Samsung Advanced Institute of Technology

We analyze 2015？2023 trends in air quality in South Korea using surface (AirKorea network) 16 and satellite measurements, including the new GEMS geostationary instrument. Primary air 17 pollutants (CO, SO2, NO2) have decreased steadily at rates consistent with the national CAPSS 18 emissions inventory. Volatile organic compounds (VOCs) show no significant trend. GEMS 19 glyoxal (CHOCHO) identifies large industrial sources of VOCs while formaldehyde (HCHO) 20 points to additional biogenic sources. Surface ozone (O3) peaks in May？June and the maximum 21 8-hour daily average (MDA8) exceeds the 60 ppbv standard everywhere. The AirKorea average 22 May？June 90th percentile MDA8 O3 increased at 0.8 ppbv a？1, which has been attributed to 23 VOC-sensitive conditions. Satellite HCHO/NO2 ratios indicate that the O3 production regime 24 over Korea is shifting from VOC- to NOx-sensitive conditions as NOx emissions decrease. The 25 O3 increase at AirKorea sites is because most of these sites are in the Seoul Metropolitan Area 26 where vestiges of VOC-sensitive conditions persist; we find no such O3 increases over the rest of 27 Korea where conditions are NOx-sensitive or in the transition regime. Fine particulate matter 28 (PM2.5) has been decreasing at 5% a？1 in both AirKorea and satellite observations but the nitrate 29 (NO3？) component has not been decreasing. Satellite NH3/NO2 ratios show that PM2.5 NO3？ 30 formation was NH3-sensitive before 2019 but is now becoming NOx-sensitive as NOx emissions 31 decrease. Our results indicate that further NOx emission decreases in Korea will reap benefits for 32 both O3 and PM2.5 NO3？ as their production is now dominantly NOx-sensitive.