Comparative Analysis of Secondary Organic Aerosol Formation during PM2.5 Pollution and Complex Pollution of PM2.5 and O3 in Chengdu, China

Nowadays, many cities in China are suffering from both fine particulate matter (PM2.5, particulate matter with an aerodynamic diameter smaller than 2.5 µm) and ozone (O3) pollution. Secondary organic aerosol (SOA) is an important component of PM2.5 and is closely related to the oxidation processes. To investigate the characteristics and formation pathways of SOA during different types of haze pollution episodes, carbonaceous components of PM2.5 and volatile organic compounds (VOCs) were monitored continuously in Chengdu in April 2019, when Chengdu experienced not only PM2.5 pollution (SPP) but also a complex pollution of PM2.5 and O3 (CoP). In the CoP episode, the concentrations of SOA increased by 51.2% as compared to SPP, and the SOA concentrations were positively correlated with PM2.5 mass concentrations. These suggest that SOA drove the increase in PM2.5 levels during the haze event to some extent. The preliminary VOC source analysis based on the feature ratio showed that vehicle emission and fuel volatilization sources were the main sources of VOCs at this urban site. In addition, coal emissions and biomass burning were also important contributors. High-carbon alkanes and aromatic hydrocarbons significantly contributed to the SOA formation. These results provide a preliminary understanding of SOA formation during different types of pollution episodes in Chengdu, which can help us to further understand air pollution in this typical region.