Temporal asymmetry in aerosol optical characteristics:A case study at ahigh-altitude station, Hanle, in Ladakh region

Diurnal features of aerosol optical depth (AOD) at a high-altitude station, Hanle (4500 m amsl) in the western Himalayas, were studied using direct/diffuse solar irradiance measurement from a Skyradiometer (Prede) during October 2007 to December 2010. The study reveals a diurnal asymmetry in the measured aerosol characteristics, with three types of diurnal variation in AOD. Among them, Types I and II are prominent during pre-monsoon, while Type III dominates during post-monsoon. Type I appears to be associated with new-particle formation process from gaseous precursors, in addition to the combination of anthropogenic and desert-dust aerosols, probably brought by the prevailing westerly/south-westerly winds during the pre-monsoon season. The diurnal feature of the Type II may be attributed by the transported desert-dust aerosols brought by the prevailing winds. Further, Type III may be associated with the aged background aerosols over the region, pertaining to a small contribution from gaseous precursors

Aerosol radiative forcing over a high-altitude station Merak, in the trans-Himalayan region during advection of anthropogenic events from the Indo-Gangetic Plain

Advection of anthropogenic aerosols from the Indo-Gangetic Plain (IGP) and dust aerosols from distant deserts towards a high-altitude station Merak, in the trans-Himalayan region are reported during June–July 2011. In order to differentiate the advection event, aerosol optical properties were examined during aged background conditions at the site. During the aged background conditions, aerosol optical depth (AOD at 500 nm) and Angstrom exponent (α) at the station were ∼0.06 and 1.36, respectively which were increased to 0.13 and 1.62, respectively during the advection event. Further, a strong signature of fine-mode aerosol volume size distribution, dominated by absorbing aerosols, was observed during the advection event. The average atmospheric forcing during the aged background condition was found to be 0.57 Wm−2 (with corresponding heating rate of 0.05 Kday−1) and these results were enhanced to 2.58 Wm−2 (with corresponding heating rate of 0.22 Kday−1) during the advection event. The present study reveals that during the advection event, heating rate in the atmosphere was increased by about four times than the aged background condition. Such atmospheric warming in the region may influence the melting of the Himalayan glaciers and consequently it may effect the local atmospheric circulatio

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