Water soluble organic carbon (WSOC) makes up a large fraction of organic carbon, which attracted great attention due to its light absorption properties and human health effects. Sources and light absorption properties of WSOC in 10 cities across China were studied by dual carbon isotope analysis and UV−visible spectrophotometer, respectively. Despite the dominate contribution of non-fossil sources, the fossil sources contribution of WSOC in China was higher than other regions across the world. The average MAE365 and fossil sources contribution of WSOC was 1.13 ± 0.37 m2/gC and 39.9 ± 9.4%, both of which were higher in Northern China. The non-fossil sources contribution of WSOC and MAE365, WSOC exhibited significant seasonal variations with highest values during cold seasons, which was likely associated with corn residues burning. Compared to warm seasons, the MAE365, WSOC showed a positive relationship with relative contribution of fossil sources and with higher values during cold seasons, indicating the fossil derived WSOC had higher light absorption capacity and enhance the overall color of WSOC during cold seasons. To constraining the regional climate and health impact of WSOC, this study suggests that mitigation strategy should consider the spatiotemporal variations in the sources, formation pathways and light absorption properties of WSOC.

The isotope tracer technique plays a key role in identifying the sources and atmospheric processes affecting pollution. The sources of brown carbon (BrC) at Guangzhou during 2017-2018 was characterized by positive matrix factorization with carbon isotope constraints and multiple linear regression analysis. The primary emission factors of fossil fuel combustion (FF) and biomass burning (BB) accounted for 34% and 27% of dissolved BrC absorption at λ = 365 nm, respectively. The total mean light absorption contributed by secondary sources was 39%. The absorption of FF-origin BrC was relatively stable and dominant in the summer monsoon period, whereas the absorption of BrC from BB and secondary nitrate formation increased and contributed larger fractions during the winter monsoon period. Transported BrC was estimated using an index of 7 Be/(7 Be+n 210 Pb). Higher values were generally accompanied by lower BrC absorption, whereas lower values were associated with higher BrC absorption, indicating that BrC absorption of aerosols transported from the upper-atmosphere is lower than that of aerosols transported near the surface. Based on the positive correlations between 210 Pb and BrC absorption and non-fossil dissolved organic carbon in the winter monsoon period, we estimated that the contribution of invasive BrC (include ground and upper-atmosphere level) to total absorption during the period of elevated BrC was approximately 50%, which was likely related to BB organic aerosols and secondary nitrate formation processes. This study supports radionuclides as a novel method for characterizing the sources and transport of BrC that can be applied in future atmospheric research.