In-situ secondary organic aerosol formation from ambient air in suburban Eastern China: Substantially distinct characteristics between summer and winter

Abstract

Elucidating the sources and formation mechanisms of secondary organic aerosol (SOA) is essential to access its impact on air quality and climate, yet the seasonal variations of SOA formation from ambient air remain poorly constrained. In this study, in-situ SOA formation from ambient air at a suburban site in Nanjing in the Yangtze River Delta of eastern China during summer and winter was investigated using an oxidation flow reactor (OFR), with equivalent photochemical age in the OFR ranging from several hours to 3 weeks. Substantially distinct SOA mass enhancements and precursor characteristics were observed during these two seasons. In summer, the average OH exposure-dependent OA enhancements were ∼0.77–2.64 μg m⁻³, much higher than ∼0.24 μg m⁻³ in winter. The known SOA precursors contributed approximately 31 % and 18 % of the measured SOA during summer daytime and nighttime and 25 % and 89 % during winter daytime and nighttime, respectively. In summer, biogenic volatile organic compounds (BVOCs) contributed to approximately 56 % of the estimated SOA during the daytime while aromatics accounted for 76 % at night. By contrast, C₆-C₉ aromatics were the dominant SOA precursors during the daytime and nighttime in winter. The unexplained SOA may be attributed to the unmeasured semi-volatile and intermediate volatile organic compounds (S/IVOCs) and precursors emitted from volatile chemical products (VCPs). This study highlights the significant seasonal differences in SOA formation potential in suburban eastern China.