NO₂, and ozone at several locations and at different altitude regions. An analysis between OMI and TROPOMI tropospheric NO₂ columns show a considerable difference (> 30%) in NO₂ VCD retrieval around the remote locations, e.g., the Himalaya, the remote Tibetan plateau, and oceanic regions. Further, an investigation of the ozone production regime showed NO₂ limited regime over India's major part, while VOC limited regime over thermal power plants regions during the lockdown.

How to cite: Rawat, P. and Naja, M.: Remote sensing study of Ozone, NO2, and CO: Contrary effect of Indian lockdown in the free troposphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5532, https://doi.org/10.5194/egusphere-egu21-5532, 2021.

We present a method for the synergetic use of IASI (Infrared Atmospheric Sounding Interferometer) profile and TROPOMI (TROPOspheric Monitoring Instrument) total column data products. Our method uses the output of the individual retrievals and consists of linear algebra a posteriori calculations (i.e. calculation after the individual retrievals). We show that this approach is largely equivalent to applying the spectra of the different sensors together in a single retrieval procedure, but with the substantial advantage of being usable together with different individual retrieval processors, of being very time efficient, and of directly benefiting from the high quality and most recent improvements of the individual retrieval processors.

For demonstrating the method, we focus on atmospheric methane (CH₄) and use IASI profile products generated by the processor MUSICA (MUlti-platform remote Sensing of Isotopologues for the investigation of the Cycle of Atmospheric water). We perform a theoretical evaluation and show that the a posteriori combination method yields total column averaged CH₄ products (XCH₄) that have the same good sensitivity as the respective TROPOMI products and upper tropospheric and lower stratospheric (UTLS) CH₄ profile data with the same good sensitivity as the IASI product. In addition, the combined product offers sensitivity for the tropospheric partial column, which is not provided by the individual TROPOMI nor the individual IASI product. The theoretically predicted synergetic effects are verified by comparisons to CH₄ reference data obtained from collocated XCH₄ measurements at five globally distributed TCCON (Total Carbon Column Observing Network) stations, CH₄ profile measurements made by 24 individual AirCore soundings, and lower tropospheric CH₄ data derived from continuous observations made at two nearby Global Atmospheric Watch (GAW) mountain stations. The comparisons clearly demonstrate that the combined product can reliably detect XCH₄ signals and allows to distinguish between tropospheric and UTLS CH₄ partial column averaged mixing ratios, which is not possible by the individual TROPOMI and IASI products. We find indications of a weak positive bias of +1.7% +/- 1.2% of the combined lower tropospheric data product with respect to the references. For the UTLS CH₄ partial columns we find no significant bias and a scatter with respect to the references of below 1%. We also briefly demonstrate the possibility of generating a combined IASI + TROPOMI water vapour isotopologue ratio product (HDO/H₂O), which allows the detection of boundary layer HDO/H₂O ratios independently from free tropospheric ratios.

The approach has the particular attraction, that IASI and TROPOMI successor instruments will be jointly aboard the upcoming Metop Second Generation satellites (guaranteeing observations from the 2020s to the 2040s). There will be several 100,000 globally distributed and perfectly collocated observations (over land) of IASI and TROPOMI successor instruments per day, for which combined products can be generated in a computationally very efficient way.

How to cite: Schneider, M. and the Synergetic IASI+TROPOMI CH4 product generation and validation team: Synergetic use of IASI and TROPOMI for generating a tropospheric methane profile product, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9757, https://doi.org/10.5194/egusphere-egu21-9757, 2021.

Actionable feedback to industrial operators is extremely valuable to help them reduce their greenhouse gas emissions. With this goal in mind, GHGSat launched in 2016 a demonstration satellite called GHGSat-D (“Claire”). It was the first satellite built specifically to detect and quantify methane emissions from individual sites.

With the launches of GHGSat-C1 (“Iris”) in September 2020 and of GHGSat-C2 (“Hugo”) planned in January 2021, GHGSat will have three methane-sensing meter-scale resolution satellites in orbit. In addition to those satellites, GHGSat has also deployed an aircraft version of the instrument to survey specific areas with even lower detection threshold thanks to its higher spatial resolution.

This presentation will show the improvements done since GHGSat-D that allow our instruments to reach column precision of  1% of background. With this enhanced sensitivity, sources such as oil and gas facilities, mines, landfills and dams can be measured from space. Emission quantification of various sources will be presented and will demonstrate that GHGSat-C1 is approaching its target detection threshold of 100 kg/h. We will also illustrate the complementarity of GHGSat’s instruments with Sentinel-5P, the first ones able to detect individual sources with low emission rates, the second able to measure daily and with high accuracy global methane concentrations. We will also discuss the data calibration and validation plan of our instruments. Finally, an update on the future expansion of GHGSat’s constellation will be given.

How to cite: Strupler, M., Deglint, H., Gains, D., Jervis, D., MacLean, J.-P., McKeever, J., Ramier, A., Shaw, W., Tarrant, E., Varon, D., and Young, D.: Meter-scale retrieval of industrial methane emissions using GHGSat’s satellite constellation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6590, https://doi.org/10.5194/egusphere-egu21-6590, 2021.

Atmospheric moisture is a crucial factor for the redistribution of heat in the atmosphere, with a strong coupling between atmospheric circulation and moisture pathways responsible most climate feedback mechanisms. Conventional satellite and in situ measurements provide information on water vapour content and vertical distribution; however, observations of water isotopologues make a unique contribution to a better understanding of this coupling.

In recent years, observations of water vapour isotopologue from satellites have become available from nadir thermal infrared measurements (TES, AIRS, IASI) which are sensitive to the free troposphere and from shortwave-infrared (SWIR) sensors (GOSAT, SCIAMACHY) that provide column-averaged concentrations including sensitivity to the boundary layer. The TROPOMI instrument on-board Sentinel 5P (S5p) measures SWIR radiance spectra that allow retrieval of water isotopologue columns but with much improved spatial and temporal coverage compared to other SWIR sensors promising a step-change for scientific and operational applications.

Here we present the retrieval algorithm development for stable water isotopologues from TROPOMI as part of the ESA S5p Innovation programme.  We also discuss the validation of these types of satellite products with fiducial in situ measurements, and challenges compared with other satellite measurements. Finally, we outline the roadmap for assessing the impact of TROPOMI data against state-of-the-art isotope enabled models.

How to cite: Trent, T., Boesch, H., Somkuti, P., Schneider, M., Khosrawi, F., Diekmann, C., Röhling, A., Sodemann, H., and Thurnherr, I.: Retrieval of Stable Water Vapour Isotopologues from the TROPOMI Instrument, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9864, https://doi.org/10.5194/egusphere-egu21-9864, 2021.

The total ozone column (TOC) is retrieved using multiple optical satellite instrumentation (including TOMS, OMI, TROPOMI, GOME, GOME-2, and SCIAMACHY, to name a few). The spatial resolution of total ozone satellite measurements is quite low (e.g., 7x3.5km for TROPOMI, 13x24km for OMI, and 30x60km for SCIAMACHY). In some cases (say, close to the ozone hole boundary) it is of importance to have information on the total ozone at a higher spatial resolution. In this work we propose the use of multiple optical instruments performing the measurements in the ozone Chappuis ozone bands (400-650nm) for the total ozone column determination. This makes it possible to extend the number of instruments, which can be used for the total ozone determination (say, also using current/historic measurements by MODIS/Aqua&Terra, S-GLI/SCOM-C, VIIRS/Suomi-NPP, MSI/S-2, OLCI/S-3, MERIS/ENVISAT). In particular, MERIS and SCIAMACHY have been operated from the same satellite platform and had similar swaths (960km for SCIAMACHY and 1150km for MERIS). This means the method of total ozone retrieval based on combination of SCIAMACHY (30x60km) and MERIS (0.3x0.3km) observations over highly reflective ground (say, in Antarctica, where the ozone hole is located) is of value. The total ozone retrievals using Chappuis ozone bands is based on the fact that the top-of-atmosphere reflectance observed over a highly reflective ground (say, snow) has a minimum in the visible located around 600nm. This feature is due to due to the absorption of light by the atmospheric ozone (Gorshelev et al., 2014). The contribution of both ground and atmospheric light scattering to the top-of-atmosphere (TOA) does not have extrema in the vicinity of 600nm. Therefore, there is a possibility to remove both atmospheric and ground light scattering effects to the TOA reflectance over highly reflective underlying surface and derive the atmospheric transmittance due to the ozone absorption effects, which can be used for the TOC determination. Such a method has been explored using MERIS/ENVISAT (Jolivet et al., 2016) and OLCI/S-3 (Kokhanovsky et al., 2020) in the past. This paper is aimed at further improvement of the technique as applied to OLCI/S-3A,B. We have performed intercomparisons of OLCI TOC retrievals with TOC derived from ground and other satellite (e.g., OMI, TROPOMI, GOME-2) measurements. The TOC retrievals using OLCI have been performed over entire Antarctica allowing the generation of TOC at various spatial resolutions including standard 1x1 degree resolution.

Gorshelev, V., et al., 2014: High spectral resolution ozone absorption cross-sections – Part 1: Measurements, data analysis and comparison with previous measurements around 293 K, Atmos. Meas. Tech., 7, 609–624, https://doi.org/10.5194/amt-7-609-2014.

Jolivet D., et al., 2016: TORMS : total ozone retrieval from MERIS in view of application to Sentinel-3,  Living Planet Symposium, Proceedings of the conference held 9-13 May 2016 in Prague, Czech Republic. Edited by L. Ouwehand. ESA-SP Volume 740, ISBN: 978-92-9221-305-3, p.358

Kokhanovsky, A. A., et al., 2020: Retrieval of total ozone over Antarctica using Sentinel -3 Ocean and Land Colour Instrument, JQSRT, 2020, 251, https://doi.org/10.1016/j.jqsrt.2020.107045.

How to cite: Kokhanovsky, A., Iodice, F., Lelli, L., and Retscher, C.: The determination of the total ozone column using satellite measurements in the Chappuis ozone absorption bands over highly reflective underlying surfaces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3068, https://doi.org/10.5194/egusphere-egu21-3068, 2021.

In this work we present the validation results of the daily observations of the Total Ozone Column (TOC) obtained by the TROpospheric Monitoring Instrument (TROPOMI), and the Dobson spectrophotometer No. 118 located in Athens, Greece, (WOUDC Station ID: 293) during the period November 2017 to February 2021. Simultaneous observations of both instruments are used for this validation.

The increased spatial resolution of TROPOMI observations in relation to the push-broom configuration (non-scanning) of the instrument (swath width of ~2600 km) offers the opportunity to study the spatial analysis of the observed differences in a large area around the ground-based station. By using the ground-based station in Athens we attempt to analyze spatial and temporal behavior of the TOC differences between Dobson and TROPOMI data in an area enclosed by a 500 km radius during the period from August 2019 to February 2021.

How to cite: Varotsos, C., Xue, Y., Christodoulakis, J., Kouremadas, G., Fotaki, E.-F., and Lampros, N.: Expanding the spatial coverage of a ground-based station to validate satellite total ozone data; The case of TROPOMI and Athens Dobson, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7690, https://doi.org/10.5194/egusphere-egu21-7690, 2021.

The TOPAS (Tikhonov regularized Ozone Profile retrievAl with SCIATRAN) algorithm to retrieve vertical profiles of ozone from space-borne observations in nadir viewing geometry has been developed at the Institute of Environmental Physics (IUP) of the University of Bremen and applied to TROPOMI L1B spectral data version 2. The data set covers the period from June 2018 to October 2019. But it is not available continuously, but for only single weeks of all 3 months. TROPOMI spectral radiance from channel UV1 and UV2 between 270 nm and 331 nm are used for the retrieval. Since the ozone profiles are very sensitive to absolute calibration at short wavelengths, a re-calibration of the measured radiances is required using comparisons with simulated radiances with ozone limb profiles from collocated MLS/Aura used as input. The time-independent re-calibration bases on simulations for cloud-free pixels of four orbits distributed over the time period. Studies with synthetic spectra show that individual profiles in the stratosphere can be retrieved with the accuracy of about 10%. In the troposphere, the retrieval errors are larger depending on the a-priori profile used. The vertical resolution is between 6 and 10 km above 18 km altitude and 15 – 25 km below. There are around 6 degree of freedom between 0 – 60 km. The TOPAS ozone profiles retrieved from TROPOMI were validated using data from ozone sondes and stratospheric ozone lidars. Above 18 km, the comparison with sondes shows excellent agreement within less than ± 5% for all latitudes. The standard deviation of mean differences is about 10%. Below 18 km, the relative mean deviation in the tropics and northern latitudes is still quite good remaining within ± 20%. At southern latitudes larger differences of up to +40% occur between 10 and 15 km. Here the standard deviation is about 50% between 7 and 18 km and about 25% below 7 km. The validation of stratospheric ozone profiles with ground-based lidar measurements also shows very good agreement. The relative mean deviation is below ± 5% in the 18 – 45 km range with a standard deviation of 10%. A pilot application for one day of TROPOMI data with a comparison to MLS and OMPS confirmed the lidar validation results. The relative mean difference between TROPOMI and MLS or OMPS is largely below ± 5% between 20 – 50 km except for the very high latitudes where differences are getting larger.

How to cite: Mettig, N., Weber, M., Rozanov, A., Arosio, C., Burrows, J. P., Veefkind, P., Thomspon, A. M., Querel, R., Leblance, T., Godin-Beekmann, S., Kivi, R., and Tully, M. B.: TOPAS ozone profile retrieval from TROPOMI L1B version 2 dataset, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3970, https://doi.org/10.5194/egusphere-egu21-3970, 2021.

Part of the space segment of EU’s Copernicus Earth Observation programme, the Sentinel-5 Precursor (S5P) mission is dedicated to global and European atmospheric composition measurements of air quality, climate and the stratospheric ozone layer. On board of the S5P early afternoon polar satellite, the imaging spectrometer TROPOMI (TROPOspheric Monitoring Instrument) performs nadir measurements of the Earth radiance within the UV-visible and near-infrared spectral ranges, from which atmospheric ozone profile data are retrieved. Developed at the Royal Netherlands Meteorological Institute (KNMI) and based on the optimal estimation method, TROPOMI’s operational ozone profile retrieval algorithm has recently been upgraded. With respect to early retrieval attempts, accuracy is expected to have improved significantly, also thanks to recent updates of the TROPOMI Level-1b data product. This work reports on the initial validation of the improved TROPOMI height-resolved ozone data in the troposphere and stratosphere, as collected both from the operational S5P Mission Performance Centre/Validation Data Analysis Facility (MPC/VDAF) and from the S5PVT scientific project CHEOPS-5p. Based on the same validation best practices as developed for and applied to heritage sensors like GOME-2, OMI and IASI (Keppens et al., 2015, 2018), the validation methodology relies on the analysis of data retrieval diagnostics – like the averaging kernels’ information content – and on comparisons of TROPOMI data with reference ozone profile measurements. The latter are acquired by ozonesonde, stratospheric lidar, and tropospheric lidar stations performing network operation in the context of WMO's Global Atmosphere Watch and its contributing networks NDACC and SHADOZ. The dependence of TROPOMI’s ozone profile uncertainty on several influence quantities like cloud fraction and measurement parameters like sun and scan angles is examined and discussed. This work concludes with a set of quality indicators, enabling users to verify the fitness-for-purpose of the S5P data.

How to cite: Keppens, A., Lambert, J.-C., Hubert, D., Compernolle, S., Verhoelst, T., Niemeijer, S., Fjaeraa, A. M., ter Linden, M., Sneep, M., de Haan, J., and Veefkind, P. and the CHEOPS-5p validation team: Validation of TROPOMI nadir ozone profile retrievals: Methodology and first results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7385, https://doi.org/10.5194/egusphere-egu21-7385, 2021.

Solar eclipses reduce the measured top-of-atmosphere (TOA) reflectances as derived by Earth observation satellites, because the solar irradiance that is used to compute these reflectances is commonly measured before the start of the eclipse. Consequently, air quality products that are derived from these spectra, such as the ultraviolet (UV) Absorbing Aerosol Index (AAI), are distorted. Sometimes, such eclipse anomalies propagate into anomalies in temporal average maps without raising an eclipse flag, potentially resulting in false conclusions about the mean aerosol effect in that time period. The availability of air quality satellite data in the penumbral and antumbral shadow during solar eclipses, however, is of particular interest to users studying the atmospheric response to solar eclipses.
Given the time and location of a point on the Earth’s surface, we explain how to compute the eclipse obscuration fraction taking into account wavelength dependent solar limb darkening. With the calculated obscuration fractions, we restore the TOA reflectances and the AAI in the penumbral shadow during the annular solar eclipses on 26 December 2019 and 21 June 2020 measured by the TROPOMI/S5P instrument.
We find that the Moon shadow anomaly in the uncorrected AAI is caused by a reduction of the measured reflectance at 380 nm, rather than a color change of the measured light. We restore common AAI features such as the sunglint and desert dust, and we confirm the restored AAI feature on 21 June 2020 at the Taklamakan desert by measurements of the GOME-2C satellite instrument on the same day but outside the Moon shadow.
We conclude that our correction method can be used to detect real AAI rising phenomena and has the potential to restore any other product that is derived from TOA reflectance spectra. This would resolve the solar eclipse anomalies in satellite air quality measurements in the penumbra and antumbra, and would allow for studying the effect of the eclipse obscuration on the local atmosphere from space.

How to cite: Trees, V., Wang, P., and Stammes, P.: Restoring the top-of-atmosphere reflectance during solar eclipses: a proof of concept with the UV Absorbing Aerosol Index measured by TROPOMI, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3181, https://doi.org/10.5194/egusphere-egu21-3181, 2021.

We present the total column water vapor (TCWV) retrieval for the TROPOspheric Monitoring Instrument (TROPOMI) observations in the blue band. The retrieval was first developed to retrieve TCWV from Global Ozone Monitoring Experience 2 (GOME-2). We have modified the settings of the retrieval to adapt it for TROPOMI observations. The TROPOMI TCWV retrieval algorithm consists of two major steps. The first step is the retrieval of water vapor slant columns by applying the differential optical absorption spectroscopy (DOAS) technique to TROPOMI observations in the blue band. The retrieved water vapor slant columns are then converted to vertical columns using air mass factors (AMFs). An iterative optimization has been developed to dynamically find the optimal a priori water vapor profile for AMF calculation. The dynamic search algorithm makes use of the fact that the vertical distribution of water vapor is strongly correlated to the total column amount. This makes the algorithm better suited for climate studies compared to typical satellite retrievals with static a priori or vertical profile information from the chemistry transport model (CTM). Details of the TCWV retrieval are presented. The TCWV retrieval algorithm is applied to TROPOMI observations. The results are validated by comparing to Ozone Monitoring Instrument (OMI), GOME-2 and Special Sensor Microwave Imager Sounder (SSMIS) satellite observations. TCWV derived from TROPOMI observations agree well with the other data sets with Pearson correlation coefficient (R) ranging from 0.94 to 0.99. The correlation is slight better during winter time of the northern hemisphere. Small discrepancies are found among TROPOMI, OMI, GOME-2 and SSMIS observations. The discrepancies are mainly due to differences in measurement time and cloud filtering. More detailed validation against ground based sun-photometer observations are presented separately in this session*.

*see the respective abstract by Katerina Garane.

How to cite: Chan, K. L., Slijkhuis, S., Garane, K., Valks, P., and Loyola, D.: TROPOMI observations of total column water vapor, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2673, https://doi.org/10.5194/egusphere-egu21-2673, 2021.

The very important role of water vapor on the greenhouse effect makes it a species that needs to be continuously and globally monitored, as well as thoroughly studied. The TROPOMI/S5P Total Column Water Vapor (TCWV) is a new product retrieved from the blue wavelength band (435 –455nm), using an algorithm that was originally developed for GOME-2. The algorithm is based on the DOAS technic and is separately presented in this session*.

The TROPOMI/S5P TCWV product is available for the time period May 2018 to August 2020, almost 2.5 years. For the validation purposes of this work, the co-located precipitable water Level 2.0 (quality-assured) measurements from the NASA AERONET (AErosol RObotic NETwork) were used. The network uses CIMEL sunphotometers located at about 1300 stations globally to monitor precipitable water, among other products. The two datasets, satellite and ground-based, were co-located and the percentage differences of the comparisons were calculated and statistically analyzed. The correlation coefficient of the two products is found to be 0.9 and the mean bias of the relative percentage differences is of the order of 2% for the mid-latitudes and the tropics but increases close to the poles. The effect of various influence quantities, such as air mass factor, solar zenith angle, clouds and albedo are also studied.

*see the respective abstract by Ka Lok Chan (EGU21-2673)

How to cite: Garane, K., Chan, K. L., Koukouli, M. E., Loyola, D., and Balis, D.: First validation results of the new TROPOMI/S5P Total Column Water Vapor product using AERONET ground-based measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2841, https://doi.org/10.5194/egusphere-egu21-2841, 2021.

The ESA S5p+Innovation programme aims at supporting the development of new TROPOMI scientific products. As part of this activity, a glyoxal tropospheric column algorithm, relying on heritage from SCIAMACHY, GOME-2 and OMI, has been adapted to TROPOMI and further developed. This product provides information on volatile organic compounds (VOC) emissions as glyoxal is mainly released in the atmosphere as an intermediate product of VOC oxidation, but also directly emitted from biomass burning events.

We present here the BIRA-IASB S5p glyoxal product, which relies on a DOAS approach: spectral fits in the 435-460 nm window provide glyoxal slant columns, which are then converted into tropospheric columns by means of air mass factors and application of a background correction. In particular, the algorithm has been improved to mitigate the impact of scene brightness inhomogeneity and of non-linearity in case of strong NO2 absorption. The retrieved columns are provided along with total error estimates resulting from the propagation of uncertainties at every step in the algorithm chain.

We also highlight the excellent consistency between the retrievals from TROPOMI and those from OMI and GOME-2A/B obtained with a similar algorithm. In addition, the good quality of the product is demonstrated with comparisons with MAX-DOAS glyoxal observations at a few stations in Asia and Europe.

How to cite: Lerot, C., Hendrick, F., De Smedt, I., Theys, N., Vlietinck, J., Yu, H., Van Roozendael, M., Stavrakou, J., Müller, J.-F., Alvarado, L., Richter, A., Valks, P., Loyola, D., Irie, H., Kumar, V., Wagner, T., and Retscher, C.: TROPOMI glyoxal tropospheric column retrievals: description, inter-satellite comparison and validation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14432, https://doi.org/10.5194/egusphere-egu21-14432, 2021.

Chlorine dioxide is an indicator for chlorine activation in the stratosphere, of importance for understanding spring-time ozone depletion processes in the polar regions of both hemispheres. Within the EUMETSAT AC SAF working group, chlorine dioxide (OClO) was retrieved from the GOME-2 instruments on MetOp-A and MetOp-B platforms, respectively over the time periods 2007-2016 and 2012-2016. Moreover, recent work performed as part of the S5p+ Innovation programme has led to the creation of an additional dataset derived from the TROPOMI instrument, extending the OClO time series in 2018-2020.

This study analyses the quality of both OClO slant column (SCD) datasets by comparing them to ground-based DOAS zenith-sky measurements at a selection of 8 stations in Arctic and Antarctic regions: Eureka (80°N), Ny Alesund (79°N), Kiruna (68°N), Harestua (60°N), Marambio (64°S), Belgrano (78°S), Neumayer (71°S) and Arrival Heights (78°S). To allow for comparison with satellite data, ground-based OClO spectral analyses are performed using yearly fixed reference spectra recorded at low SZA in the absence of chlorine activation. Furthermore, an additional bias-correction is applied in post-processing to generate a consistent long-term OClO data record covering the 2007-2020 period.

Daily comparisons of satellite and ground-based SCD data pairs corresponding to similar SZA conditions are performed, assuming similar stratospheric light paths in satellite nadir and ground-based zenith-sky geometries. Daily mean OClO SCD time-series show that satellite and ground-based observations agree well at all stations in terms of short-term variability and seasonal variation. Linear regression plots show a correlation coefficient R of about 0.97, a slope of 0.9 and an intercept of less than 1x10¹³ molec/cm² for TROPOMI, while for GOME-2 results are more noisy and tend to be biased low, with correlation coefficients between 0.76 and 0.88, slopes between 0.65 and 0.74 and intercepts up to 2.4 x10¹³ molec/cm².

How to cite: Pinardi, G., Van Roozendael, M., Hendrick, F., Meier, A., Richter, A., Wagner, T., Gu, M., Friess, U., Strong, K., Brognar, K., Alwarda, R., Querel, R., Yela, M., Prados-Roman, C., and Valks, P.: Validation of satellite OClO products from S5P/TROPOMI and MetopA and B/GOME2 , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8446, https://doi.org/10.5194/egusphere-egu21-8446, 2021.

Solar ultraviolet (UV) radiation has a broad range of effects concerning life on Earth. Because of its high photon energy, UV radiation influences human health, terrestrial and aquatic ecosystems, air quality, and materials in various ways.  The Sentinel 5 Precursor (S5P) mission on a sun-synchronous orbit with an ascending node equatorial crossing at 13:30, which in conjunction with a wide swath of 2600 km provides near-global daily coverage. S5P’s TROPOMI instrument measures radiation backscattered from the Earth–atmosphere system and provides observations of atmospheric composition with the best spatial resolution presently. Among other things, TROPOMI measurements are used for calculating the UV radiation reaching the Earth's surface over the sunlit part of the globe.

This UV-radiation product is processed at the Finnish Meteorological Institute Copernicus Collaborative Ground segment. The product was released via FinHUB in summer 2020. The TROPOMI L2 UV product contains 36 UV parameters in total, including irradiances at four different wavelengths and dose rates for erythemal and vitamin D synthesis action spectra. All parameters are calculated for overpass time, for solar noon time, and for theoretical clear-sky conditions with no clouds or aerosols. Daily doses and accumulated irradiances are also calculated by integrating over the sunlit part of the day. In addition to UV parameters, quality flags related to the UV product and processing are generated.Validation with ground based instruments have shown that the agreement is very good, typically within 10%.

The S5P is the first Copernicus mission dedicated to atmospheric observations, and it will be complemented by Sentinel 4 with geostationary orbit and Sentinel 5 on Sun-synchronous morning orbit with planned launches in the coming years.  It is expected that surface UV-radiation products from these instruments will continue the present time series.

The TROPOMI surface UV radiation product responds to the increasing need for information regarding the tropospheric chemistry and biologically active wavelengths of the solar spectrum reaching the surface. In this presentation we introduce the TROPOMI UV radiation product and future developments, discuss about the quality of the product and demonstrate the usefulness of the satellite UV-data by showing resent applications including among others the exceptionally high UV-radiation conditions in mid latitudes due to persistent Antarctic ozone hole in December 2020 and modeling of seasonal cycle of COVID-19. By combining  the TROPOMI UV data with observations of trace gases from the same instrument, there is also a potential for new kind of applications, where satellite data can be used in novel ways to study photochemical processes in the troposphere.

How to cite: Kujanpää, J., Lakkala, K., Lindfors, A., Kalakoski, N., Sundström, A.-M., Ialongo, I., Arola, A., Hassinen, S., and Tamminen, J.: TROPOMI UV radiation product and recent applications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15853, https://doi.org/10.5194/egusphere-egu21-15853, 2021.

Sentinel-5 Precursor and Sentinel-4 are atmospheric Copernicus missions focused on trace gas, greenhouse gas, aerosol and cloud retrieval and operate in the UV/VIS/NIR/(SWIR) spectral region. A key ingredient for the retrieval of the aforementioned trace gases and greenhouse gases is a precise knowledge of the presence of clouds. On top of that, clouds are by themselves interesting to measure and monitor because of their contribution to the radiation budget, and hence, impact on climatological applications. In this contribution, we present the algorithms for retrieving the operational cloud products from TROPOMI onboard Sentinel-5 Precursor and the UVN spectrometer onboard Sentinel-4. These are called OCRA (Optical Cloud Recognition Algorithm) and ROCINN (Retrieval of Cloud Information using Neural Networks) and both have their heritage with GOME/ERS-2 and GOME-2 MetOp-A/B/C, where they have already been successfully implemented in an operational environment.

OCRA employs a broad band color space approach to determine a radiometric cloud fraction and ROCINN retrieves cloud top height, cloud optical thickness and cloud albedo from NIR measurements in and around the oxygen A-band, taking as a priori input the cloud fraction computed by OCRA.
The cloud parameters retrieved by ROCINN are provided for two different cloud models. One which treats clouds more realistically as layers of scattering water droplets (clouds-as-layers, CAL) and one which treats clouds as simple Lambertian reflectors (clouds-as-reflecting boundaries, CRB).
The current status of the algorithms is presented along with recent developments and improvements.

How to cite: Lutz, R., Molina García, V., Romahn, F., Argyrouli, A., and Loyola, D.: The Operational Cloud Products for Sentinel-5 Precursor and Sentinel-4, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9450, https://doi.org/10.5194/egusphere-egu21-9450, 2021.

Space-born atmospheric composition measurements, like those from Sentinel-5p TROPOMI, are strongly affected by the presence of clouds. Dedicated cloud data products, typically retrieved with the same sensor, are therefore an important tool for the provider of atmospheric trace gas retrievals. Cloud products are used for filtering and modification of the modelled radiative transfer.

In this work, we assess the quality of the cloud data derived from Copernicus Sentinel-5 Precursor TROPOMI radiance measurements. Three cloud products are considered: (i) L2_CLOUD OCRA/ROCINN CAL (Optical Cloud Recognition Algorithm/Retrieval of Cloud Information using Neural Networks; Clouds-As-Layers), (ii) L2_CLOUD OCRA/ROCINN CRB (same; Clouds-as Reflecting Boundaries), and (iii) the S5p support product FRESCO-S (Fast Retrieval Scheme for Clouds from Oxygen absorption bands for Sentinel). These cloud products are used in the retrieval of several S5p trace gas products (e.g., ozone columns and profile, total and tropospheric nitrogen dioxide, sulfur dioxide, formaldehyde). The quality assessment of these cloud products is carried out within the framework of ESA’s Sentinel-5p Mission Performance Centre (MPC) with support from AO validation projects focusing on the respective atmospheric gases.

Cloud height data from the three S5p cloud products is compared to radar/lidar based cloud profile information from the ground-based networks CLOUDNET and ARM. The cloud height from S5p CLOUD CRB and S5p FRESCO are on average 0.6 km below the cloud mid-height of CLOUDNET measurements, and the cloud top height from S5p CLOUD CAL is on average 1 km below CLOUDNET’s cloud top height. However, the comparison is different for low and high clouds, with S5p CLOUD CAL cloud top height being only 0.3 km below CLOUDNET’s for low clouds.  The radiometric cloud fraction and cloud (top) height are compared to those of other satellite cloud products like Aura OMI O₂-O₂. While the latitudinal variation is often similar, offsets are encountered.

Recently, major S5p cloud product upgrades were released for S5p OCRA/ROCINN (July 2020) and for S5p FRESCO (December 2020), leading to a decrease of the ROCINN CRB cloud height and an increase of the FRESCO cloud height on average. Moreover, a major change in the ROCINN surface albedo treatment leads to a clear improvement of the comparison with CLOUDNET at the complicated sea/land/ice/snow site Ny-Alesund.

How to cite: Compernolle, S., Argyrouli, A., Lutz, R., Sneep, M., Lambert, J.-C., Fjaeraa, A. M., Granville, J., Hubert, D., Keppens, A., Loyola, D., O'Connor, E., Rasson, O., Romahn, F., Stammes, P., Verhoelst, T., and Wang, P.: Validation of Sentinel-5p TROPOMI cloud data with ground-based Cloudnet and other satellite data products, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5811, https://doi.org/10.5194/egusphere-egu21-5811, 2021.

The first European Sentinel satellite for monitoring the composition of the Earth’s atmosphere, the Sentinel 5 Precursor (S5p), carries the TROPOspheric Monitoring Instrument (TROPOMI) to map trace species of the global atmosphere at high spatial resolution. Retrievals of tropospheric trace gas columns from satellite measurements are strongly influenced by clouds. Thus, cloud retrieval algorithms were developed and implemented in the trace gas processing chain to consider this impact.

In this study, different cloud products available for NO₂ retrievals based on the TROPOMI level 1b data version 1 and an updated TROPOMI level 1b test data set of version 2 (Diagnostic Data Set 2B, DDS2B) are analyzed. The data sets include a) the TROPOMI level 2 OCRA/ROCINN (Optical Cloud Recognition Algorithm/Retrieval of Cloud Information using Neural Networks) cloud products CRB (cloud as reflecting boundaries) and CAL (clouds as layers), b) the FRESCO (Fast Retrieval Scheme for Clouds from Oxygen absorption bands) cloud product,  c) the cloud fraction from the NO₂ fitting window, d) the VIIRS (Visible Infrared Imaging Radiometer Suite) cloud product, and e) the MICRU (Mainz Iterative Cloud Retrieval Utilities) cloud fraction. The cloud products are compared with regard to cloud fraction, cloud height, cloud albedo/optical thickness, flagging and quality indicators in all 4 seasons. In particular, the differences of the cloud products under difficult situations such as snow or ice cover and sun glint are investigated.

We present results of a statistical analysis on a limited data set comparing cloud products from the current and the upcoming lv2 data versions and their approaches. The aim of this study is to better understand TROPOMI cloud products and their quantitative impacts on trace gas retrievals.

How to cite: Latsch, M., Richter, A., Burrows, J. P., Wagner, T., Sihler, H., van Roozendael, M., Loyola, D., Valks, P., Argyrouli, A., Lutz, R., Veefkind, P., Eskes, H., Sneep, M., Wang, P., and Siddans, R.: Intercomparison of Cloud Products based on S5P/TROPOMI Level 1b Data Version 1 and the updated Version 2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4946, https://doi.org/10.5194/egusphere-egu21-4946, 2021.

Operational retrievals of tropospheric trace gases from space-borne spectrometers are made using 1D radiative transfer models. To minimize cloud effects generally only partially cloudy pixels are analysed using simplified cloud contamination treatments based on radiometric cloud fraction estimates and photon path length corrections based on oxygen collision pair (O2-O2) or O2A-absorption band measurements. In reality, however, the impact of clouds can be much more complex, involving unresolved sub-pixel clouds, scattering of clouds in neighbouring pixels, and cloud shadow effects, such that 3D radiation scattering from unresolved boundary layer clouds may give significant biases in the trace gas retrievals. In order to quantify this impact, we use the MYSTIC 3D radiative transfer model to generate synthetic data. The realistic 3D cloud fields, needed for MYSTIC input, are generated by the ICOsahedral Non-hydrostatic (ICON) atmosphere model for a region including Germany, the Netherlands and parts of other surrounding countries. The retrieval algorithm is applied to the synthetic data and comparison to the known input trace gas concentrations yields the retrieval error due to 3D cloud effects. 
In this study, we study NO2, which is a key tropospheric trace gas measured by TROPOMI and the future atmospheric Sentinels (S4 and S5). The work starts with a sensitivity study for the simulations with a simple 2D box cloud. The influence of cloud parameters (e.g., cloud top height, cloud optical thickness), observation geometry, and spatial resolution are studied, and the most significant dependences of retrieval biases are identified and investigated. Several approaches to correct the NO2 retrieval in the cloud shadow are explored and ultimately applied to both synthetic data with realistic 3D clouds and real observations.

How to cite: Yu, H., Kylling, A., Emde, C., Mayer, B., Van Roozendael, M., Stebel, K., and Veihelmann, B.: Impact of 3D cloud structures on tropospheric NO2 column measurements from UV-VIS sounders, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7523, https://doi.org/10.5194/egusphere-egu21-7523, 2021.

The NASA-TROPOMI aerosol algorithm (TropOMAER), is an adaptation of the currently operational OMI near-UV (OMAERUV & OMACA) inversion schemes, that take advantage of TROPOMI’s unprecedented fine spatial resolution at UV wavelengths, and the availability of ancillary aerosol-related information to derive aerosol loading in cloud-free and above-cloud aerosols scenes. In this presentation we will introduce the NASA TROPOMI aerosol algorithm and discuss initial evaluation results of retrieved aerosol optical depth (AOD) and single scattering albedo (SSA) by direct comparison to AERONET AOD direct measurements and SSA inversions. We will also demonstrate TropOMAER retrieval capabilities in the context of recent continental scale aerosol events.

How to cite: Torres, O., Jethva, H., Ahn, C., Jaross, G., and Loyola, D.: The NASA-TROPOMI Aerosol Algorithm: Evaluation of first results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8521, https://doi.org/10.5194/egusphere-egu21-8521, 2021.

To better understanding the role of aerosols in climate change and their direct effects on human health, aerosol optical properties have been monitored by various satellite sensors. Successful operations of the Tropospheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor satellite allow an improved understanding of the wide-ranging variation in aerosol distribution and properties with high spatial resolution since 2018. The Geostationary Environmental Monitoring Spectrometer (GEMS), onboard Geokompsat-2B (GK-2B) satellites, is the first air quality monitoring sensor in geostationary earth orbit and was successfully launched on February 19, 2020. GEMS measures hourly hyperspectral radiances with the spectral resolution of 0.6 nm in UV and visible range (300 – 500 nm) and the spatial resolution of 3.5 x 8 km² over Asia during the daytime to provide air quality information. TROPOMI which has similar specifications to GEMS, has the advantages of the sensitivity of aerosol absorption and aerosol height information in UV-Vis wavelengths. GEMS aerosol algorithm was applied to the Level 1B data of TROPOMI to retrieve aerosol optical properties such as aerosol optical depth (AOD), UV aerosol index (UVAI), single scattering albedo (SSA), and aerosol loading height (ALH). We present GEMS aerosol retrieval results to discuss high aerosol loading cases over East Asia and analysis results as a case study. Our results show that the GEMS aerosol products have the advantage to capture the fine-scale features of aerosol properties in high spatial resolution. Further, the results are compared to other aerosol products obtained from the Advanced Himawari Imager (AHI) onboard Himawari-8. Qualitatively good agreements and fine-scale features are shown in this case study.

How to cite: Cho, Y., Kim, J., Go, S., Kim, M., Lim, H., and Torres, O.: Retrieval of Aerosol Optical Properties over East Asia from TROPOMI using GEMS Aerosol Algorithm , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6841, https://doi.org/10.5194/egusphere-egu21-6841, 2021.

The quantification of the abundance of particulate matter in the atmosphere has a great societal importance, as they directly impact the environment and Earth climate. Aerosols modify the radiative budget of the planet by scattering the sun radiation and preventing part of it from reaching the Earth surface . Depending on the vertical distribution of aerosols in the atmosphere, they may also modify precipitation rates since they act as cloud condensation nuclei. When located near the surface, aerosols are the most harmful air pollutants causing chronic diseases and premature deaths of millions of people everyyear around the world (WHO 2018). Aerosols can directly impact the economy by e.g., transport disruption or health care costs.

Satellite measurements offer a great potential for observing aerosol distribution in the atmosphere from the regional to the global scale. However, these measurements are mostly done in two dimensions (2D): horizontal distributions of aerosol optical depth (AOD) by passive sensors as MODIS or latitudinal transects of vertical profiles of aerosol backscatter spaced in longitude by about 2000 km as done by the CALIOP spaceborne lidar. Recently, horizontal maps of mean aerosol layer altitudes are also retrieved by analyzing the radiation spectra. However, the full 3D distribution of aerosols at daily scale has been only observed for coarse particles such as desert dust from the analysis of thermal infrared spectra from the IASI sensor (Cuesta et al., 2015, 2020). In the present work, we have extended the retrieval of the 3D distribution for fine aerosols for the first time, applying the approach on the biomass burning aerosol emitted from Australian major wildfires in December 2019. For this, we have analyzed the spectrum of reflectance at the oxygen A-band around 760 nm together with some part of the visible spectrum measured by the TROPOMI sensor onboard the Sentinel 5-Precursor satellite. Since these measurements are done in the near infrared and visible, they are sensible to fine aerosols and oxygen absorption in the A-band provides information on the vertical distribution of these particles. The retrieval is based on a Tikhonov-Philips altitude-dependent regularisation which adapts the constraints iteratively to each observed scene as done by Cuesta et al., (2015). In the current presentation, we will present the first retrievals of the 3D distribution of biomass burning aerosols for cloudfree conditions. We compare our TROPOMI-derived 3D distributions with MODIS 2D AOD maps, AERONET AOD retrievals and CALIOP vertical profile transects. Finally, we analyse the 3D pathways of transport followed by these biomass burning aerosols during these events, based on our new retrieval.

How to cite: Lemmouchi, F.: Three-dimensional distribution of biomass burning aerosols from Australianwildfires revealed by TROPOMI satellite observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16535, https://doi.org/10.5194/egusphere-egu21-16535, 2021.

Sentinel-5p/TROPOMI instrument provides hyperspectral measurements in UV, VIS and infrared spectral range. Though the main purpose of the satellite is trace gases characterization, it is capable of aerosol and surface studies. In particular, S5p/TROPOMI measurements in UV provide unique information about absorption and elevation properties of aerosol. Moreover, measurements in wide spectral range are very sensitive to aerosol size and surface type.

In the framework of ESA S5P+I AOD/BRDF project an innovative algorithm for aerosol and surface retrieval from S5p/TROPOMI instrument is being developed. It integrates the advanced GRASP algorithm with the heritage AOD and DLER algorithm previously applied to TOMS, GOME(-2), SCIAMACHY and OMI sensors. The innovative algorithm is expected to provide surface BRDF and AOD with the accuracy required by most trace gas retrieval algorithms.

Here we present the results of aerosol and surface validation and inter-comparison obtained within ESA S5p+I project. New advanced possibility of aerosol and surface characterization from S5p/TROPOMI instrument will be discussed.

How to cite: Litvinov, P., Dubovik, O., Chen, C., Lopatin, A., Lapyonok, T., Fuertes, D., Bindreiter, L., Lanzinger, V., Holter, C., Hangler, A., Aspetsberger, M., de Graaf, M., Tilstra, G., Stammes, P., and Retscher, C.: Surface and aerosol retrieval from S5P/TROPOMI: new possibilities and expected performance, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14770, https://doi.org/10.5194/egusphere-egu21-14770, 2021.

Global and regional air quality measurements play an important role in the everyday life of people, inasmuch as atmospheric constituents such as ozone (O₃), carbon monoxide (CO), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), methane (CH₄), and aerosols may cause severe threats to human health and agriculture productivity. Space-based sensors on satellites are able to detect these atmospheric constituents directly and indirectly at high spatial and temporal scales. The TROPOspheric Monitoring Instrument (TROPOMI) on the Copernicus Sentinel-5 Precursor (Sentinel-5P) satellite provides measurements of O₃, NO₂, SO₂, CH₄, CO, formaldehyde (HCHO), aerosols, and cloud in ultraviolet-visible (UV-VIS), near infrared (NIR), and shortwave infrared (SWIR) spectral ranges. The Ozone Monitoring Instrument (OMI) aboard the Aura mission measures ozone, aerosols, clouds, surface UV irradiance, and trace gases including NO₂, SO₂, HCHO, BrO, and OClO using UV electromagnetic spectrum bands. The Ozone Mapping Profiler Suite (OMPS) on the Suomi National Polar-Orbiting Partnership (Suomi-NPP or SNPP) provides environmental data products including O₃, NO₂, SO_2, and aerosols. The Microwave Limb Sounder (MLS) on Aura has been monitoring atmospheric chemical species (CO, volcanic SO₂, O₃, N₂O, BrO), temperature, humidity, and cloud ice since 2004. MLS measurements help understand stratospheric ozone chemistry, and the effects of air pollutants injected into the upper troposphere and low stratosphere. The Thermal And Near infrared Sensor for carbon Observation - Fourier Transform Spectrometer (TANSO-FTS) on the Greenhouse Gases Observing Satellite (GOSAT) covers a wide spectral range from VIS to thermal infrared (TIR), which enables remote observations of the greenhouse gases carbon dioxide (CO₂) and CH₄. Furthermore, atmospheric constituent data are also available in the second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) NASA's atmospheric reanalysis data collection. MERRA-2 uses an upgraded version of the Goddard Earth Observing System Model, version 5 (GEOS-5) data assimilation system, enhanced with more aspects of the Earth system.

The NASA Goddard Earth Sciences Data and Information Services Center (GES DISC) supports over a thousand data collections in the focus areas of Atmospheric Composition, Water & Energy Cycles, and Climate Variability. Some of these data collections include atmospheric composition products from the ongoing TROPOMI, OMI, OMPS, MLS, TANSO-FTS, and MERRA-2 missions and projects. The GES DISC web site (https://disc.gsfc.nasa.gov) provides multiple tools designed to help data users easily search, subset, visualize, and download data from these diverse sources in a unified way. We will demonstrate several methodologies employing these tools to monitor air quality.

How to cite: Zeng, J., Shen, S., Johnson, J., Savtchenko, A., Iredell, L., Wei, J., Gerasimov, I., and Meyer, D.: Air Quality Measurement and Analysis by TROPOMI, OMI, MLS, OMPS, TANSO-FTS , and MERRA-2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8076, https://doi.org/10.5194/egusphere-egu21-8076, 2021.

To improve our knowledge of the coupling of atmospheric circulation, composition and regional climate change, and to provide the urgently needed observations of the on-going changes and processes involved, we have recently proposed the Changing-Atmosphere Infra-Red Tomography Explorer (CAIRT) to ESA as Earth Explorer 11 candidate. CAIRT will be the first limb-sounder with imaging Fourier-transform infrared technology in space. By observing simultaneously the atmosphere from the troposphere to the lower thermosphere (about 5 to 115 km altitude), CAIRT will provide global observations of temperature, ozone, water vapour, as well as key halogen and nitrogen compounds. The latter will help to better constrain coupling with the upper atmosphere, solar variability and space weather. Observation of long-lived tracers (such as N₂O, CH₄, SF₆, CF₄) will provide information on transport, mixing and circulation changes. CAIRT will deliver essentially a complete budget of stratospheric sulfur (by observations of OCS, SO₂, and H₂SO₄-aerosols), as well as observations of ammonia and ammonium nitrate aerosols. Biomass burning and other pollution plumes, and their impact on ozone chemistry in the UTLS region, will be detected from observations of HCN, CO and a further wealth of volatile organic compounds. The potential to measure water vapour isotopologues will help to constrain water vapour and cloud processes and interactions at the Earth’s surface. The high-resolution measurements of temperature will provide the momentum flux, phase speed and direction of atmospheric gravity waves. CAIRT thus will provide comprehensive information on the driving of the large-scale circulation by different types of waves. Tomographic retrievals will provide temperature and trace gas profiles at a much higher horizontal resolution and coverage than achieved from space so far. Flying in formation with the Second Generation Meteorological Operational Satellite (MetOp-SG) will enable combined retrievals with observations by the New Generation Infrared Atmospheric Sounding Interferometer (IASI-NG) and Sentinel-5, resulting in consistent atmospheric profile information from the surface up to the lower thermosphere. Our presentation will give an overview of the proposed CAIRT mission, its objectives and synergies with other sensors.

How to cite: Sinnhuber, B.-M., Höpfner, M., Friedl-Vallon, F., Sinnhuber, M., Stiller, G., von Clarmann, T., Preusse, P., Plöger, F., Riese, M., Ungermann, J., Chipperfield, M., Errera, Q., Funke, B., López Puertas, M., Godin-Beekmann, S., Peuch, V.-H., Polichtchouk, I., Raspollini, P., Riel, S., and Walker, K.: The Changing-Atmosphere Infra-Red Tomography Explorer CAIRT – a proposal for an innovative whole-atmosphere infra-red limb imaging satellite instrument, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7141, https://doi.org/10.5194/egusphere-egu21-7141, 2021.

Since its selection as a NASA Earth Venture mission in late 2016, the Geostationary Carbon Cycle Observatory (GeoCarb) has been in steady development. Launch is planned for 2024, and the instrument will be hosted on a commercial platform in geostationary orbit. Featuring a geostationary view over the western hemisphere, GeoCarb will be able to provide atmospheric total-column trace gas amounts to help answer scientific questions related to the carbon cycle of North and South America such as the quantification of regional- and urban-scale carbon dioxide emissions.

GeoCarb’s instrument design features a two-arm grating-type spectrometer with four separate bands at wavelengths 0.765 µm, 1.606 µm, 2.065 µm and 2.323 µm in order to measure atmospheric absorption features of oxygen (O2), carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO). With the spacecraft position being fixed relative to Earth and the instrument’s scan mirror assembly, GeoCarb will be able to selectively point at locations visible from its position over the American continents. As a result, very different sampling strategies can be employed,  compared to polar orbiting instruments which are generally limited to revisit periods of days and weeks. For routine operations, the North and South American land masses will be scanned at least once per day – depending on the final choice of scanning strategy, large portions of the American continents could be measured twice per day. Thanks to the flexible scanning capability, there is also the possibility for special campaigns which can feature many repeated measurements over targets of special interest throughout a single day.

In this presentation, we summarize the most recent development status of the GeoCarb instrument and the various retrieval algorithms that will be used for data product generation. We will share updates on the impact of sub-slit scene inhomogeneity on retrieval results, and how a slit homogenizer can mitigate those effects. Further, we report on our analyses regarding the correction of the so-called keystone optical aberration. Finally, we provide a detailed overview of GeoCarb’s capabilities for the retrieval of solar-induced chlorophyll fluorescence.

How to cite: Somkuti, P., O'Dell, C., McGarragh, G., Crowell, S., Burgh, E., Adamkovics, M., and Crisp, D.: Latest instrument and algorithm developments from the GeoCarb mission, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8948, https://doi.org/10.5194/egusphere-egu21-8948, 2021.

NOAA’s Geostationary Extended Observations (GeoXO) satellite system is the ground-breaking mission that will advance Earth observations from geostationary orbit.  GeoXO will supply vital information to address major environmental challenges of the future in support of U.S. weather, ocean and climate operations. The GeoXO mission will continue and significantly expand observations provided by the GOES-R Series. GeoXO will bring new capabilities demonstrated by NASA, ESA, and KARI into an operational environment to address emerging environmental issues and challenges that threaten human health and the economy.

The recommended observations on GeoXO include hyperspectral observations in the ultraviolet, visible, and infrared, visible/infrared imaging during day and night time, and lightning mapping. The combination of these observing systems will provide an exciting new ability to continuously measure trace gases and aerosols over much of North America. Potential GeoXO atmospheric composition products offer new opportunities for understanding and predicting air quality, weather, climate, and their linkages.

This presentation will highlight GeoXO’s recommended atmospheric composition capabilities and describe NOAA’s efforts to engage the user community in planning for the applications of these future datasets.

How to cite: Frost, G., Kondragunta, S., Kopacz, M., Lindsey, D., Heidinger, A., and Sullivan, P.: NOAA’s Contribution to the Geo-Ring: The New Geostationary Extended Observations (GeoXO) Atmospheric Composition Capability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9200, https://doi.org/10.5194/egusphere-egu21-9200, 2021.