Wintertime Trends of Fine Particulate Matter (PM2.5) in South Korea, 2012-2022: Response of Nitrate and Organic Components to Decreasing NOx Emissions

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 mu g m(-3) per year after correcting for meteorology. However, Seoul PM(2.5 )declined only after 2019 and its composition has shifted toward particulate nitrate (pNO(3 )(-)) and organic aerosol (OA). Trends in pNO(3) (-), OA, nitrogen dioxide (NO2), and ozone (O-3) suggest that nighttime formation of the nitrate radical (NO3) from the NO2 + O(3 )reaction is a key driver of pNO(3) (-) and secondary OA (SOA) formation. Increasing O-3 as nitrogen oxide (NOx) emissions decline has increased nighttime NO3 production over the 2012-2022 period, promoting pNO(3) (- )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.