Investigation of sources and formation mechanisms of fine particles and organic aerosols in cold season in Fenhe Plain, China

Abstract

Air quality has been improved significantly from 2017 to 2020 in Fenhe Plain, one of the most polluted regions in China. However, severe haze episodes from days to weeks were still frequently observed in cold seasons, and the sources and formation mechanisms especially organic aerosols are poorly understood due to the limited highly time-resolved measurements. Here we have a comprehensive characterization of chemical composition, variations and sources of fine particles in fall and winter in Fenhe Plain mainly using a PM2.5 Aerosol Chemical Speciation Monitor. Our results showed that organics and nitrate were two major species of PM2.5, on average accounting for 35.4% and 27.0%, respectively, and they played consistently important roles across different pollution levels. Positive matrix factorization revealed three primary sources from traffic, biomass burning and coal combustion emissions, and two secondary factors that are associated with photochemical and aqueous-phase processing. While secondary OA (SOA) dominated OA for the entire study (63%), large increases in primary OA from 19% to 41% were observed during heating period. Particularly, we found a high contribution of photochemical OOA (45%) during severely polluted periods in heating season demonstrating the presence of strong photochemical production in winter. Most aerosol species presented similarly pronounced diurnal cycles with low concentrations during daytime indicating the major influences from planetary boundary layer and primary emissions. The bivariate polar plots further illustrated the different sources of primary and secondary species, i.e., local emissions versus regional transport from the south and southeast. By analyzing four case studies, we found three major formation mechanisms of severe haze episodes that were driven by photochemical processing, aqueous-phase production, and local emissions, respectively. Therefore, different mitigating strategies are suggested in Fenhe Plain depending on the types of severe haze episodes.