Source apportionment of PM2.5 and sulfate formation during the COVID-19 lockdown in a coastal city of southeast China

Abstract

Revealing the changes in chemical compositions and sources of PM2.5 is important for understanding aerosol chemistry and emission control strategies. High time-resolved characterization of water-soluble inorganic ions, elements, organic carbon (OC), and elemental carbon (EC) in PM2.5 was conducted in a coastal city of southeast China during the COVID-19 pandemic. The results showed that the average concentration of PM2.5 during the city lockdown (CLD) decreased from 46.2 μg m−3 to 24.4 μg m−3, lower than the same period in 2019 (PM2.5: 37.1 μg m−3). Concentrations of other air pollutants, such as SO2, NO2, PM10, OC, EC, and BC, were also decreased by 27.3%–67.8% during the CLD, whereas O3 increased by 28.1%. Although SO2 decreased from 4.94 μg m−3to 1.59 μg m−3 during the CLD, the concentration of SO42− (6.63 μg m−3) was comparable to that (5.47 μg m−3) during the non-lockdown period, which were attributed to the increase (16.0%) of sulfate oxidation rate (SOR). Ox (O3+NO2) was positively correlated with SO42−, suggesting the impacts of photochemical oxidation. A good correlation (R2 = 0.557) of SO42− and Fe and Mn was found, indicating the transition-metal ion catalyzed oxidation. Based on positive matrix factorization (PMF) analysis, the contribution of secondary formation to PM2.5 increased during the epidemic period, consisting with the increase of secondary organic carbon (SOC), while other primary sources including traffic, dust, and industry significantly decreased by 9%, 8.5%, and 8%, respectively. This study highlighted the comprehensive and nonlinear response of chemical compositions and formation mechanisms of PM2.5 to anthropogenic emissions control under relatively clean conditions.