Combined influences of sources and atmospheric bleaching on light absorption of water-soluble brown carbon aerosols

Abstract

Light-absorbing Brown Carbon (BrC) aerosols partially offset the overall climate-cooling of aerosols. However, the evolution of BrC light-absorption during atmospheric transport is poorly constrained. Here, we utilize optical properties, ageing-diagnostic δ¹³C-BrC and transport time to deduce that the mass absorption cross-section (MAC_WS-BrC) is decreasing by ~50% during long-range oversea transport, resulting in a first-order bleaching rate of 0.24 day^‒1 during the 3-day transit from continental East Asia to a south-east Yellow Sea receptor. A modern ¹⁴C signal points to a strong inverse correlation between BrC light-absorption and age of the source material. Combining this with results for South Asia reveals a striking agreement between these two major-emission regions of rapid photobleaching of BrC with a higher intrinsic absorptivity for BrC stemming from biomass burning. The consistency of bleaching parameters constrained independently for the outflows of both East and South Asia indicates that the weakening of BrC light absorption, thus primarily related to photochemical processes rather than sources, is likely a ubiquitous phenomenon.