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

Abstract. We analyze 2015–2023 trends in air quality in South Korea using surface (AirKorea network) and satellite measurements, including the new GEMS geostationary instrument. Primary air pollutants (CO, SO₂, NO₂) have decreased steadily at rates consistent with the national CAPSS emissions inventory. Volatile organic compounds (VOCs) show no significant trend. GEMS glyoxal (CHOCHO) identifies large industrial sources of VOCs while formaldehyde (HCHO) points to additional biogenic sources. Surface ozone (O₃) peaks in May–June and the maximum 8-hour daily average (MDA8) exceeds the 60 ppbv standard everywhere. The AirKorea average May–June 90^th percentile MDA8 O₃ increased at 0.8 ppbv a⁻¹, which has been attributed to VOC-sensitive conditions. Satellite HCHO/NO₂ ratios indicate that the O₃ production regime over Korea is shifting from VOC- to NO_x-sensitive conditions as NO_x emissions decrease. The O₃ increase at AirKorea sites is because most of these sites are in the Seoul Metropolitan Area where vestiges of VOC-sensitive conditions persist; we find no such O₃ increases over the rest of Korea where conditions are NO_x-sensitive or in the transition regime. Fine particulate matter (PM_2.5) has been decreasing at 5 % a⁻¹ in both AirKorea and satellite observations but the nitrate (NO₃⁻) component has not been decreasing. Satellite NH₃/NO₂ ratios show that PM_2.5 NO₃⁻ formation was NH₃-sensitive before 2019 but is now becoming NO_x-sensitive as NO_x emissions decrease. Our results indicate that further NO_x emission decreases in Korea will reap benefits for both O₃ and PM_2.5 NO₃⁻ as their production is now dominantly NO_x-sensitive.