Chemical characterization of rainwater at a high-altitude site "Nainital" in the central Himalayas, India.

The present study investigates the chemical composition of rainwater (RW) from a high-altitude site "Nainital" (1958 m above msl) in the central Himalaya region, to understand the influence of local, regional, and long-range transport of pollutants. A total of 55 (2 in pre-monsoon and 53 in monsoon) RW samples were collected during the study period (June-September 2012) and were analyzed for major anions and cations using an ion chromatograph. The pH of precipitation events ranged from 4.95 to 6.50 (average 5.6 ± 0.3) was observed during the monsoon period (near to the acidic), whereas during the pre-monsoon, the pH was 6.25 ± 0.49 (alkaline) over the study region; it is due the mixture of anthropogenic as well as the natural chemical constituents. The average ionic concentration (sum of measured chemical constituents) was ∼3 times higher during the pre-monsoon (986 ± 101 μeq/1) compared to that in the monsoon season (373 ± 37 μeq/1). This is mainly due to the presence of more natural aerosols in the pre-monsoon season which is also reflected in the pH of rainwater (average 6.25 ± 0.50) as well as ionic concentration. The chemical composition suggests that Ca2+ was the major contributor (34%) among cations, followed by Na+ (10%), K+ (8%), and Mg2+ (9%), whereas Cl-, NO3-, and SO42- contributed ∼13, 11, and 9%, respectively, among anions. The average ratio of acidic species (SO42-/NO3-) is 1.56, suggesting 61 and 39% contribution of SO42- and NO3-, respectively, which is very close to the estimated contribution of H2SO4 (60-70%) and HNO3 (30-40%) in the precipitation samples. Neutralization factors for Ca2+, Mg2+, and NH4+ in RW at Nainital are 4.94, 1.21, and 0.37, respectively, indicating their crucial role in neutralization of acidic species. The non-sea-salt (NSS) contribution to total Ca2+, K+, and Mg2+ is estimated to be ∼98, 97, and 74%, respectively, suggesting the dominance of crustal sources for cations. In contrast, the NSS contribution to the total Cl- and SO42- is 16 and 69% indicating their anthropogenic origin, respectively. Principle component analysis also suggests that the first factor (i.e., natural sources, mainly dust, and sea-salts) accounts for ∼33% variance, whereas the second factor (i.e., fossil fuel and biomass burning) accounts for ∼18% variance of the measured ionic composition. The remaining contributions are attributed to the mixed emission sources and transport of pollutants from Indo-Gangetic Plain (IGP) and western parts of India. The results of the present study reveal a significant contribution of crustal and anthropogenic sources in the RW and neutralization processes in the central Himalaya.