Brown carbon (BrC) contributes significantly to aerosol light absorption, thus can affect the earth’s radiation balance and atmospheric photochemical processes. In this study, we examined the light absorption properties and molecular compositions of water-soluble (WS-BrC) and water-insoluble (WI-BrC) BrC in PM_2.5 collected from a rural site in the Guanzhong Basin – a highly polluted region in Northwest China. Both WS-BrC and WI-BrC showed elevated light absorption coefficients (Abs) in winter (4-7 times of those in summer) mainly attributed to enhanced emissions from residential biomass burning (BB) for house heating. While the average mass absorption coefficients at 365 nm (MAC₃₆₅) of WS-BrC were similar between daytime and nighttime in summer (0.99±0.17 and 1.01±0.18 m² g^-1, respectively), the average MAC₃₆₅ of WI-BrC was more than a factor of 2 higher during daytime (2.45±1.14 m² g^-1) than at night (1.18±0.36 m² g^-1). This difference was partly attributed to enhanced photochemical formation of WI-BrC species, such as oxygenated polycyclic aromatic hydrocarbons (OPAHs). In contrast, the MACs of WS-BrC and WI-BrC were generally similar in winter and both showed little diel differences. The Abs of wintertime WS-BrC correlated strongly with relative humidity, sulfate, and NO₂, suggesting that aqueous-phase reaction is an important pathway for secondary BrC formation during the winter season in Northwest China. Nitrophenols on average contributed 2.44±1.78% of the Abs of WS-BrC in winter, but only 0.12±0.03% in summer due to faster photodegradation reactions. WS-BrC and WI-BrC were estimated to account for 0.83±0.23% and 0.53±0.33%, respectively, of the total down-welling solar radiation in the UV range in summer, and 1.67±0.72% and 2.07±1.24%, respectively, in winter. The total absorption by BrC in the UV region was about 55-79% relative to the elemental carbon (EC) absorption.

PM2.5,Brown carbon,aerosol optical properties,Molecular Composition