Summertime and wintertime atmospheric processes of secondary aerosol in Beijing

Secondary aerosol constitutes a large fraction of fine particles in urban air of China. However, its formation mechanisms and atmospheric processes remain largely uncertain despite considerable studies in recent years. To elucidate the seasonal variations of fine particles composition and secondary aerosol formation, an Aerodyne quadrupole aerosol chemical speciation monitor (Q-ACSM) combined with other online instruments were used to characterize the submicron particulate matter (diameter < 1 μm, PM₁) in Beijing during summer and winter 2015. Our results suggest that the photochemical oxidation was the major pathway for sulfate formation during summer, whereas aqueous-phase reaction became an important process for sulfate formation during winter. High concentration of nitrate (17% of the PM₁ mass) was found during winter explained by enhanced gas-to-particle partitioning at low temperature, while high nitrate concentration (19%) was also observed under the conditions of high relative humidity (RH) during summer likely due to the hydrophilic property of NH₄NO₃ and hydrolysis of N₂O₅. As for SOA formation, photochemical oxidation perhaps played an important role for summertime oxygenated OA (OOA) formation and wintertime less oxidized OOA (LO-OOA) formation. The wintertime more oxidized OOA (MO-OOA) showed a good correlation with aerosol liquid water content (ALWC), indicating more important contribution of aqueous-phase processing than photochemical production to MO-OOA. Meanwhile, the dependence of LO-OOA and the mass ratio of LO-OOA to MO-OOA on atmospheric oxidative tracer (i.e., O_x) both degraded when RH were greater than 60%, suggesting that RH or aerosol liquid water may also affect the LO-OOA formation.