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

Abstract

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°N, 72.6°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 μeq 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 μeq L^-1). The ionic composition of low solute events show large variability due to varying contribution of anthropogenic species (NH⁺₄: 1%-74%; NO^-₃: 1%-25%; and SO^2-₄: 8%-68%) to the respective ion balance. In high solute events, ionic abundances are dominated by mineral dust (Ca²⁺ and HCO^- ₃) and sea-salts (Na⁺ and Cl^-). These differences are also reflected in the p^H 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- SO^2-₄/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 NH ⁺ ₄ and NO ^- ₃ 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 SO^2-₄ are comparable.