Seasonal light absorption properties of water-soluble brown carbon in atmospheric fine particles in Nanjing, China

Publication date: August 2018
Source:Atmospheric Environment, Volume 187
Author(s): Yanfang Chen, Xinlei Ge, Hui Chen, Xinchun Xie, Yuntao Chen, Junfeng Wang, Zhaolian Ye, Mengying Bao, Yanlin Zhang, Mindong Chen
Recently atmospheric brown carbon (BrC) is recognized as an important contributor to light absorption and positive climate forcing. In this work, daily fine particulate matter (PM2.5) samples were collected over a full year (May 2015–May 2016) in Nanjing, and seasonal light absorption properties of water-soluble BrC were investigated. We found that winter samples had the strongest light absorption among four seasons. The light absorption at 365 nm (Abs365) for all seasons linked closely with secondary organic carbon (SOC), indicating a dominant contribution from secondary sources to BrC. However primary biomass burning might also contribute to BrC as revealed by the good correlations of Abs365 versus levoglucosan fragments and/or K⁺, and such influence was more evident during summer. Furthermore, an Aerodyne soot-particle aerosol mass spectrometer (SP-AMS) was employed to determine the elemental ratios of BrC. We found that except in winter, the Abs365 in general positively correlated with the average oxidation states (OSc) of BrC, suggesting more BrC were produced at higher OSc. The mass absorption efficiency at 365 nm (MAE365) showed no clear dependences on OSc during spring, summer and fall, but decreased against OSc during winter, indicating chemical aging may lead to photo-bleaching of BrC in winter. Moreover, positive responses of Abs365 and MAE365 to N/C ratios were found during all seasons, indicating nitrogen-containing organics can be important BrC chromophores. Potential source areas of BrC were further discussed to improve our understanding of BrC sources in this region.

Graphical abstract

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