Chemistry of PM2.5 in haze events in two East Asian cities during winter-spring 2019

East Asian experiences frequent severe haze episodes (EPs) during winter-spring, and their causes are not fully understood. Two EPs (daily average PM2.5 > 35 mu g m(-3)) occurred at Tianjin (TJ), China, and Daejeon (DJ), Korea, during late February (EP-1) and early March (EP-2) 2019. The mean total PM2.5 concentrations at the TJ and DJ sites during these events (82.9 and 66.2 mu g m(-3), respectively) were approximately three times higher than those observed during non-haze events (27.3 and 23.0 mu g m(-3), respectively). MODIS satellite RGB images suggested that aerosols were transported regionally from east Asian countries to the DJ site during EPs. After NO3-, the major contributors to PM2.5 during the EPs at both sites were NH4+, SO42- , and organic carbon (OC), indicating that secondary aerosols were important in severe haze formation. During the EPs, the distributions of secondary chemical species were similar at the two sites, with coefficient of divergence (COD) values of <0.2, whereas primary chemical species had dissimilar distributions with COD values of >0.2. This suggests that secondary aerosols may be impacted by regional influences, whereas primary aerosols are subject to more local influence. The average COD values in the first two days (27-28 February) relative to the remaining days (1-7 March) were <0.2 at the DJ site during EP-2, indicating no additional local production of secondary and primary species. Sulfur oxidation ratios (SOR) were lower at the TJ site than at the DJ site, while nitrogen oxidation ratios (NOR) were higher. Gradual increases observed in SOR and NOR at the TJ site early in the EPs imply significant secondary formation of SO42- and NO3-. Strong correlations between aerosol liquid-water content (ALWC) and SOR and NOR at the TJ site indicate that aqueous-phase reactions were important in the formation of SO42- and NO3-. Although high concentrations of SO42- and NO3- were observed at the DJ site, correlations between ALWC and SOR/NOR were weak with low regression gradients, implying that SO42- and NO3- may have been associated with regional transport. Low COD values, and SOR and NOR values similar to those at the beginning of EP-2, imply that additional formation of SO42- or NO3- at the DJ site is not active, with stagnant atmospheric conditions prevailing.