The roles of aqueous-phase chemistry and photochemical oxidation in oxygenated organic aerosols formation

The formation mechanism and evolution process of secondary organic aerosol (SOA) remain poorly understood due to measurement uncertainties and chemical complexities. Here, we present the characterization of non-refractory fine particles (NR-PM2.5) analyzed by means of a time-of-flight ACSM (ToF-ACSM) at a rural site in the North China Plain during winter. Our results show that air quality in rural areas was heavily polluted by primary organic aerosols (POA), which accounted for 83% of total organic aerosol (OA). The oxygenated organic aerosols (OOA) were mainly generated from local emissions. As pieces of evidence, firstly, less-oxidized OOA (LO-OOA) had good correlations with primary species like biomass burning OA (BBOA) (R2 = 0.54), EC (R2 = 0.46), and chloride (R2 = 0.46), implying LO-OOA was formed with the POA emission processes; secondly, there was a significant increase in OOA concentration after sunrise, indicating a lot of local generation during the daytime; thirdly, the potential source region of OOA was restricted in that of POA, implying that OOAs were converted from where primary emission exists. The formation processes of LO-OOA and more-oxidized OOA (MO-OOA) are different. Aqueous-phase chemistry had a dominant effect on the formation of LO-OOA due to the better correlation between aerosol liquid water content (ALWC) with LO-OOA (R2 = 0.54) than MO-OOA (R2 = 0.15). In comparison, both aqueous -phase and photochemical processes acted on the MO-OOA formation, when odd oxygen (Ox = O3+NO2) was low (Ox 35 ppb), photochemical oxidation played an essential role in MO-OOA formation