Drew C. Pendergrass, D. J. Jacob, Yujin J. Oak, Ruijun Dang, Laura Hyesung Yang, Ellie Beaudry, Nadia K. Colombi, Shixian Zhai, Hwajin Kim, Jin‐soo Choi, Jinsoo Park, Soontae Kim, Ke Li, Hong Liao
Abstract We analyze 2011–2022 trends in wintertime fine particulate matter (PM 2.5 ) and its composition in South Korea using surface network data and machine learning. PM 2.5 decreased nationwide by 1.2 μg 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 (NO 2 ), and ozone (O 3 ) suggest that nighttime formation of the nitrate radical (NO 3 ) from the NO 2 + O 3 reaction is a key driver of pNO 3 − and secondary OA (SOA) formation. Increasing O 3 as nitrogen oxide (NO x ) emissions decline has increased nighttime NO 3 production over the 2012–2022 period, promoting pNO 3 − and SOA formation. As NO x emissions in South Korea continue to decline, transition from NO x ‐saturated to NO x ‐limited conditions for NO 3 formation should lead to rapid decreases in nighttime PM 2.5 formation.