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https://doi.org/10.5194/acp-2019-1002
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1002
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 05 Dec 2019

Submitted as: research article | 05 Dec 2019

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This preprint is currently under review for the journal ACP.

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

Jianjun Li1,2, Qi Zhang2, Gehui Wang1,3,4, Jin Li1, Can Wu1,3, Lang Liu1, Jiayuan Wang1,2, Wenqing Jiang2, Lijuan Li1,2, Kin Fai Ho1,5, and Junji Cao1 Jianjun Li et al.
  • 1Key Lab of Aerosol Chemistry & Physics, SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China
  • 2Department of Environmental Toxicology, University of California, Davis, CA 95616, USA
  • 3Key Laboratory of Geographic Information Science of the Ministry of Education, School of Geographic Sciences, East China Normal University, Shanghai 200241, China
  • 4Institute of Eco-Chongming, 3663 N. Zhongshan Rd., Shanghai 200062, China
  • 5The Jockey Club School of Public Health and Primary Care, The Chinese University of Hong Kong, Hong Kong, China

Abstract. 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) and water-insoluble (WI) 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 of those in summer) mainly attributed to enhanced emissions from residential biomass burning (BB) for 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 mainly 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 WS-BrC correlated strongly with relative humidity, sulfate, and NO2, suggesting that aqueous-phase reactions 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.

Jianjun Li et al.

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Short summary
We examined the light absorption properties and molecular composition of water-soluble (WS) and -insoluble (WI) BrC in PM2.5 collected from Northwest China. We found that photochemical formation contributes significantly to light absorption of WI-BrC in summer whereas aqueous-phase reactions play an important role in secondary WS-BrC formation in winter. BrC was estimated to account for 1.36 % and 3.74 %, respectively, of the total down-welling solar radiation in the UV range in summer and winter.
We examined the light absorption properties and molecular composition of water-soluble (WS) and...
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