Formaldehyde column retrievals from the GEMS mission and evaluation against TROPOMI data

<p>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.</p><p>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.</p><p>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. MAX-DOAS measurements are used to validate and interpret the observed hourly variations.</p>