Chemistry of Precipitation Events and Inter-Relationship with Ambient Aerosols over a Semi-Arid Region in Western India     

N. Rastogi  M. M. Sarin 《Journal of Atmospheric Chemistry》2007,56(2):149-163

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 (: 1%–74%; : 1%–25%; and : 8%–68%) to the respective ion balance. In high solute events, ionic abundances are dominated by mineral dust (Ca²⁺ and ) 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- 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 and 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 are comparable.  相似文献