Chemical composition of fine organic aerosols during a moderate pollution event in summertime in Beijing: Combined effect of primary emission and secondary formation

In order to clarify the influence of primary emission and secondary formation on the organic compositions of fine aerosols during the atmospheric pollution process in summer in Beijing, an intensive field measurement were carried out at an urban site in Beijing from July 1 to July 30, 2017. Inorganic ions (SO42-, NO3, and NH4+) and organic compounds (OM) were the most important components of PM2.5 throughout the whole sampling period, with inorganic ions having the highest concentrations during the polluted period and OM having the highest concentration during the clean period. The apportioned quantitatively primary and secondary sources accounted for an average 86 +/- 12% of the measured organic carbon (OC) at the urban site, based on positive matrix factorization modeling and the tracerbased method. Coal combustion was an important primary source for OC in the polluted episode, and biomass burning was the main contributor during the clean period. beta-Caryophyllene SOC (secondary OC) was the main secondary contributor for OC during summertime in Beijing. The significant amount of SOC enhanced with an increased ozone and acidification indicated that the atmospheric oxidants and the insitu acidity of particles were important factors in the formation of SOC during period of high pollution in Beijing.