The Copernicus Atmosphere Monitoring Service (CAMS), operated by the European Centre for Medium-Range Weather Forecasts (ECMWF) on behalf of the European Commission, provides daily analyses and 5-day forecasts of atmospheric composition including ozone, carbon monoxide, nitrogen dioxide, sulphur dioxide, methane and aerosols in near-real time as well as a reanalysis of atmospheric composition going back to 2003. This is accomplished by combining a state-of-the art transport and chemistry model with satellite data from a wide range of sensors, among them retrievals from the TROPOMI instrument on the S5P satellite which add valuable information in the CAMS system. Ozone retrievals from TROPOMI have been assimilated routinely by CAMS since December 2018 and were followed by the operational assimilation of other TROPOMI products (e.g. NO2 and SO2). In this talk we present a status update on the use of TROPOMI products in the global CAMS system, with special emphasis on the recently activated assimilation of NO2 and SO2 layer height data, and illustrate the benefits the TROPOMI data bring to the CAMS analysis.

The assimilation of TROPOMI NO2 retrievals in the CAMS system became possible after including a simplified version of the NO2 chemistry in the tangent linear and adjoint routines of the CAMS model. The impact of the NO2 assimilation is assessed by comparison against surface air quality observations and against a control run without data assimilation. CAMS assimilates the operational NRT TROPOMI SO2 retrievals for volcanic eruptions, which are total column products that do not provide any information about the height of volcanic plumes, and also explores the use of the SO2 layer height product produced in the frame of the ESA-funded Sentinel-5P Innovation–SO2 Layer Height Project whose use can lead to improved SO2 forecasts. Furthermore, CAMS is moving closer to the operational assimilation of TROPOMI CO products, retrieved in the SWIR part of the spectrum, that bring extra information into the CAMS CO analysis which already assimilates CO products from IASI and MOPITT, leading to an improved vertical distribution of CO in the CAMS system.

More details about CAMS can be found on http://www.copernicus-atmosphere.eu and CAMS data are freely available from the Atmosphere Data Store (ADS) ads.atmosphere.copernicus.eu.

The Tropospheric Monitoring Instrument (TROPOMI) aboard ESA's Copernicus Sentinel-5 satellite (S5-P) provides the Carbon Monoxide (CO) total column data product with daily global coverage and a spatial resolution of 7x7 km2 (improved to 5.5x7km2 since August 2019). Sources of atmospheric CO are incomplete combustion e.g due to traffic, industrial activity and wild fires. CO is widely used as a tracer to analyse the atmospheric transport of the pollutant in the Earth atmosphere. Although the CO product comprises only total column amount of the trace gas, the data product includes height information because of the vertical column sensitivity of product given by the column averaging kernel.

In this study, we present a scheme to retrieve vertical CO profiles from TROPOMI data over a selected region and/or time range and under the assumption of static vertical distribution of CO, To this end, we make use of the fact that the vertical sensitivity of total column measurements changes in the data ensemble because of different observation geometries and cloud occurrence ins the observed scenes. We show results for three example cases. First, we estimate a vertical CO profile for the "rabbit foot fire" in Canada on 8th of December 2018 that shows elevated CO pollution in an altitude of of about 5 km in agreement with the airborne measurements of BB-Flux team. Second, we estimate the altitude of pollution from biomass burning events in Siberia that was transported over the ocean to Canada on the 17th of August in 2018. Our analysis agrees well with the CAMS-IFS model simulation showing an altitude of the smoke layer of about 7.5 km. Last but not least, we analyse half month of TROPOMI CO observations over the Amazon region before and during the burning season. This example demonstrates the numerical efficiency of our assimilation scheme that can be used to combine large amount of individual TROPOMI measurements. Again our findings agree well with the calculation of CAMS-IFS. Hence, our study demonstrate the validity of CO height information in the TROPOMI CO data product due to different observation geometries, and clear-sky and cloudy conditions. Our study paves the way for more elaborated assimilation schemes like the CAMS-IFS model to fully explore the wealth of information presented in the S5P CO data product.

Sentinel-5 Precursor (S5P) is the first atmospheric composition mission of EU's Copernicus Earth Observation programme. Launched in October 2017, its unique payload TROPOMI (TROPOspheric Monitoring Instrument) measures on the global scale, on a daily basis, and at unprecedented horizontal resolution, the atmospheric abundance of species related to air quality, ozone, climate forcing, UV radiation and volcanic hazards, namely: the vertical profile and column of tropospheric and stratospheric O₃, the column of tropospheric, stratospheric and total NO₂, the total column of HCHO, SO₂, CO and CH₄, cloud properties, and aerosol UV absorbing index and layer height. Additional data products are in development and verification phase. S5P contributes observational data to operational information services on our environment, e.g., the Copernicus Atmosphere Monitoring Service (CAMS) and the Copernicus Climate Change Service (C3S), both run at ECMWF on behalf of the EC. Therefore, fitness-for-purpose of the data is essential and its quality must be monitored operationally over the mission lifetime.

Procured by an international consortium supported by ESA and national agencies of The Netherlands, Germany and Belgium, the S5P Mission Performance Centre (S5P-MPC) ensures the necessary routine validation for all S5P operational data products available to the Copernicus operational services and to the public. The S5P-MPC Routine Operations Validation Service integrates several complementary validation approaches, including comparisons of S5P data to fiducial reference measurements, to ground-based monitoring networks, to other satellites, and to alternative retrievals. Routine monitoring of S5P data quality relies in large parts on the Validation Data Analysis Facility (VDAF) and its Automated Validation Server (AVS). The latter ingests ground-based data collected from ESA’s FRM projects and from monitoring networks contributing to WMO’s Global Atmosphere Watch, performs data comparisons according to state-of-the-art protocols and validation metrics, and reports S5P data quality indicators automatically on the VDAF-AVS website. Baseline quality monitoring with the VDAF-AVS is complemented with in-depth validation carried out by dedicated teams to produce consolidated validation results. The latter are published every three months in the Routine Operations Consolidated Validation Report (ROCVR), available publicly as well. Ad hoc validation support is also provided to the teams working on the evolution of S5P retrieval algorithms and data processors. The approaches developed and implemented, the successful operation of the service, and the continuous evolution of the VDAF-AVS over the last 5 years constitute a pathfinder for the operational validation of the upcoming constellation of atmospheric composition Sentinels.

In this contribution, after a brief introduction to the S5P-MPC Routine Operations Validation Service, we report the latest validation results for each of the aforementioned TROPOMI atmospheric data products generated operationally and freely available to the public. We conclude that all data products meet mission requirements, and that for several data products recent reprocessings of Level-1b and Level-2 data improve their agreement with independent measurements. We also raise particular quality features to the attention of potential users so that they can judge the fitness of S5P data for their own applications. Latest updates of consolidated validation results and other S5P validation resources can be accessed through the portal of the TROPOMI Validation Data Analysis Facility at http://mpc-vdaf.tropomi.eu .

Introduction
The Tropospheric Monitoring Instrument (TROPOMI) on board Sentinel 5 precursor(S5P) features a new aerosol product that is dedicated to retrieval of the height of absorbing tropospheric aerosols. At present, daily global observations of aerosol height are not available on an operational basis. Aerosol profiles are provided by active sensors, particularly by ground-based lidar systems or by the space-borne Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP) with a high vertical resolution, but they observe at specific locations or in narrow tracks only. Passive sensors, such as TROPOMI, can cover the entire earth in a single day and therefore could become in the future a great choice for many scientific applications like radiative forcing studies, long-range transport, modeling and studies of cloud formation processes. Therefore, validation of the Aerosol Layer Height (ALH) product is essential to understand the quality and performance of the sensor and algorithm. The ground based lidar networks (e.g., EARLINET/ACTRIS in Europe) along with ceilometers (e-profile) are designed with long-term observation in mind, and therefore a suitable source for a long-term validation for the TROPOMI ALH data. The goal of this study is to develop a NRT (Near Real Time) procedure for comparison between Aerosol layer heights derived from satellite passive remote sensing measurements(S5P/TROPOMI) and from ground based active remote sensing measurements.
Validation methodology and collocation criteria
The validation of products with a typical resolution of several kilometres against point-like ground-based measurements involves uncertainties. A key question is how well the ground-based observation represents a larger area around the measurement site and to a large extent depends on the characteristics of the station location (urban, sub-urban, mountains, valleys, etc).
For the comparison of TROPOMI/ ALH against aerosol height from ground based active remote sensing measurements, the coincidence criteria have been checked, by setting a search of the satellite data base within 50, 100, 150 km radius circles centred in the geolocation of the ground-based station and then average the values found.
The ground based measurements nearest to the S5P overpass time within 30 minutes and also within a 1,2,3 hours temporal interval have been considered during the sensitivity studies to select every available day of measurement, ensuring a sufficiently large collocation database.
The scientific community is making remarkable efforts in developing automatic ALH retrieval algorithms applied to lidar observations. In this study, we present the results of the validation analysis of the ALH satellite-based product using independent, ground-based cloud-free observations taken at 18 sites in the frame of the European Aerosol Research Lidar Network (EARLINET) during September 2017- October 2021 and NATALI (Neural network Aerosol Typing Algorithm based on LIdar data) software (https://doi.org/10.5194/acp-18-14511-2018).
EARLINET data was processed in a centralized way using the Single Calculus Chain (SCC), with specific configurations and settings; Level 2 data is compliant to all the QC procedures currently working on the ACTRIS/EARLINET database; also, the ACTRIS COVID-19 pandemic data collection https://doi.org/10.21336/gen.682q-8163 was used. Geometrical features of lofted aerosol layers (top and bottom) are calculated by applying the gradient method on the 1064 nm backscatter coefficient profile. Backscattering, extinction and depolarization information (one hour averaged) within the layers with thicknesses of more than 300 m are considered further and analysed using NATALI software to determine the type of aerosol predominant in the layer. For overpass comparisons, ALH satellite pixels are average within 0.5 degrees radius around ground-based lidar location. The figure shows an example the comparison of the satellite ALH data and the aerosol layer heights retrieved from multiwavelength Raman lidar measurements, station (INOE) in Magurele, Romania, 6km south of Bucharest. The overpass time is once or twice per day around noon local time. The vertical bars for each data point highlight the variability of the values within the averaged subset.

The Copernicus mission Sentinel-5 Precursor (S5P) is designed to investigate atmospheric composition and to perform trace gas and greenhouse gas retrievals for air quality monitoring. Spectral information covering the UV/VIS/NIR region also allows to retrieve basic cloud information, which is an important input parameter to an accurate trace gas retrieval because of potential albedo and/or shielding effects of the clouds. Operational cloud products have been provided by the German Aerospace Center (DLR) for heritage instruments like GOME, SCIAMACHY and GOME-2, are being provided for the polar orbiting S5P (operational since 2018) and will be provided for the geostationary Sentinel-4 (to be launched in early 2024). The cloud retrieval algorithms applied are called OCRA (Optical Cloud Recognition Algorithm) and ROCINN (Retrieval of Cloud Information using Neural Networks). OCRA applies a UV/VIS broad band color space approach to determine a radiometric cloud fraction while ROCINN retrieves cloud-top height, cloud optical thickness and cloud albedo from measurements in the near infrared in and around the oxygen A-band. 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). While the operational S5P cloud products are continuously and routinely validated with ground-based observations from CLOUDNET, this contribution will focus on satellite-to-satellite comparisons. Cloud retrievals with the ROCINN_CAL model are compared to cloud products from the VIIRS instrument on-board the Suomi-NPP platform which is flying in close formation with S5P.

The TOPAS (Tikhonov regularized Ozone Profile retrievAl with SCIATRAN) algorithm to retrieve vertical ozone profiles from space-borne nadir UV radiance measurements was developed and applied to TROPOMI L1B version 2 spectral data. Spectral measurements from 270 to 329 nm (band UV1 and UV2) were used after they were spectrally re-calibrated using comparisons to simulated radiances with collocated ozone profiles from MLS/Aura as input.

The TOPAS retrieved ozone profiles from TROPOMI have a vertical resolution varying between 6 and 9 km in the stratosphere (Mettig et al. 2021). Below 18 km the sensitivity is limited and the vertical resolution is reduced.
The TOPAS ozone profiles were validated using collocated stratospheric ozone lidar and ozonesonde measurements. The validation with stratospheric ozone profiles shows very good agreement with a mean bias of to within ± 5% in the 18 - 45 km altitude range and a standard deviation of 10%. In the troposphere, the validation with ozonesonde profiles shows a larger bias with up to +40% between 10 and 15 km and a wider spread of results as well. The operational ozone profile product developed by the KNMI is available since November 2021. First results of the validation of TOPAS ozone profiles and the operational TROPOMI profiles with TOPAS results will be presented.

To increase the information content and the vertical resolution below 30 km, CrIS (on Suomi NPP) measurements in the infrared spectral range between 9.35 and 9.9 μm were included in the TOPAS retrieval. The combination of collocated UV TROPOMI and IR CrIS pixels, which have a time difference of only 3 minutes, improves the vertical resolution in the troposphere (above 3 km) to about 10 km. Validation of the combined ozone profiles with tropospheric lidars and ozonesondes shows improved tropospheric profiles and tropospheric ozone column values.


References:
Mettig, N., Weber, M., Rozanov, A., Arosio, C., Burrows, J. P., Veefkind, P., Thompson, A. M., Querel, R., Leblanc, T., Godin-Beekmann, S., Kivi, R., and Tully, M. B.: Ozone profile retrieval from nadir TROPOMI measurements in the UV range, Atmos. Meas. Tech., 14, 6057–6082, https://doi.org/10.5194/amt-14-6057-2021, 2021.