  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 $\pm$
0.033 (typical to this site), total suspended particulate (TSP) concentration
was in the range 15 - 40 micro g m^(-3) (mean value 27.1 $\pm$ 8.3 micro g
m^(-3)). Interestingly, aerosol BC had a mean concentration of 1.36 $\pm$ 0.99
micro g m^(-3), contributed to ~5.0 $\pm$ 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 $\pm$ 0.03), indicating significant aerosol absorption. The estimated
aerosol radiative forcing was as low as 4.2 W m^(-2) at the surface, +0.7 W
m^(-2) at the top of the atmosphere, implying an atmospheric forcing of +4.9 W
m^(-2). 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 $\sim$88 W m^(-2),
which is attributed to the high BC mass fraction.
