Attribution of black and brown carbon aerosols to solar light absorption at surface and column atmosphere in Kathmandu, Nepal

Kathmandu is a region with high black carbon (BC) and brown carbon (BrC) emissions and aerosol loadings, making a considerable contribution to light absorption on both global and regional scales (Cho et al., 2017). Various emission sources of BC and BrC aerosols (e.g., uncontrolled motor vehicles, brick kilns, fugitive soil dust, and biomass and garbage burning) and changing in light-absorbing properties of BC and BrC during aging process in the ambient could affect the relative contribution of BC and BrC to aerosol light absorption, but remains sparsely investigated and poorly understood, which results in substantial uncertainties in their climatic impact (Dasari et al., 2019). In this study, we estimated the contribution of BC and BrC to aerosol light absorption from surface in-situ and aerosol robotic network (AERONET) columnar observations at Kathmandu.

Absorption coefficient of BC was segregated from total absorption coefficient measured by aethalometer by assuming AAE of BC is 1.0 (Lack et al., 2013). Absorption coefficient of BrC was then estimated by subtracting the absorption coefficient of BC from total absorption coefficient at all wavelengths (370, 470, 520, 590, 660, 880 nm). BC substantially contributed 92.0-93.0 % to light absorption at 520-660 nm and the remaining was by BrC. However, the contribution of BrC significantly increased to 12.3-19.3 % at near ultraviolet wavelengths (370-470 nm), with a value of absorption angstrom exponent of 2.8. This result shows that even if BC overwhelms light absorption at visible wavelengths, BrC should also be regarded as the major absorber affecting atmospheric warming over Kathmandu. For investigating their impact on climate more relevant, the contribution of BC and BrC to absorption aerosol optical depth in the atmospheric column will be also presented.