Chemical characterization, formation mechanisms and source apportionment of PM2.5 in north Zhejiang Province: The importance of secondary formation and vehicle emission

Abstract

PM2.5 affects air quality, therefore, chemical evolution, formation mechanism and source identification of PM2.5 are essential to help figure out mitigation measures. PM2.5 and its constituents were comprehensively characterized with highly time-resolved measurements from 2019 to 2020 in north Zhejiang Province (Shanxi, SX) for the first time, with an emphasis on the contribution of secondary formation and vehicle emission to PM2.5. Secondary inorganic ions (sulfate: 3.86 μg/m3, nitrate: 7.82 μg/m3 and ammonium: 4.59 μg/m3, SNA) were found to be the major components (54%) in PM2.5 (29.70 μg/m3). The highly consistence of nitrate, sulfate and secondary organic compounds (SOC) with Ox (NO2 + O3) or RH indicated the importance of photochemical oxidation and heterogeneous reaction in different scenarios. Higher atmospheric oxidative potential facilitated the SOC formation in spring. The PM2.5 mass was apportioned to eight sources resolved by positive matrix factorization (PMF): secondary nitrate (9.63 μg/m3), secondary sulfate (5.14 μg/m3), vehicle emission (7.26 μg/m3), coal combustion (2.39 μg/m3), biomass burning (1.38 μg/m3), soil dust (0.86 μg/m3), industry emission (0.50 μg/m3), and ship emission (0.32 μg/m3). Secondary nitrate (35%) and sulfate (19%) formation and vehicle emission (26%) were the main factors contributing to the PM2.5. Furthermore, the contribution of secondary nitrate formation increased with elevating PM2.5 concentration. Regional transport was synthetically studied by chemical and backward trajectory analysis, reflecting that secondary nitrate contributed severely to the air quality at SX, while vehicle emission contribution enhanced when atmosphere was stagnant. This study first provides long-term comprehensive chemical characterization and source apportionments of PM2.5 pollution in north Zhejiang, which may provide some guidance for the air pollution control.