Ozone in the urban atmosphere can accumulate from photochemistry involving nitrogen oxides, carbon monoxide, and volatile organic compounds mainly emitted from fossil fuel combustions and volatile chemical products. Concentrated ozone near the earth's surface has a detrimental impact on the ecosystem and our health. The Los Angeles Basin is a classic example of an urban region with long‐standing ozone pollution, frequently in violation of its standard, designated by the US Environmental Protection Agency (EPA). Studies have reported substantial declines of ozone and their precursor concentrations over the past half century in this area due to extensive pollution controls. Since 2000, however, the ozone design value has remained the same and has not been able to fully reach the air quality standard set by the EPA despite decreases in its precursor concentrations. To understand this phenomenon, we utilize a chemical transport modeling approach to reproduce historical ozone trends and to conduct sensitivity runs relative to a 2010 baseline. Based on our analysis of the model simulations comparing the weekends and weekdays, rather than focusing only on the weekly averages, we diagnose that the Los Angeles Basin is actually going under a significant transition in photochemistry toward a lower ozone concentration. This reveals how air quality management strategies are slowly moving towards a pathway of reducing ozone concentrations in the future. Our study, however, suggests that further reductions of NOx emissions can lead to continued improvement of ozone pollution in this area.

Nitrogen oxides detected in urban regions are primarily emitted by transportation methods and are crucial precursors for air pollutants and climate forcers such as ozone and fine particulate matter. We investigate the trends of nitrogen dioxide (NO2) obtained from a satellite instrument and surface monitors over the megacity, Seoul, South Korea, from 2005 to 2019. Both satellite Ozone Monitoring Instrument NO2 and surface in situ concentrations decreased by up to 30% between 2015 and 2019 while significant trends were not observed between 2005 and 2015. Further analysis shows the continual reduction of NO2 concentrations prior to and during the COVID-19 pandemic in 2020. This study highlights the efficacy of South Korean pollution control policies targeting vehicular emissions. However, this study also found inconsistencies between trends observed in the official bottom-up emission inventory and data collected from space and surface sites. Further research will be urgently needed to understand the causes for the discrepancies.

<p>Nitrogen oxides (NO<sub>x</sub>) 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 NO<sub>2</sub> abundances in the troposphere are needed.</p><p>Passive remote sensing of NO<sub>2</sub> 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 NO<sub>2</sub> 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 NO<sub>2</sub>, they all are from satellites in sun-synchronous orbits and provide little insight into the diurnal evolution of NO<sub>2</sub>. This has changed with the launch of the Korean GEMS instrument that is the first to provide hourly NO<sub>2</sub> measurements over Asia.</p><p>In this study, spectra from the GEMS instrument were analysed for tropospheric NO<sub>2</sub> using the IUP-Bremen NO<sub>2</sub> 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 NO<sub>2</sub> 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 NO<sub>2</sub> over different cities in Asia.</p>