Diurnal Variation of SOA Formation Potential from Ambient Air at an Urban Site in Beijing

Abstract

Secondary organic aerosol (SOA) is an important component of atmospheric fine particles (PM2.5). The study of the diurnal variation of SOA formation potential is important for understanding the evolution of SOA and its contribution to fine particle pollution. The oxidation flow reactor (OFR) was used to study the SOA formation potential of ambient air in summer at an urban site in Beijing. The high concentration of OH radicals in the reactor can oxidize the volatile organic compounds (VOCs) and lead to SOA formation. The hour average SOA formation potential varied between 3.9-9.4 μg·m-3 in a day and had a higher value at night than in the daytime. The lowest value of SOA formation potential was about 3.9 μg·m-3 observed at 16:00 in the afternoon. This variation of SOA formation potential is consistent with the typical VOCs, such as toluene, and inversely related to the concentration of ozone. In addition to the impact of change in the height of the boundary layer, experimental data showed that the reduction of VOCs in photo-oxidation in the daytime was an important reason for the decrease of SOA formation potential in daytime. Compared to similar studies in developed countries, the SOA formation potential was higher in Beijing due to the higher concentrations of VOCs and might make an important contribution to the fine particle pollution in Beijing.