[Characteristics of Light Absorption, Sources, and Seasonal Variation of Atmospheric Brown Carbon in Northern Nanjing].

Brown carbon (BrC) refers to a group of organic compounds in fine atmospheric particles (PM) that are able to absorb light in the ultraviolet and visible range. They have a significant impact on the visibility of air and on the earth's climate. In this study, we used a black carbon analyzer (Model AE33) to conduct field measurements in northern suburban Nanjing from March 2021 to February 2022. We measured the light absorption coefficients of BrC in PM and quantified the contributions of primary (BrC) and secondary brown carbon (BrC) in BrC by using the minimum correlation method (MRS), combined with the backward trajectories,potential source contribution function (PSCF) analysis, and diurnal patterns to analyze the seasonal characteristics of BrC. The results showed that the annual average light absorption of BrC was(7.76±7.17)Mm (at 370 nm), and its contribution to the total aerosol light absorption was (22.0±8.8)%. BrC light absorption at different wavelengths all showed a U-shape seasonal variation of high in spring and winter and low in summer and fall. MRS analysis showed that the annual average contributions of BrC and BrC were (62.9±21.4)% and (37.1±21.4)% (at 370 nm), respectively; however, the contribution of BrC increased with the increase in wavelength, and it became dominant in longer wavelengths such as 660 nm. Backward trajectory and PSCF analysis showed that BrC was heavily influenced by air masses from the sea in spring, summer, and fall but was influenced greatly by local and regional continental emissions in winter. Traffic emissions in spring, summer, and fall were more intense to contribute to BrC than that in winter, whereas coal and biomass combustion had a greater impact on BrC in winter. Detailed analysis revealed that gas-phase photochemistry and aqueous chemistry had different influences on BrC formation in different seasons. It was mainly from gas-phase photochemistry in summer but was dominated by aqueous process in winter; both processes, however, were important pathways to BrC in spring and fall.