Black carbon aerosols over an urban region: Radiative forcing and climate impact

Abstract

Black carbon (BC) mass concentration in Ahmedabad, an urban location, varies from 2 μg m−3 during summer to 11 μg m−3 during winter and postmonsoon seasons. Aerosol optical depth (AOD) is higher (0.63) in summer when compared to winter (0.31). BC mass concentrations in Ahmedabad are governed by local sources and meteorology (boundary layer, winds, rainfall, and long‐range transport). Single‐scattering albedo (SSA) deduced using measured BC mass concentration as input in an aerosol optical properties model varies from ∼0.7 during winter and postmonsoon to 0.93 in monsoon over Ahmedabad. Surface (SFC) and atmosphere (ATM) aerosol radiative forcing (ARF) in premonsoon and monsoon are ∼50% lower than those obtained during winter and postmonsoon despite higher AODs. ATM forcing is more positive for lower SSA and AOD, while it is less positive for higher AOD and SSA. It is shown that when the amounts of BC and water vapor are high over continental regions, the net (shortwave + longwave) ATM warming will be higher. BC aerosols alone contribute on average 60% and 25% of shortwave and longwave ATM forcing. Seasonal mean heating rates are higher than 1.5 K/d in winter and postmonsoon. Heating rates including BC aerosols are at least a factor of 3 higher than when BC aerosols are absent, thus highlighting the crucial role BC aerosols play in modifying the radiation budget and climate. Thus, it is possible that BC aerosols because of their radiative and climate impacts could be contributing to the decreasing trend in rainfall over India.