Nature and Sources of Ionic Species in Precipitation across the Indo-Gangetic Plains, India

ABSTRACT

The spatial distribution of rainwater chemistry over the densely-populated and highly polluted Indo-Gangetic Plains (IGP) was investigated using samples (total = 687) collected during three consecutive summer monsoon seasons from 2009 to 2011. The concentrations of secondary ionic species (SO₄^2– and NO₃^–) were measured along with the other major ions (F^–, Cl^–, Na⁺, K⁺, Ca²⁺, Mg²⁺ and NH₄⁺) and pH and specific conductivity. The weighted mean pH (± std) and conductivity of rainwater were 5.73 (± 0.17) and 31.6 (± 31.0) µS cm^–1, respectively. Approximately 16% of rainwater samples were acidic (pH < 5.61) with a mean pH = 5.38 of acid rain and rest of them were more alkaline (pH > 5.61) (mean pH = 6.34 for the more basic samples). Specific conductivity was ~39% lower (20.6 µS cm^–1) for the acidic rain compared to the more basic (33.6 µS cm^–1) samples. The mean sum of all of the measured ions is 351.6 ± 130.1 µeq L^–1 with the highest contributions being Ca²⁺ (30%) and SO₄^2– (15%). Mean [SO₄^2–] (52 µeq L^–1) and [NO₃^–] (29 µeq L^–1) were approximately five and ten times higher, respectively, compared to background hemispheric values. Secondary ions had the highest deposition fluxes (SO₄^2–, 25.2 kg ha^–1 y^–1 and NO₃^–: 18.3 kg ha^–1 y^–1). The mean ratio of H⁺/(NO₃^– + SO₄^2–) was 0.02 indicating ~98% of the acidity was neutralized. Ca²⁺, (57%), Mg² (25%), NH₄⁺ (15%) and K⁺ (4%) were important neutralizing species. Positive Matrix Factorization (PMF) was applied to the deposition fluxes. Five factors were identified and identified as ammonia neutralized, sea salt, soil, biomass burning, and calcium neutralized.