Abstract:
First time observations of spectral aerosol optical depths (AODs) at Mohal (31.9◦ N, 77.11◦ E;
altitude 1154 m amsl) in the Kullu valley, located in the northwestern Indian Himalayan region, have
been carried out during Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB),
as a part of the Indian Space Research Organisation–Geosphere Biosphere Program (ISRO–GBP).
AODs at six wavelengths are obtained using Microtops-II Sunphotometer and Ozonometer. The
monthly mean values of AOD at 500 nm are found to be 0.27 ± 0.04 and 0.24 ± 0.02 during March
and April, 2006 respectively. However, their monthly mean values are 0.33 ± 0.04 at 380 nm and
0.20 ± 0.03 nm at 870 nm during March 2006 and 0.31 ± 0.3 at 380 nm and 0.17 ± 0.2 at 870 nm
during April 2006, showing a gradual decrease in AOD with wavelength. The Angstrom wavelength
exponent ‘α’ had a mean value of 0.72 ± 0.05, implying reduced dominance of fine particles. Further,
the afternoon AOD values are higher as compared to forenoon values by ∼ 33.0% during March
and by ∼ 9.0% during April 2006 and are attributed to the pollutant lifted up from the valley by
the evolving boundary layer. Besides the long-range transportation of aerosol particles by airmass
from the Great Sahara and the Thar Desert regions to the observing site, the high values of AODs
have also been influenced by biomass burning and frequent incidents of forest fire at local levels.
