Chemical characteristics of individual rain events from a semi-arid region in India: Three-year study [An article from: Atmospheric Environment]

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Description:
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^o03'N, 72^o38'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 Ca^2^+ (5.5-1057@meql^-^1), Mg^2^+ (1.5-251@meql^-^1) and HCO"3^- (8-836@meql^-^1); non-sea-salt (nss) components of Ca^2^+ and Mg^2^+ varying as 94-99% and 0-84%; and linear regression parameters among (nss-Ca^2^++nss-Mg^2^+) and HCO"3^- (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 SO^2^-"4 and NO"3^- vary over a wide range, from 4.8 to 431 and 1.0 to 115@meql^-^1, respectively; and that of NH"4^+ varied from 1.7 to 220@meql^-^1 and K^+ as 0.6-53@meql^-^1. A significant impact of anthropogenic sources on rainwater composition is evident based on the nss-fraction of SO^2^-"4 (67-99%, VWM=86%) and equivalent ratio of nss-SO"4^2^-/NO"3^- (0.8-6.7, VWM=2.5). With the exception of NH"4^+, 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.