Air quality trends and regimes in South Korea inferred from 2015-2023 surface and satellite observations

Abstract

Air pollutant emissions in South Korea have been changing rapidly over the past decade. Here we analyze the resulting 2015-2023 trends in air quality and regimes using surface (AirKorea), aircraft (KORUS-AQ), and satellite (low Earth orbit, geostationary) measurements. Surface concentrations of primary pollutants have decreased at rates (CO: -2.6 +/- 0.7 % a-1, SO2: -6.4 +/- 0.8 % a-1, NO2: -4.4 +/- 0.8 % a-1) consistent with the national Clean Air Policy Support System (CAPSS) emissions inventory and satellite observations. CAPSS indicates no trend in volatile organic compound (VOC) emissions, consistent with satellite observations of formaldehyde (HCHO) and glyoxal (CHOCHO), but surface aromatic concentrations show a 5.0 +/- 3.9 % a-1 decrease. Peak season (May-June) maximum 8 h daily average (MDA8) surface ozone (O3) exceeds the 60 ppbv standard everywhere at AirKorea sites, with an increase of 0.8 +/- 0.9 ppbv a-1 in the 90th percentile averaged across all sites indicating VOC-limited conditions for O3 production. However, satellite HCHO/NO2 ratios indicate a shift from VOC- to NOx-limited as NOx emissions decrease. Most AirKorea sites are in the Seoul Metropolitan Area (SMA), where vestiges of VOC-limited conditions persist; we find no O3 increases over the rest of South Korea. Fine particulate matter (PM2.5) has been decreasing at 5.0 +/- 1.6 % a-1, but the nitrate (NO3-) component has not. Satellite NH3/NO2 ratios show that PM2.5 NO3- formation was NH3-sensitive before 2019 but is now becoming NOx-sensitive as NOx emissions decrease. Our results indicate that further NOx emission decreases will reap benefits for both O3 and PM2.5 NO3- as their production is now dominantly NOx-sensitive.