Both emissions and ageing altered brown carbon aerosols in the East Asian outflow

Brown carbon aerosols (BrC) significantly contribute to regional climate warming in East Asia. However, their sources and atmospheric transformation remain poorly constrained due to limited observations. In this study, we clarified the seasonal dynamics of BrC and quantified the sources of relating carbonaceous components, at the gateway of the East Asian air outflow for seasonal variations. Our findings reveal that fossil fuel combustion dominates the sources of BrC containing carbonaceous components in winter, while biomass burning and local biogenic sources become more prominent in spring and summer, respectively. We provide benchmark optical properties of BrC for climate model simulations, demonstrating that the absorption coefficient and mass absorption cross-section of water-soluble fraction from land-originated air masses (0.47 Mm^-1 and 0.53 m² gC^-1, respectively) are more than twice those of sea-originated air masses (0.11 Mm^-1 and 0.21 m² gC^-1, respectively). Additionally, we show that BrC undergoes photochemical degradation during transport with a half-life of approximately 1.2 days. A significant reduction in BrC levels during the COVID-19 lockdown period highlights the potential of stringent emission controls to mitigate air pollution and its associated climate impacts. By shedding light on the seasonal dynamics, diverse sources, and atmospheric ageing of BrC, the study provides valuable insights for emission reduction strategies and improving BrC representation in climate models.