Abstract:
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- ³ (mean value 27.1 ± 8.3 μg m- ³).
Interestingly, aerosol BC had a mean concentration of 1.36 ± 0.99 μg m- ³ 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-² at the surface, +0.7 W m 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 ~88 W m-² , which is attributed to the high BC mass fraction.
