Predicting air quality requires understanding the processes that emit pollutants, how these are transported in the atmosphere, the chemical and physical transformations take place, and the potential impact on health and environment. Over the last 20 years, low-Earth orbit (LEO) atmospheric composition observations have provided some amazing satellite measurements of pollutants in the atmosphere, mainly at continental-to-global, weekly-to-seasonal scales. The new-generation geostationary (GEO) satellite perspective, with high spatial resolution and hourly measurements, represents a major step forward in capability for understanding how air quality processes change diurnally at the local scale. South Korea's Geostationary Environment Monitoring Spectrometer (GEMS) was launched in February 2020 over Asia and is the first member of the GEO constellation that will eventually include the Tropospheric Emissions: Monitoring Pollution (TEMPO) mission over North America, and Sentinal-4 over Europe. Common objectives for these missions and a constellation framework will provide a quasi-continuous northern hemisphere view and unprecedented capability to meet the needs of air quality research and applications. The measurement hourly time resolution is truly the new perspective that the GEO platform provides, and in this presentation, we use a combination of satellite observations from GEMS and chemical transport model simulations to investigate the diurnal variation of pollution over several Asian regions. These studies were performed as part of the GEMS Validation Team, and it should be stressed that they use preliminary versions of the tropospheric nitrogen dioxide (NO2) and formaldehyde (HCHO) data products that are still in the process of being validated and refined. As such all findings here are preliminary.

When considering the GEMS whole-Asia field-of-regard, the most striking impression of the NO2 diurnal variation is of how large it is in magnitude as well as how much the spatial distribution changes hour-by-hour. This questions our understanding of the distributions of reactive species based on the representativeness of once-a-day LEO observations. For NO2 in particular, photochemistry appears to be the dominant driver of diurnal variability on the regional scale. At finer local scales, the picture becomes complicated with large day-to-day differences driven by changes in emissions and meteorology. To analyze the city-scale, we concentrate on the Seoul, S. Korea, area. This was chosen to allow comparison with the Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) aircraft spectrometer measurements taken during the Korea-United States Air Quality Study (KORUS-AQ) field campaign in 2016. On several occasions, GeoTASO was able to map out the distribution of NO2 columns during multiple flights on a single day. These showed maximum columns in the early afternoon following the buildup of emissions during the morning and subsequent horizontal mixing of pollution. GEMS data sometimes show this same diurnal pattern, but the day-to-day changes are also very large, to the extent that one day may show a morning maximum in NO2 and the next day, a morning minimum. To help understand these daily differences in diurnal patterns at regional and local scales, we use the Multi-Scale Infrastructure for Chemistry and Aerosols (MUSIC-V0). This uses a global modeling framework with regional grid refinement to resolve chemistry at emission and exposure relevant scales. The model is run at 1/16-degree spatial resolution over Korea to match the resolution of the GEMS data at 7 km × 8 km x 2 pixel. The model shows reasonable agreement with the GEMS data and captures the different diurnal patterns at the different spatial scales and the degree of day-to day variability. The model also allows the drivers of variability due to emissions, meteorology, and photochemistry to be considered separately. We further analyze the related chemical cycles between different nitrogen species and begin to compare these with the GEMS HCHO data.

Nitrous acid (HONO) is a key atmospheric species primarily due to its role as a source of hydroxyl radicals (OH) through its rapid photolysis. OH is the atmosphere’s primary oxidant: it plays a central role in breaking down pollutants and greenhouse gases, and at the same time, it is a key ingredient to photochemical smog and ozone formation. Despite recent scientific progress, the emission budget and formation mechanisms of HONO are poorly constrained and consequently the impact of HONO emissions on tropospheric chemistry remains particularly uncertain although it is believed to be important.
Recent global space measurements of HONO in freshly emitted biomass burning plumes using the Sentinel-5 Precursor/TROPOMI instrument have provided unprecedented information on pyrogenic HONO (Theys et al., 2020), and have opened new research possibilities.
Here, we present our progress to improve the HONO space-based data. For the spectral analysis, the recently developed Covariance-Based Retrieval Algorithm (COBRA; Theys et al., 2021) is applied to retrieve HONO from TROPOMI. The results are compared to the DOAS retrievals, and discussed. In a second step, we aim at improving the selectivity of HONO plumes by combining the results with other TROPOMI trace gas and aerosols products. Then first attempt towards a quantitative HONO vertical column product is presented. Finally, we explore the potential of the geostationary GEMS instrument to provide information on HONO.
In the second part of the presentation, the satellite HONO data set is used to assess the spatio-temporal variability of HONO from fires. For a limited number of case studies spanning different ecosystems and fire magnitude, the relationship of HONO with other geophysical datasets, related to fire activity and aerosol loadings, is studied.

Theys, N., R. Volkamer, J.-F. Müller, K. J. Zarzana, N. Kille, L. Clarisse, I. De Smedt, C. Lerot, H. Finkenzeller, F. Hendrick, T. K. Koenig, C. F. Lee, C. Knote, H. Yu, and M. Van Roozendael: Global nitrous acid emissions and levels of regional oxidants enhanced by wildfires, Nat. Geosci., 13, 681-686 (2020). https://doi.org/10.1038/s41561-020-0637-7
Theys, N., Fioletov, V., Li, C., De Smedt, I., Lerot, C., McLinden, C., Krotkov, N., Griffin, D., Clarisse, L., Hedelt, P., Loyola, D., Wagner, T., Kumar, V., Innes, A., Ribas, R., Hendrick, F., Vlietinck, J., Brenot, H., and Van Roozendael, M.: A sulfur dioxide Covariance-Based Retrieval Algorithm (COBRA): application to TROPOMI reveals new emission sources, Atmos. Chem. Phys., 21, 16727–16744, https://doi.org/10.5194/acp-21-16727-2021, 2021.

Atmospheric formaldehyde (HCHO) is a secondary product in the destruction of non-methane volatile organic compounds (NMVOCs), through both natural and anthropogenic processes. With a relatively short lifetime of a few hours, the HCHO concentrations are usually localised close to their source. Measuring HCHO from space is therefore highly relevant in obtaining information on NMVOC emissions and their role in air quality and climate. HCHO retrievals from space have so far been limited to polar orbiting sensors with a fixed local overpass time.
The Geostationary Environment Monitoring Spectrometer (GEMS), launched on-board the GEO-KOMPSAT-2B satellite in February 2020 is the first geostationary sensor dedicated to air quality and atmospheric composition measurements. GEMS (observing South-East Asia hourly) will be complemented by TEMPO in 2022 (United States) and Sentinel-4 in 2023 (Europe and Northern Africa). Those instruments will provide an unprecedented hourly revisit time in their respective spatial domains. However, geostationary sensors make fundamentally different demands on the HCHO algorithm as compared to polar sensors.
In this work, we present DOAS tropospheric column retrieval results for HCHO from GEMS. In order to fit the SCD, a precise wavelength calibration is applied and potential changes in the instrumental line shape are accounted for. Polarisation spectral structures and scene heterogeneity effects are included, and a background correction and destriping procedure dedicated to geostationary observations is also developed. Air mass factors are calculated using auxiliary data consistent with the TROPOMI operational product. We compare our first results with those from TROPOMI in the early afternoon and with the GEMS HCHO operational product. Finally, we examine the diurnal variations observed with GEMS over different emission sources. The MAGRITTEv1.1 regional chemistry-transport model over Asia, as well as MAX-DOAS measurements in Xianghe and Phimae are used to validate and interpret the observed hourly variations.

Bromine monoxide (BrO) is a halogen radical capable of influencing atmospheric chemical processes, in particular the abundance of ozone, e. g. in the polar boundary layer and above salt lakes, in the stratosphere as well as in volcanic plumes. Furthermore, the molar bromine to sulphur ratio in volcanic gas emissions is a proxy for the magmatic composition of a volcano and potentially an eruption forecast parameter.
The high spatial resolution of the S5-P/TROPOMI instrument (up to 3.5x5.5km2) and its daily global coverage offer the potential to detect BrO and its corresponding ratio with sulphur dioxide (BrO/SO2) even during minor eruptions and for continuous passive degassing volcanoes.
Here, we present a global overview of BrO/SO2 molar ratios in volcanic plumes derived from a systematic long-term investigation covering four years (Januar 2018 to December 2021) of TROPOMI data.
We retrieved column densities of BrO and SO2 using Differential Optical Absorption Spectroscopy (DOAS) and calculated mean BrO/SO2 molar ratios for various volcanoes. As expected, the calculated BrO/SO2 molar ratios differ strongly between different volcanoes, but also between measurements at one volcano at different points in time, ranging from several 10-5 up to several 10-4. In our three-year study of S5P/TROPOMI data we successfully recorded elevated BrO column densities at 506 volcanic events. We were able to derive significant BrO/SO2 ratios at 26 different volcanoes on 378 occasions, thus adding an important volcanic parameter to these volcanoes.

Over the past century ammonia (NH3) emissions have increased with human population growth and fertilizer usage. The abundant NH3 emissions lead to climate change, reduction in biodiversity and affect the human health. Up-to-date information of NH3 emissions is essential to better understand the impact of NH3. In this study we adapted the existing DECSO (Daily Emissions Constrained by Satellite Observations) algorithm for use of NH3 observations from the Cross-track Infrared Sounder (CrIS) to estimate NH3 emissions.
The DECSO algorithm has been developed to derive emissions for short-lived species based on satellite observations. This is a very efficient system, which can be operated in near-real time. DECSO is specifically designed to use daily satellite observations of column concentrations for fast updates on emission estimates of short-lived atmospheric constituents at a mesoscopic scale. The algorithm needs only one forward model run from a chemical transport model to calculate the sensitivity of concentration to emissions using trajectory analysis to account for transport away from the source. An extended Kalman filter is used, in which emissions are translated to concentrations via the CHIMERE Chemical Transport Model (CTM) and compared to the satellite observations. Previous studies have applied DECSO to NO2 observations from different satellites and demonstrated that the temporal and spatial variability of total surface NOx emissions are well captured.
To apply DECSO to NH3 observations of CrIS, we have adapted the interface between DECSO and the satellite observations. All parameters to describe the various error covariances used in the data assimilation have been analysed and adapted when necessary in the DECSO-NH3 version. We have estimated NH3 emissions over Europe for 2020 on a daily basis. Due to the sparseness of daily satellite observations of NH3, monthly emissions of NH3 are analysed. We compare the derived NH3 emissions with other existing emission inventories, such as CAMS-REG-ANT.

Ammonia (NH3) is widely recognised as a major primary pollutant, deteriorating water, soil and air quality. While the importance of monitoring and regulating atmospheric NH3 emissions has been underlined for decades by experts in the field and endorsed or ratified by a multitude of international organizations, it is only recently that the issue is making its way onto the political agendas. Over a decade ago, it was discovered that high-resolution infrared satellite sounders can measure atmospheric NH3, leading to major progress in our understanding of this atmospheric compound and its sources, and to new possibilities for benchmarking or enforcing regulations.

Currently, several polar-orbiting instruments are in orbit that measure NH3 global distributions twice a day. We first use the long-term NH3 time-series available from the Infrared Atmospheric Sounding Interferometer (IASI) mission (end of 2007 up to now) to derive global, regional and national trends. Reported national trends are analysed in the light of changing anthropogenic and pyrogenic NH3 emissions, meteorological conditions and the impact of concomitant sulphur and nitrogen oxides emissions.

For the first time, the presence of a weekly cycle in the atmospheric NH3 burden is identified from space over north-western Europe. A week-end effect is found with, however, a strong seasonality due to agricultural activities and associated regulations. These results are consolidated looking at ground-based measurements from the Dutch National Air Quality Monitoring Network.

Finally, we present the first observations of NH3 from the Geostationary Interferometric Infrared Sounder (GIIRS) onboard the Chinese FY-4A satellite. GIIRS measures almost all of Asia ten times per day. We analyse the daily cycle of NH3 in detail over two small regions in Pakistan and China, and how it varies across different seasons.