Chemical Composition of Atmospheric Aerosols From High and Low Altitude Sites in Northern and Western India

The chemical composition of ambient aerosols, collected during wintertime from four designated sites: Ahmedabad (23.0oN, 72.6oE, 49 m asl), Mt Abu (24.6oN, 72.7oE, 1680 m asl), Hisar (29.2oN, 75.7oE, 216m asl) and Nainital (29.4oN, 79.5oE, 1940 m asl), has been studied to understand the potential role of regional emission sources as well as the long-range transport of chemical constituents through free troposphere. The two high altitude sites, Mt Abu and Nainital, exhibit free tropospheric characteristics during wintertime; whereas the urban sites (Ahmedabad and Hisar) are within boundary layer. The ratios of major ionic species, measured in water extracts of aerosols, are considered to be advantageous in order to remove the effect of aerosol mass loading on the atmospheric concentrations of species at different sites. At low-altitude-urban sites (Ahmedabad and Hisar), geometric mean of Ca2+/Na+ ratios (4.3 and 4.5) and HCO3-/ Ca2+ ratios (1.7 and 1.5) are comparable; whereas significant differences are observed in Cl-/Na+ (1.1 and 0.6), SO42-/Ca2+ (1.5 and 3.5) and SO42-/NO3- (2.1 and 1.0). These differences are attributed to relative dominance of anthropogenic emissions over the northern site (Hisar). In contrast, major differences arise over Mt Abu and Nainital with respect to Cl-/Na+ (1.0 and 0.2), Ca2+/Na+ (9.0 and 4.4) and HCO3-/ Ca2+ (1.8 and 0.9) ratios; whereas SO42-/Ca2+ (2.3 and 3.1) and SO42-/NO3- (5.3 and 5.9) ratios are comparable. Such regional differences for a high altitude site could arise due to semi-arid climate and high abundance of mineral dust at Mt Abu. Higher SO42-/NO3- ratios over high altitude sites than those over low altitude sites are attributed to the relative difference in the size distribution of SO42- (fine mode) and NO3- (coarse mode) aerosols.