Retrieval of tropospheric NO2 columns from GEMS observations using the Sentinel-4 breadboard algorithm 

<p>Nitrogen oxides (NO_x) play an important role in tropospheric chemistry and are key pollutants in particular in industrialised regions. While some natural emission sources exist such as lightning and bacterial soil activities, anthropogenic emissions dominate, mainly from transport, energy production, heating and industrial sources. To better understand the role of nitrogen oxides in the troposphere and to monitor the effects of measures taken to reduce emissions, continuous and global measurements of NO₂ abundances in the troposphere are needed.</p><p>Passive remote sensing of NO₂ from space is possible as it has strong and structured absorption features in the UV and visible part of the solar spectrum. Global measurements of tropospheric NO₂ have been achieved from a series of instruments including GOME, SCIAMACHY, GOME2, OMI and TROPOMI. While these data sets provide a wealth of information on NO₂, they all are from satellites in sun-synchronous orbits and provide little insight into the diurnal evolution of NO₂. This has changed with the launch of the Korean GEMS instrument that is the first to provide hourly NO₂ measurements over Asia.</p><p>In this study, spectra from the GEMS instrument were analysed for tropospheric NO₂ using the IUP-Bremen NO₂ retrieval code developed as breadboard algorithm for the upcoming European geostationary instrument Sentinel-4. Very good agreement is found between GEMS and concurrent measurements from TROPOMI. Validation using ground based MAX-DOAS measurements in Incheon, Republic of Korea during the GMAP-2021 campaign shows good correlation but a systematic underestimation, similar to what is reported for TROPOMI data. A number of sensitivity studies have been performed to explore the changes of the retrievals when using different stratospheric correction schemes, different a priori NO₂ profiles, and different surface reflectivity assumptions. The results will be presented and discussed, in particular in view of their impact on the diurnal variations retrieved for NO₂ over different cities in Asia.</p>