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=91n=91), collected for three years (2000–2002) during the period of SW-monsoon (June–September) from a semi-arid region (Ahmedabad, 23°03′N, 72°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 μeq l−1), Mg2+ (1.5–251 μeq l−1) and HCO3− (8–836 μeq l−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, r2=0.96r2=0.96) all tend to suggest that crustal sources dominate the chemical composition of precipitation. The concentrations of acidic constituents SO2−4 and NO3− vary over a wide range, from 4.8 to 431 and 1.0 to 115 μeq l−1, respectively; and that of NH4+ varied from 1.7 to 220 μeq l−1 and K+ as 0.6–53 μeq l−1. A significant impact of anthropogenic sources on rainwater composition is evident based on the nss-fraction of SO2−4 (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.