Abstract
ABSTRACT The chemistry of rainwater over mid-Brahmaputra Valley was studied for three consecutive years (2012–2014; n = 285). The samples were analyzed for major chemical parameters viz. pH, electrical conductivity (EC), and ions (SO42–, NO3–, Cl–, F–, Br–, Ca2+, NH4+, Mg2+, Na+, K+, and Li+), organic acids (HCOO– and CH3COO–) and dissolved organic carbon (DOC). The mean pH for the entire study period was found to be 5.66, which ranged from 4.51 to 7.68, and the volume weighted (VW) mean pH was found to be 5.16. Over 55% of the samples showed pH between 5 and 6, and a few samples had pH Ca2+ > SO42– > NO3– > Cl– > Na+ > K+ > Mg2+ > H+ > HCO3– > Br– > F– > Li+, indicating dominance of alkaline ions over acidic ions such that 94% of mineral acid was neutralized. The secondary ions, NH4+, SO42–, and NO3–, showed high wet deposition fluxes. The chemistry exhibits explicit seasonality. The airmass clusters of monsoon and non-monsoon seasons, and the associated chemistry varied, which showed influence of long-range transport. The interspecies correlations varied between the monsoon and non-monsoon time samples meaning variation in the source strengths of the contribution sources of the chemical species of the rainwater. Positive Matrix Factorization (PMF) was applied to the data which extracted six factors that explained the sources and chemistry of the rainwater constituents which are of sea, agriculture, coal burning, biomass burning, and secondary origin.
Highlights
For many decades, rainwater chemistry has drawn significant attention because of its acidity and the associated consequences on ecosystems
The arithmetic and volume weighted (VW) mean pH and electrical conductivity (EC) of rainwater observed during the study period are given in Supplementary Table S1
Rain events characterized by low pH (< 5.6) often have significant contributions from NO3– and SO42– (e.g., Rastogi and Sarin, 2005)
Summary
Rainwater chemistry has drawn significant attention because of its acidity and the associated consequences on ecosystems. This difference would suggest the influence of strong acidic components like NO3–, SO42–, and Cl– derived from anthropogenic activities, and it indicates minimum neutralization of these acids by alkaline ions in the atmosphere. A year-long study of water-soluble ions in airborne PM (sampled during 2012– 13) across the Tezpur region revealed high loading of SO42– and Cl– (Bhuyan et al, 2016b) that might have been scavenged by rainfall.
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