Tower-based profiles of wintertime secondary organic aerosols in the urban boundary layer over Guangzhou.

Secondary organic aerosol (SOA) accounts for a large fraction of fine particulate matter (PM _2.5 ), but the lack of vertical observations of SOA in the urban boundary layer (UBL) limits a comprehensive understanding of its sources and formation mechanisms. In this study, PM _2.5 samples were simultaneously collected at 3 m, 118 m, and 488 m on the Canton Tower in Guangzhou during winter. Typical SOA tracers, including oxidation products of isoprene (SOA _I ), monoterpene (SOA _M ), sesquiterpene (SOA _S ), and toluene (ASOA), were investigated alongside meteorological parameters and gaseous/particulate pollutants. Total concentrations of SOA tracers showed an increasing trend with height, with daytime levels exceeding nighttime levels. C ₅ -alkene triols and 2-methylglyceric acid displayed a significant increase with height, potentially affected by nighttime chemistry in the residual layer, determining the overall vertical trend of SOA _I tracers. Concentrations of later-generation SOA _M (SOA _M_S ) tracers also increased with height, while those of first-generation SOA _M (SOA _M_F ) tracers decreased, indicating relatively aged SOA _M in the upper layers. SOA _S and ASOA tracers exhibited higher enhancement under polluted conditions, likely impacted by biomass burning and anthropogenic emissions. The yields of SOA _I tracers varied with temperature in the vertical profile. The formation of SOA _M_F tracers was negatively correlated with relative humidity, liquid water content, and pH, affecting their vertical distributions. The effect of O ₃ on SOA formation enhanced significantly with height, influenced by air mass transport, and likely contributed to the higher yields of SOA in the upper layer. However, at ground level, SOA formation was primarily driven by high local emissions of both NO _x and volatile organic compounds. We also observed the roles of SO ₂ in SOA generation, particularly at 118 m. This study demonstrates the vertical diurnal characteristics of SOA tracers in the UBL, highlighting the varying effects of meteorological conditions and anthropogenic pollutants on SOA formation at different heights.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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