The formation and transport of HNO3 over the Yellow Sea and its impact on the January 2018 PM2.5 episode in Seoul†

Abstract

The formation of HNO₃ gas over the Yellow Sea and its impact on particulate nitrate concentrations in Seoul during the PM_2.5 episode in Seoul in January 2018 were studied by analyzing monitoring data and running WRF-CMAQ model simulations. Monitoring data showed that particulate nitrate made up 41% of the PM_2.5 during the episode, 2.7 times more than in other periods, and its level was strongly correlated with PM_2.5 mass, indicating that the particulate nitrate is the primary cause of the episode. During the episode, eastern China was in slight to moderate ammonia-rich conditions and Seoul had slight ammonia-rich conditions. In contrast, severe ammonia-lean conditions prevailed in the Yellow Sea. Despite the decline of solar radiation at night, the production rate of HNO₃ in the Yellow Sea still increased slightly due to the higher nocturnal ozone levels compared to the daytime. On the other hand, it decreased in eastern China and western Korea during the night as O₃ dissipated by NO titration, resulting in the conversion rates of NO₂ to HNO₃ in the Yellow Sea being several times higher than those in eastern China and western Korea. The HNO₃ gas formed in the Yellow Sea was transported to Seoul under westerly winds, contributing to particulate nitrate formation. Furthermore, the rate of formation of HNO₃ gas in the Yellow Sea was 65% higher than that in Seoul during the night, which indicated that the regional influence dominated the local influence in determining the particulate nitrate concentrations in Seoul.