Enhanced PM2.5 episodes in a small residential city of South Korea: Effects of biomass burning and secondary formations

In this study, the characteristics and possible causes of high-concentration PM2.5 episodes occurring in a small residential city with low anthropogenic emissions were investigated. The largest contributor to PM2.5 mass over the entire study period was organic carbon (OC). The OC apportioned by biomass burning (OCbb) was estimated using levoglucosan, and it was found to be highest in winter when the forest fire frequently occurred. In spring and fall, the burning of Asian crop residue was likely to be an important contributor. The contribution of secondary organic carbon (OCsec) was found to be consistently high throughout the year, and the high correlation between OCbb and OCsec suggests that aging of biomass burning emissions may promote the formation of OCsec in this study area. Regarding the high PM2.5 concentration episodes, the increments in secondary inorganic constituents and OCsec surpassed those of primary components and the decreased O3 and increased RH were observed, indicating that secondary aqueous-phase formations have a significant effect on enhancing PM2.5 in this region.