Physico-chemical and optical properties of aerosols at a background site (~4 km a.s.l.) in the western Himalayas

Abstract

This paper presents first of its kind results on the aerosol mass concentrations and size distributions, their chemical properties and columnar aerosol optical depth (AOD) over a high-altitude background site of Lahaul and Spiti valley (32.4°N, 77.6°E, 4080 m a.s.l.) in the western Himalayas, obtained from collocated and concurrent measurements made during August (summer) to October (autumn) of the year 2017. The results depicted very low values of columnar AOD (mean ~ 0.07) with ~4% contribution of BC (mean BC ~ 168  ng m−3) to near surface composite aerosol mass. The total suspended particulate (TSP) matter showed significant variations between 5 μg m−3 and 40 μg m−3, having dominance of mineral dust components (~67%) during the entire study period. The ratio of organic carbon (OC) to elemental carbon (EC) varied over a wide range from 7.8 to 18.9, with a mean value of 12.5 ± 2.95. The higher fraction of water-soluble OC in the total OC component indicated the aged nature of the carbonaceous species. An enrichment factor estimated from the concentrations of trace elements also revealed the influence of anthropogenic source contribution. The regions located west and northwest of the measurement site, which includes the regional hot-spots of Indo-Gangetic Plains, west Asia and the Middle East are possible source regions to modulate the aerosols over the observational site through long range transport. These observations have far reaching implications in view of the role of aerosols on regional radiative balance and impact on snow/glacier coverage.