Nighttime waste wood burning and its impact on urban air quality: multi-Year measurements in central Seoul, Gwanghwamun, South Korea

This study examines variations in PM2.5 mass, chemical composition, and oxidative potential (OP) at Gwanghwamun Square, Seoul, using intensive 3-h integrated sampling campaigns performed in September 2020 and 2022. The mean PM2.5 concentrations declined significantly from 20.66 µg/m3 in 2020 to 12.04 µg/m3 in 2022, indicating a 42% reduction in overall particle mass. Despite this progress, the mass-normalized oxidative potential (DTTm) remained statistically unchanged (0.04 ± 0.04 vs. 0.03 ± 0.05 µM/µg, showing that particle toxicity per unit mass did not decrease. However, organic aerosol components exhibited marked increases in 2022. Organic carbon (OC) and water-soluble organic carbon (WSOC) increased by 22% and 82%, respectively, and primary organic carbon (POC) experienced more than a twofold rise. Levoglucosan (LEVO) concentrations grew by 37%, especially during nighttime periods, and the levoglucosan-to-POC ratio (LEVO/POC) surpassed 35 × 10− 3 µg/µg at night, providing clear evidence of intensified biomass burning contributions. Annual flammable waste wood generation increased substantially in 2022, further supporting the conclusion that nighttime waste wood combustion contributed to higher organic aerosol levels and persistent OP values. These findings emphasize that reducing PM2.5 mass alone is inadequate; targeted interventions to control nighttime biomass burning are essential for lowering health-relevant aerosol toxicity in urban Seoul.