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

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 (Abs(365)) 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 Abs(365) 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 Abs(365) in general positively correlated with the average oxidation states (OS,) of BrC, suggesting more BrC were produced at higher OS,. The mass absorption efficiency at 365 nm (MAE(365)) showed no clear dependences on OS, during spring, summer and fall, but decreased against OS, during winter, indicating chemical aging may lead to photo bleaching of BrC in winter. Moreover, positive responses of Abs(365) and MAE(365) 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.