Karachi, being one of the largest urban setup in the world under the sprawl of rapid and unplanned urbanization, is at the risk of contaminated atmosphere. In the present study, chemical contaminants were assessed and monitored in wet and dry atmospheric depositions of Karachi to identify the adverse impact of anthropogenic activities mainly and climatic influence in general affecting atmospheric composition of Karachi. Samples of rainwater had been collected for five years (i.e. 2008, 2010 – 2013), to estimate the ionic load in wet deposition. Similarly, dust samples were collected from different sites for the analyses and estimation of contaminants in dry deposition. Sampling sites with intense vehicular activities, neighboring to industrial zone and sites in congested localities where routine dumping and burning of garbage were in progress, were given due consideration during the sampling campaigns. Major ions (F-, Cl-, SO42-, NO2-, NO3-, HCOO-, CH3CHOO-, C2O42-, Li+, Na+, K+, Mg2+ and Ca2+) were analyzed through Ion Chromatography as per EPA method 300, UV/Visible spectrophotometer was used for the analysis of NH4+ and TOC analyzer was used for the monitoring of TOC in rainwater samples. XRF analysis was carried out for the quantitative scanning of complete elemental profile of dry samples. Heavy metals were also monitored along with other elements. Air mass back trajectories were generated for the rainy events using NOAA HYSPLIT model for confirming the climatic control on the composition of rainwater. GIS was used as a tool to interpret data through thematic maps. Source apportionment of contaminants was supported by evaluating different factor like Enrichment Factor (EF), Neutralization Factor (NF), Correlation Matrix, and Factor Analysis (FA) using PCA (Principal Component Analysis). It was concluded through the detailed systematic analysis and processing of the data through several statistical and GIS models that acidic ions are present in high concentrations which are concealed by the neutralizing agents like Na+ and Ca2+ in atmosphere. Elemental profile indicates the alarming enrichment of Pb and Zn in atmospheric deposition samples of industrial areas and sites with high vehicular activities. This study provides very first systematic efforts to set a baseline data of contaminants in the ambient atmospheric wet and dry depositions of Karachi city.
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