Chemical composition and source apportionment of rainwater over Bangladesh during the monsoon

The composition of rainwater is highly dependent upon the air quality of the area. Thirty-four samples of monsoon rainwater were obtained during the monsoon season of June to September, 2015, in Bangladesh's heavily polluted megacity, Dhaka, and a regional background island area, Bhola. The physical properties of rainwater, such as electrical conductivity and pH, were evaluated. The concentrations of various cations and trace metals such as Na+, K+, Ca2+, Mg2+, and Zn, Fe, Mn, respectively, in rainwater were determined using atomic absorption spectroscopy (AAS). In addition, ion chromatography was used to calculate concentrations of anions such as Cl-, SO42-, NO3-, and HCO3. The source proportion of chemical species in rainwater was determined using percent source contribution, correlation analysis, enrichment factor analysis, and air mass trajectory analysis. Among Dhaka and Bhola, there were differences in physicochemical properties and anthropogenic contribution. The average concentration of SO42- in rainwater from Dhaka and Bhola was 12.53 and 8.48 mu eq L-1, respectively, while the NO3- concentrations were 40.64 and 33.93 mu eq L-1. Dhaka's rainwater contained 8.85 mg L-1 of total dissolved solids, and Bhola's rainwater contained 6.78 mg L-1. Zn was found in higher concentration in Dhaka's rain water, while Fe was found in higher concentration in Bhola. Neutralization factor analysis revealed that Ca2+ and Mg2+ were the predominant neutralization components of rainwater at both the locations. The obtained high enrichment factors for SO42- and NO3- were a strong indicator of anthropogenic origins. It is the first time that the aqueous atmospheric chemistry of monsoon rainwater in Bangladesh has been reported over urban and background locations.