Interpreting Fine Particulate Matter (PM2.5) Trends in South Korea, 2011-2022

We analyze 2011-2022 trends in fine particulate matter (PM_2.5) and its composition in South Korea using a combination of surface and satellite observations interpreted with statistical models and with the GEOS-Chem chemical transport model. We use a synthetic PM_2.5 network, trained on air quality data available from 2011, to investigate PM_2.5 trends prior to 2015 when national monitoring begins. Using a stepwise multi-linear regression (MLR) technique to remove meteorological influence, we find that PM_2.5 has declined by 0.9 µg m^-3 a^-1 over the study period, with considerable spatial and temporal variability. In the first five years of PM_2.5 monitoring from the surface network (2015-19), meteorologically-normalized annual trends over the Seoul Metropolitan Area (SMA) were flat and winter PM_2.5 concentrations increased. This reflected increases in wintertime nitrate offsetting the decrease in sulfate. Since 2019, however, all PM_2.5 components have decreased. Satellite observations of the NH₃/NO₂ ratio indicate that PM_2.5­ nitrate formation was ammonia-limited prior to 2019 but has become increasingly NO_x-limited as NO_x emission has decreased and may now finally be responding to NO_x emission controls. Weekday/weekend comparisons of PM_2.5 show higher weekday levels prior to 2015 but higher weekend levels after 2015, mostly from nitrate and organic components, implying an increasing role of oxidants (as opposed to primary emissions) as sources of PM_2.5­. Nighttime ozone concentrations steadily increased over the period, explaining why wintertime PM_2.5 nitrate increased in 2015-2019 even as NO_x emissions decreased.