Influence of urban–coastal activities on organic acids and major ion chemistry of wet precipitation at a metropolis in Pakistan

Anthropogenic and natural emissions in the atmosphere directly affect the rainwater chemistry as its chemical speciation is representative of emission status in the surrounding area. A comprehensive assessment was carried out in 2008 for the wet precipitation in the context of chemical composition, in Karachi, a mega-city of Southeast Asia to delineate the urbanization impact on the local environment. Rainwater samples were analyzed for conductivity, pH, HCO ₃ ⁻ , Mg ²⁺ , Ca ²⁺ , NO ₂ ⁻ , NO ₃ ⁻ , SO ₄ ²⁻ , Na ⁺ , NH ₄ ⁺ , K ⁺ , F ⁻ , Cl ⁻ , HCOO ⁻ , CH ₃ COO ⁻ , C ₂ O ₄ ²⁻ , 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. Acidic content of rainwater had been neutralized by the local alkaline particulates and aerosols introducing alkalinity in rainwater of Karachi City. pH ranged from 3.30 to 7.91 having a mean value of 6.84 ± 0.93. The most dominant ionic species was Na ⁺ , followed by SO ₄ ²⁻ , Ca ²⁺ , Cl ⁻ , HCO ₃ ⁻ , K ⁺ , NH ₄ ⁺ , Mg ²⁺ , NO ₃ ⁻ , CH ₃ COO ⁻ , and HCOO ⁻ in sequence. HCOO ⁻ and CH ₃ COO ⁻ were the dominant carboxylic acids found in this region, and their mean concentrations were 4.9 ± 7.3 and 9.4 ± 16.0 μeq/L, respectively. These organic acids together contributed 7% to the TOC in precipitation. The formate/acetate ratio was 0.52. Combustion and vehicular exhaust generated acetate may contribute to elevated levels of these organic acids. Statistical tools and source apportionment analysis confirmed the strong impact of anthropogenic pollution on wet precipitation of the mega-city.