Aerosol radiative forcing due to enhanced black carbon at an urban site in India

Abstract

During a comprehensive aerosol field campaign, simultaneous measurements were made of aerosol spectral optical depths, black carbon mass concentration (M_b), total (M_t) and size segregated aerosol mass concentrations over an urban continental location, Bangalore (13° N, 77°E, 960 m msl), in India. Large amounts of BC were observed; both in absolute terms and fraction of total mass (~11%) and sub-micron mass (~23%) implying a significantly low single scatter albedo. The aerosol visible optical depth (τ_p) was in the range 0.24 to 0.45. Estimated surface forcing is as high as -23 W m^-2 and top of the atmosphere (TOA) forcing is +5 W m^-2 during relatively cleaner periods (τ_p ~0.24). The net atmospheric absorption translates to an atmospheric heating of ~0.8 K day^-1 for cleaner periods and ~1.5 K day^-1 for less cleaner periods (τ_p ~0.45). Our observations raise several issues, which may have impacts to regional climate and monsoon.