ABSTRACT
Although the air quality in China has been greatly improved in recent years, the air pollution remains severe. The annual mean PM2.5 concentrations have not met the second grade of the National Ambient Air Quality Standards in China and are still much higher than the guideline value of the World Health Organization. Thus, the PM2.5 concentration needs to be further reduced. Secondary organic aerosol (SOA) is an important component of PM2.5 and has an important impact on air quality, global climate change, and human health. Therefore, understanding the formation mechanism of SOA is an important basis to control SOA and further reduce PM2.5. As an important precursor of SOA, volatile organic compounds (VOCs) can be oxidized by oxidants such as ·OH, NO3[KG-*2/3]·, Cl·, and O3 to generate low volatile organic compounds and further to form SOA through gas-particle partitioning, homogeneous nucleation, aqueous phase reaction, and heterogeneous reaction processes. The formation of SOA can be affected by many factors, such as the types and initial concentrations of VOCs, VOCs/NOx ratios, relative humidity (RH), temperature (T), seed aerosols, oxidants, aqueous phase process, and photochemical process. The observed SOA concentration is always underestimated by air quality models because a comprehensive understanding of the complexity of SOA chemical composition and formation mechanisms is still lacking, especially that under the highly complex air pollution conditions in China. Therefore, the formation mechanism and influencing factors of SOA under highly complex air pollution conditions have become an important concern in the field of atmospheric sciences. Recently, much laboratory work has focused on the formation of SOA under complex conditions. The research progress of SOA formation from different anthropogenic VOCs are reviewed here, and the methods used and the impact of different influencing factors on SOA formation are introduced. Finally, the key scientific issues that exist in the research of the SOA mechanism at present are put forward, and the future research direction is projected.
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.
