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

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 delta C-13-BrC and transport time to deduce that the mass absorption cross-section (MACWS-BrC) is decreasing by similar to 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 C-14 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.

Automated summary

The study finds that the light absorption of brown carbon (BrC) aerosols can partially offset the overall climate-cooling effect of aerosols. However, the evolution of BrC light-absorption during atmospheric transport is poorly understood. Through optical properties, age-diagnostic delta C-13-BrC, and transport time analysis, it is deduced that the mass absorption cross-section of MACWS-BrC decreases by about 50% during long-range oversea transport. The results suggest that the weakening of BrC light absorption is likely a ubiquitous phenomenon primarily related to photochemical processes rather than sources.