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.
Observations from the Little Bunker
Last updated: October 16, 2025 02:57 AM
Atmospheric CO₂
424.4 ppm
+18.2
ppm/year
Atmospheric CH₄
1933.54 ppb
+12.29
ppb/year
Global Temperature
+1.48 °C
Peak: +1.73°C
World Population
8.1 billion
+67
million/year