Exacerbation of PM2.5 concentration due to unpredictable weak Asian dust storm: A case study of an extraordinarily long-lasting spring haze episode in Seoul, Korea

Severe haze episodes have occurred frequently throughout megacities in East Asia, especially during winter and early spring. Although notable progress has been attained in understanding the causes of severe haze formation, the exacerbation of PM2.5 concentration due to Asian dust has not yet been fully investigated. Here, we investigated the contribution of weak Asian dust particles to PM2.5 concentration during early spring for an extraordinarily long-lasting high PM2.5 episode in Seoul, Korea. PM2.5 concentrations exceeding national criteria (35 μg m−3) were continuously observed from February 27 to March 7, 2019, and can be attributable to the transboundary transported aerosols for the Asian continent under slow eastward-migrating anticyclones. The high-frequency of PM10-2.5 and PM2.5 mass concentrations in the high concentration regime (PM10-2.5 > 30 μg m−3; PM2.5 > 80 μg m−3) suggests that Asian dust particles transported concurrently with anthropogenic pollution aerosols contributed to elevated PM2.5 concentration during this episode. This was also evident from an elevated depolarization ratio (δ532: 0.06–0.13) and aerosol volume concentrations both in coarse and fine modes, which indicates the presence of non-spherical dust particles, from collocated aerosol lidar and optical particle counter measurements. Unlike severe Asian dust episodes (PM10 > 800 μg m−3) in Korea, relatively low Asian dust particle concentrations during high PM2.5 haze episodes have not been accurately predicted and alerted by most air quality forecasting models. This study emphasizes the need for the implementation of a series of dust emission and transport processes for better predictions of PM2.5 concentration during winter-spring high PM2.5 episodes.