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Concentrations of water-soluble-ionic-species (NH4+, Na+, K+, Ca2+, Cl-, NO3-, SO42- and HCO3-) in the ambient aerosols and their temporal variability have been studied for a 3-year period (2000-2002) from an urban site (Ahmedabad, 23.0 degrees N, 72.6 degrees E, 49 m asl) and a high-altitude station (Mt Abu, 24.6 degrees N, 72.7 degrees E, 1680 m asl) located in a semi-arid region of western India. During the drier months (January-April, September-December), the average concentrations (mu g m(-3)) of water-soluble species in the aerosols over Ahmedabad are: HCO3-: 4.95, SO42- : 4.57, Ca2+: 2.96, NO3-: 2.07, Cl-: 0.99, Na+: 0.81, K+: 0.76, NH4+: 0.48, Mg2+: 0.25. During the wet period of the SW-monsoon (May-August), the contribution of Na+ (2.42 mu g m(-3)) and Cl- (3.96 mu g m(-3)) are significantly enhanced. In spite of the geographic and altitude differences, the composition of aerosols from the two sites shows comparable ratios among the major species (Ca2+, HCO3- and SO42-). On average, during dry period, nss- 4-/nss-Ca ’ and HC03-/nss-Ca ratios over Ahmedabad are 1.6 and 1.7; and those over Mt Abu are 1.9 and 2.0, respectively. During wet phase, the corresponding ratios are 1.3 and 1.7 over Ahmedabad and 1.2 and 1.8 over Mt Abu. Although the concentrations of several species are significantly different over the two sites and exhibit large seasonal variations, their inter-annual variability is nowhere pronounced. Unlike the chemical characteristics reported over other urban regions, aerosols sampled in this study are of alkaline nature with major neutralizing component as Ca2+ rather than NH4+. As a result, significant uptake of acidic species (NO3- and SO42- derived from pollution sources) by mineral aerosols (CaCO3) is a dominant chemical transformation process over the study sites. (c) 2005 Elsevier Ltd. All rights reserved.