STUDY OF AEROSOL OPTICAL PROPERTIES OVER TWO SITES IN THE FOOTHILLS OF THE CENTRAL HIMALAYAS

Abstract. Atmospheric aerosol possesses impacts on climate system and ecological environments, human health and agricultural productivity. The environment over Himalayas and Tibetan Plateau region are continuously degraded due to the transport of pollution from the foothills of the Himalayas; mostly the Indo-Gangetic Plain (IGP). Thus, analysis of aerosol optical properties over two sites; Lumbini and Kathmandu (the southern slope of central Himalayas) using AERONET’s CIMEL sun photometer were conducted in this study. Aerosol optical depth (AOD at 500 nm), angstrom exponent (α or AE), volume size distribution (VSD), single scattering albedo (SSA) and asymmetry parameter (AP) were studied for 2013–2014 and the average AOD was found to be: 0.64 ± 0.41 (Lumbini) and 0.45 ± 0.30 (Kathmandu). The average AE was found to be: 1.25 ± 0.24 and 1.26 ± 0.18 respectively for two sites. The relation between AOD and AE was used to discriminate the aerosol types over these sites which indicated anthropogenic, mixed and biomass burning origin aerosol constituted the major aerosol types in Lumbini and Kathmandu. A clear bi-modal distribution of aerosol volume size was observed with highest volume concentration during the post-monsoon season in fine mode and pre-monsoon season in coarse mode (Lumbini) and highest value over both modes during pre-monsoon season in Kathmandu. The single scattering albedo (SSA) and asymmetry parameter (AP) analyses suggested aerosols over the Himalayan foothills sites are dominated by absorbing and anthropogenic aerosols from urban and industrial activities and biomass burning. Long-term studies are essential to understand and characterize the nature of aerosol over this research gap zone.