Inorganic Ionic Composition of Rainwater at a High Altitude Station over the Western Ghats in Peninsular India

This study investigates chemical composition of rainwater (RW) and its contribution from different sources collected over the period of two years (2016 and 2017) at a high altitude location (1380 m above mean sea level) located at Mahabaleshwar situated in the Western Ghats in Peninsular India. The volume weighted mean pH of RW was found to vary between 4.57 and 7.51 (average 5.95) indicating overall alkaline nature of the RW. Prominent ionic species in the RW were Ca2+ (25%), Na+ (19%), Cl−(23%), SO42− (10%), and Mg2+ (9%) with NH4+, NO3− and K+ together forming about 8% of ionic composition. Moreover, ample presence of dust source (Ca2+) was found that acted as a major neutraliser to the acidic ions. The order of Neutralisation Factor of ions was Ca2+ > Mg2+ > NH4+. In addition, a strong correlation between Na+ and Cl− (r ≈ 0.99) further suggested substantial supplement of marine (NaCl) component to the RW. The impact of local anthropogenic activities such as fossil fuel/biomass burning was observed apart from some contribution from the long-range transport. The high contribution of non-sea salt fractions to Ca2+, SO42−, Mg2+ and K+ showed a substantial effect of crustal and continental air masses. Results of source apportionment for the RW composition by using the Positive Matrix Factorization technique indicated four factors i.e. Marine and long range transport (Na+, Cl−), crustal (Ca2+, Mg2+), emissions from the fossil fuel and biomass burning (NO3−, SO42−) and the agriculture/farming activities (NH4+).