Primary Emissions and Secondary Aerosol Processing During Wintertime in Rural Area of North China Plain

The vast rural areas often experience more severe haze pollution than megacities during wintertime in North China Plain (NCP), yet the sources and evolution processes of aerosol particles, particularly organic aerosol (OA) remain poorly understood. Here we conducted real-time measurements of submicron aerosol (PM1) species using a high-resolution aerosol mass spectrometer at a representative rural site in NCP in winter 2019. Our results showed the dominance of organics (36%) and nitrate in PM1 (22%) in 2019. Positive matrix factorization analysis illustrated similarly important primary sources from traffic emissions, coal combustion, and biomass burning, yet secondary OA (SOA) from photochemical and aqueous-phase related processing exceeded primary OA (53% vs. 47%). Substantial decreases in primary species and considerable increases in nitrate and sulphate were observed since winter 2018, demonstrating enhanced secondary formation in winter 2019. OA composition changed significantly from clean period to fog events with the contribution of aqueous phase-related oxygenated OA increasing from 6% to 44%, while the photochemical SOA decreased correspondingly from 51% to 19%. The size distributions of aerosol species also changed by shifting toward large sizes during fog events. Elemental analysis of OA and the Van Krevelen diagram (H/C vs. O/C) illustrated the different roles of photochemical and aqueous-phase processing during daytime and nighttime, respectively, and aqueous-phase processing is subject to the formation of organic compounds with high H/C and O/C ratios. The large differences in fog processing of submicron aerosol species between 2018 and 2019 due to different temperatures were also elucidated.

Automated summary

Rural areas in the winter in North China Plain experience severe haze pollution due to aerosol particles, particularly organic aerosol (OA), but the sources and processes of these particles are not well understood. Real-time measurements in a representative rural site in 2019 showed that primary sources of OA include traffic emissions, coal combustion, and biomass burning, while secondary OA from photochemical and aqueous-phase processing was more dominant. The study also observed enhanced secondary formation in winter 2019 compared to winter 2018, with changes in aerosol composition and size distributions during fog events.