Aerosol Optical Properties over Gurushikhar, Mt. Abu: A High Altitude Mountain Site in India

Continuous and simultaneous measurements of aerosol optical properties (the scattering and absorption coefficients) were conducted, for the first time, at a mountain site, Gurushikhar, Mt. Abu, in the Aravalli Range in western India from January 2015 to December 2016. The aerosol scattering and absorption coefficients were higher in the afternoon than during the forenoon and night because of atmospheric boundary layer dynamics that, when accompanied by strong thermal convection, promote an upward movement of pollutants to the observational site from the surrounding foothills. The aerosol scattering and absorption coefficients exhibited a strong seasonal variability. The average scattering coefficients during the winter, premonsoon, monsoon and postmonsoon seasons were 78.5 ± 22.9, 61.8 ± 12.2, 49.9 ± 25.1 and 121.8 ± 47.4 Mm –1 , respectively, and the corresponding absorption coefficients were 14.9 ± 3.1, 9.1 ± 2.9, 5.1 ± 3.2 and 20.2 ± 10.2 Mm –1 . The single scattering albedo also exhibited a significant seasonal variation, with the maximum occurring during the monsoon season (0.91) and the minimum during winter (0.83). The mean annual β sca , β abs and SSA at 550 nm were 74 ± 34 Mm –1 , 12 ± 7 Mm –1 and 0.87 ± 0.04. These results indicate that Gurushikhar, a remote high altitude site, is influenced by local and long-range transported aerosols through convection and advection processes. However, the aerosol properties at this pristine location are governed by transported emissions rather than local anthropogenic sources and also display low inter-annual variability. Thus, Gurushikhar can be considered a background site for the nearby source regions in western India.