Headline: Estimating contributions of black and brown carbon to solar absorption from aethalometer and AERONET measurements in the highly polluted Kathmandu Valley, Nepal

We estimated the individual contributions of black carbon (BC) and brown carbon (BrC) to the aerosol absorption coefficient (σap) and absorption aerosol optical depth (AAOD) in the highly polluted Kathmandu Valley, Nepal, by applying the absorption Ångström exponent (AÅE) method to multi-wavelength aethalometer and AERONET sun/sky radiometer measurements. The elevated σap levels observed during the winter and pre-monsoon periods were primarily due to increased usage of diesel generators and low-grade fuel/coal. The AAODBC and AAODBrC values were substantially higher during the pre-monsoon period, almost two-fold higher than winter levels, due to seasonally increased biomass-burning activities from agricultural residue burning and forest fires. The overall contribution of BC to σap was approximately 80%–95%, whereas BrC accounted for 5%–20% at 520 nm. However, the BrC contribution to σap at 370 nm was substantially higher during the winter, ranging from 29.3% to 34.0%. The portioning of AERONET measurements indicates that BC contributed 80% (69%) while BrC contributed 20% (31%) to AAOD of carbonaceous aerosols at 520 nm (370 nm). Although the observation principles and techniques are completely independent, the BC and BrC absorption for simultaneous daytime data points shows a strong correlation between surface aethalometer-based and column AERONET-based estimates. The contributions of BC and BrC to absorption in Kathmandu Valley are similar to those observed under open biomass and garbage-burning conditions; however, the BrC absorption at both 370 and 520 nm is approximately 2- to 3-fold higher than those observed for urban areas in East Asia.