Chemical characteristics of individual rain events from a semi-arid region in India: Three-year study

A systematic study of individual precipitation (wet-only) events (n = 91), collected for three years (2000-2002) during the period of SW-monsoon (June-September) from a semi-arid region (Ahmedabad, 23 degrees 03'N, 72 degrees 38'E) in India, reveals that rainwater is characteristically alkaline in nature (pH range: 5.2-8.2, Av. = 6.7). The ionic ratio of Cl- and Na+ in each event closely follows that of sea-salt composition. The measured abundances of Ca2+ (5.5-1057 mu eq 1(-1)), Mg2+ (1.5-251 mu eq 1(-1)) and HCO3- (8-836 mu eq 1(-1)); non-sea-salt (nss) components of Ca2+ and Mg2+ varying as 94-99% and 0-84%; and linear regression parameters among (nss-Ca2+ +nss-Mg2+) and HCO3- (slope = 0.71, r(2) = 0.96) all tend to suggest that crustal sources dominate the chemical composition of precipitation. The concentrations of acidic constituents SO42- and NO3- vary over a wide range, from 4.8 to 431 and 1.0 to 115 mu eq 1(-1), respectively; and that of NH4+ varied from 1.7 to 220 mu eq 1(-1) and K+ as 0.6-53 mu eq 1(-1). A significant impact of anthropogenic sources on rainwater composition is evident based on the nss-fraction of SO42- (67-99%, VWM = 86%) and equivalent ratio of nss-SO42-/NO3- (0.8-6.7, VWM = 2.5). With the exception of NH4+, concentration of an individual species in a rain event is largely controlled by the precipitation amount. The inter-annual variations in the volume-weighted-mean composition are not significantly pronounced; making this a representative data set for the study region. (c) 2005 Elsevier Ltd. All rights reserved.