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

Abstract

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°03'N, 72°38'E) in India, reveals that rainwater is characteristically alkaline in nature (p 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 (5.5-1057 µeq l), Mg (1.5-251 µeq l) and HCO (8-836 µeq l); non-sea-salt (nss) components of Ca and Mg varying as 94-99% and 0-84%; and linear regression parameters among (nss-Ca+nss-Mg) and HCO (slope=0.71, r=0.96) all tend to suggest that crustal sources dominate the chemical composition of precipitation. The concentrations of acidic constituents SO and NO vary over a wide range, from 4.8 to 431 and 1.0 to 115 µeq l, respectively; and that of NH varied from 1.7 to 220 µeq l and K as 0.6-53 µeq l. A significant impact of anthropogenic sources on rainwater composition is evident based on the nss-fraction of SO (67-99%, VWM=86%) and equivalent ratio of nss-SO/NO (0.8-6.7, VWM=2.5). With the exception of NH, 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.