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. Brown carbon (BrC) contributes significantly to aerosol light absorption andthus can affect the Earth's radiation balance and atmospheric photochemicalprocesses. In this study, we examined the light absorption properties andmolecular compositions of water-soluble (WS-BrC) and water-insoluble(WI-BrC) BrC in PM2.5 collected from a rural site in the GuanzhongBasin – a highly polluted region in northwest China. Both WS-BrC and WI-BrCshowed elevated light absorption coefficients (Abs) in winter (4–7 times those in summer) mainly attributed to enhanced emissions from residentialbiomass burning (BB) for heating of homes. While the average mass absorptioncoefficients (MACs) at 365 nm (MAC365) of WS-BrC were similar between daytimeand 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 afactor of 2 higher during daytime (2.45±1.14 m2 g−1)than at night (1.18±0.36 m2 g−1). This difference waspartly 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 bothshowed few diel differences. The Abs of wintertime WS-BrC correlatedstrongly with relative humidity, sulfate and NO2, suggesting thataqueous-phase reaction is an important pathway for secondary BrC formationduring the winter season in northwest China. Nitrophenols on averagecontributed 2.44±1.78 % of the Abs of WS-BrC in winter but only0.12±0.03 % in summer due to faster photodegradation reactions.WS-BrC and WI-BrC were estimated to account for 0.83±0.23 % and0.53±0.33 %, respectively, of the total down-welling solarradiation in the ultraviolet (UV) range in summer, and 1.67±0.72 % and2.07±1.24 %, respectively, in winter. The total absorption by BrCin the UV region was about 55 %–79 % relative to the elemental carbon (EC)absorption.

Optical properties and molecular compositions of water-soluble and water-insoluble brown carbon (BrC) aerosols in northwest China