High Contribution of Secondary Brown Carbon to Aerosol Light Absorption in the Southeastern Margin of Tibetan Plateau

The optical properties of atmospheric secondary brown carbon (BrC) aerosol are poorly understood because of its chemical complexity, and this has hampered quantitative assessments of the impacts of this light-absorbing material on glaciers on the Tibetan Plateau. For this study, a statistical approach was developed to investigate BrC light absorption over the southeastern margin of the Tibetan Plateau. Secondary sources for BrC were more important for absorption than primary ones. A diurnal cycle in secondary BrC absorption was explained by the formation of light-absorbing chromophores by photochemical oxidation after sunrise followed by photobleaching of the chromophores under the more oxidizing conditions as the day progressed. Multimethod analyses showed that biomass burning in northern Burma and along the Sino-Burmese border was the most important source for the secondary BrC. The mean integrated simple forcing efficiency was 79 W/g, indicating that secondary BrC can cause substantial radiative effects.