Aerosol characteristics at a high-altitude location in central Himalayas: optical properties and radiative forcing

Collocated measurements of the mass concentrations of aerosol black carbon (BC) and composite aerosols near the surface were carried out along with spectral aerosol optical depths (AODs) from a high-altitude station, Manora Peak in central Himalayas, during a comprehensive aerosol field campaign in December 2004. Despite being a pristine location in the Shivalik Ranges of central Himalayas and having a monthly mean AOD (at 500 nm) of 0.059 ± 0.033 (typical to this site), total suspended particulate (TSP) concentration was in the range 15–40 μg m<SUP>−3</SUP> (mean value 27.1 ± 8.3 μg m<SUP>−3</SUP>). Interestingly, aerosol BC had a mean concentration of 1.36 ± 0.99 μg m<SUP>−3</SUP> and contributed ∼5.0 ± 1.3% to the composite aerosol mass. This large abundance of BC is found to have linkages to the human activities in the adjoining valley and to the boundary layer dynamics. Consequently, the inferred single scattering albedo lies in the range of 0.87 to 0.94 (mean value 0.90 ± 0.03), indicating significant aerosol absorption. The estimated aerosol radiative forcing was as low as −4.2 W m<SUP>−2</SUP> at the surface, +0.7 W m<SUP>−2</SUP> at the top of the atmosphere, implying an atmospheric forcing of +4.9 W m<SUP>−2</SUP>. Though absolute value of the atmospheric forcing is quite small, which arises primarily from the very low AOD (or the column abundance of aerosols), the forcing efficiency (forcing per unit optical depth) was ∼88 W m<SUP>−2</SUP>, which is attributed to the high BC mass fraction