Seasonal Variation of Black Carbon and Aerosol Optical Depth over Varanasi during 2020-21.

Black Carbon (BC) is one the most effective light-absorbing aerosols and the second strongest climate forcing agent. It plays important role in earth's radiation budget and have significant impact on uncertainty in radiative forcing. In this present study, the seasonal variation of total BC (contribution of biomass burning and fossil fuels) and AOD were analyzed over Varanasi from December 2020 to November 2021. Ground-based observations of BC and AOD have been done through Aethalometer AE-33 and MICROTOPS-II Sunphotometer respectively. The concentration of BC was found to be highest in the winter followed by summer, post-monsoon, and the lowest in monsoon seasons. The higher value of BC concentration in winter season may be due to calm meteorological conditions and lower atmospheric boundary layer (ABL) height. The contribution of biomass burning to BC was observed higher during winter and a higher concentration of fossil fuels was measured during the summer season. Along with the BC concentration, the seasonal mean value of AOD was found to be high during the post-monsoon followed by winter, summer, and monsoon seasons. The high AOD500 (aerosol optical depth at 500 nm) was observed in June and the low AOD500 was found in September. The high AOD values at shorter wavelengths indicate that particle loading occurs primarily in fine mode aerosols. The highest mean AOD870 observed during the summer which shows the presence of coarse mode particles.