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 (TP). For this study, a statistical approach was developed to investigate BrC light absorption over the southeastern margin of the TP.
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. Multi-method 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-1, indicating that secondary BrC can cause substantial radiative effects.