Chemistry of precipitation events and inter-relationship with ambient aerosols over a semi-arid region in western India

The precipitation events (n= 91), collected for 3 years ( 2000 - 2002) during the period of SW-monsoon ( Jun - Aug) from an urban site (Ahmedabad, 23.0 degrees N, 72.6 degrees E) of a semi-arid region in western India, are found to exhibit characteristic differences in terms of their solute contents. The low solute (< 700 mu eq L-1) events are either marked by heavy precipitation amount or successive events collected during an extended rain spell; whereas light precipitation events occurring after antecedent dry period are characterized by high solutes (> 700 mu eq L-1). The ionic composition of low solute events show large variability due to varying contribution of anthropogenic species ( NH4+ : 1% - 74%; NO3- : 1% - 25%; and SO42- : 8% - 68%) to the respective ion balance. In high solute events, ionic abundances are dominated by mineral dust ( Ca2+ and HCO3-) and sea-salts (Na+ and Cl-). These differences are also reflected in the pH of low solute events ( range: 5.2 - 7.4, VWM: 6.4) and high solute events ( range: 6.6 - 8.2, VWM: 7.3). The comparison of Ca2+/Na+ and nss-SO42-/Ca2+ ratios ( on equivalent basis) in rain and aerosols suggests that the ionic composition of high solute events is influenced by below-cloud scavenging; whereas evidence for in-cloud scavenging is significantly reflected in low solute events. The annual wet-deposition fluxes of NH4+ and NO3- are 330 and 480 mg m(-2) y(-1), respectively, in contrast to their corresponding dry-deposition fluxes (14 and 160 mg m(-2) y(-1)); whereas wet and dry removal of Ca2+, Mg2+ and SO42- are comparable.