Characterization of aerosol optical properties over the high-altitude station Hanle, in the trans-Himalayan region

Optical properties of aerosols over Hanle (4500 m amsl) in the western Himalayas were studied using skyradiometer observations during October 2007 to December 2010. Yearly mean value of aerosol optical depth (AOD) at 500 nm is 0.042 ± 0.002, which demonstrates the pristine environment of the station. Seasonal mean AODs at 500 nm during summer, autumn, winter, and spring are 0.044 ± 0.002, 0.031 ± 0.001, 0.031 ± 0.001, and 0.061 ± 0.002, respectively. The relatively high AOD during spring, associated with an elevated aerosol layer observed from space, supports the hypothesis of middle-upper tropospheric heating during pre-monsoon period. Seasonal mean values of Angstrom exponent (α) estimated from linear regression method varied from minimum 0.65 (spring) to maximum 1.02 (autumn). Dominance of coarse mode aerosols at the site is thus evident during spring. Analysis of AOD profiles obtained from satellite data and airmass back trajectories superimposed with fire-counts data indicated the presence of desert-dust at the altitudes of 5 to 7 km amsl during the episodes of high AOD and low α. These trajectories indicated airmasses mostly coming from different desert regions, e.g in north-west Asia and Iran in the Middle east. Further, arrival of airmasses from the densely populated and industrialized Punjab and Haryana regions from the north-west of India apparently explains the relative contribution of transported anthropogenic aerosols over the station.