资源环境科技发展态势分析平台

Brown carbon (BrC) contributes significantly to aerosol light absorption and 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 PM2.5 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 those in summer) mainly attributed to enhanced emissions from residential biomass burning (BB) for heating of homes. While the average mass absorption coefficients (MACs) at 365 nm (MAC(365)) of WS-BrC were similar between daytime and nighttime in summer (0.99 +/- 0.17 and 1.01 +/- 0.18 m(2) g(-1), respectively), the average MAC(365) of WI-BrC was more than a factor of 2 higher during daytime (2.45 +/- 1.14 m(2) g(-1)) than at night (1.18 +/- 0.36 m(2) 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 few 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 ultraviolet (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.