Abstract
We analyze 2011–2022 trends in wintertime fine particulate matter (PM2.5) and its composition in South Korea using surface network data and machine learning. PM2.5 decreased nationwide by 1.2 μg m−3 per year after correcting for meteorology. However, Seoul PM2.5 declined only after 2019 and its composition has shifted toward particulate nitrate (pNO3−) and organic aerosol (OA). Trends in pNO3−, OA, nitrogen dioxide (NO2), and ozone (O3) suggest that nighttime formation of the nitrate radical (NO3) from the NO2 + O3 reaction is a key driver of pNO3− and secondary OA (SOA) formation. Increasing O3 as nitrogen oxide (NOx) emissions decline has increased nighttime NO3 production over the 2012–2022 period, promoting pNO3− and SOA formation. As NOx emissions in South Korea continue to decline, transition from NOx-saturated to NOx-limited conditions for NO3 formation should lead to rapid decreases in nighttime PM2.5 formation.