Study of Chemical Composition in Wet Atmospheric Precipitation in Karachi, Pakistan

Anthropogenic and natural emissions in atmosphere directly affect the rainwater chemistry as its chemical speciation is representative of emission status in the surrounding area. A comprehensive study on the chemical composition of rainwater was carried out in Karachi, a mega-city of Southeast Asia to delineate the urbanization impact on the local environment. Rainwater samples were analyzed for conductivity, pH, HCO3−, F−, Cl−, NO2−, NO3−, SO42−, Na+, NH4+, K+, Mg2+, Ca2+, HCOO−, CH3COO−, C2O42−, pyruvate, malonate, propionate, glyoxylate, and total organic carbon (TOC) levels. The ionic load in rainwater samples was found to be high in the densely populated sampling sites experiencing heavy traffic activity and located adjacent to industrial zones. Na+ was the most dominant ionic species, followed by SO 4 2−, Ca2+, Cl−, HCO3−, K+, NH4+, Mg2+, NO3−, CH3COO− and HCOO− in sequence. HCOO− and CH3COO− were found to be the predominant carboxylic acids and their mean concentrations were 4.9 ± 7.3 and 9.4 ± 16.0 µeq/L, respectively. These organic acids together contributed with 7% to the TOC in precipitation. Source apportionment of the contaminants was accomplished by statistical techniques, which indicated strong impact of anthropogenic pollution on the rainwater chemistry in Karachi.