Chemistry of soluble ions in atmospheric bulk precipitation during the monsoon over the Brahmaputra Valley

This study presents spatial variations of soluble ion chemistry in the bulk precipitation over the Brahmaputra Valley covering a large city (Guwahati) and remote rural locations (Tezpur and Dhemaji) during the monsoon period of 2015. The chemical characterization of the inorganic ions (F−, Cl−, Ca2+, Mg2+, Na+, K+) along with secondary ions (NO3−, SO42−, and NH4+) of bulk samples was done. Concentrations of NO3−, SO42−, K+, Ca2+, and Mg2+ showed significant variations in all the three sites at p < 0.05 significant level. Calcium ion (Ca2+) and SO42− showed maximum concentrations in the bulk precipitation samples irrespective of different sampling backgrounds. Precipitation at Dhemaji showed the lesser total ionic composition (60.2 μeq/L), compared to Guwahati (178.4 μeq/L) and Tezpur (87.5 μeq/L). Calculation of %SSF (sea salt fraction), %CF (crustal fraction), and %AF (anthropogenic fraction) revealed NO3− and SO42− being mostly contributed by the anthropogenic sources in samples of all the locations. Principal component analysis revealed the influence of crustal, biomass burning, marine and other anthropogenic sources such as emissions from agricultural fields, cattle farming, wasteland, waste burning, and biogenic emissions as the main contributors to the bulk precipitation chemistry over the Brahmaputra Valley. A significant positive correlation between PM2.5 and anthropogenic components such as NO3−, SO42−, and NH4+ of bulk precipitation was observed. This indicates a prominent aerosol precipitation coupling, which helps in the scavenging of air pollutants over the study region.