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

Category: Aerosol and Atmospheric Chemistry

Volume: 16 | Issue: 4 | Pages: 943-957
DOI: 10.4209/aaqr.2015.06.0423
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Suresh Tiwari 1,2, Philip K. Hopke 2, Devraj Thimmaiah2, Umesh C. Dumka3, Atui K. Srivastava1, Deewan S. Bisht1, Pasumarti S.P. Rao4, Dilip M. Chate4, Manoj K. Srivastava5, Sachchida N. Tripathi6

  • 1 Indian Institute of Tropical Meteorology, New Delhi-110 060, India
  • 2 Clarkson University, Box 5708, Potsdam, NY-13699-5708, USA
  • 3 Aryabhatta Research Institute of Observational Science, Nainital-263 001, India
  • 4 Indian Institute of Tropical Meteorology, Pune-411 008, India
  • 5 Department of Geophysics, Banaras Hindu University, Varanasi-221 005, India
  • 6 Department of Civil Engineering, Indian Institute of Technology Kanpur-208016, (null), India


Sulfate (25.2 kg ha–1 y–1) was highest annual wet deposition flux.
16% rainwater was acidic (mean pH = 5.38) over IGP region.
Five times higher SO42– (52 µeq L–1) than remote site.
57% acidity neutralized by calcium ion.


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 (SO42– 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 SO42– (15%). Mean [SO42–] (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 (SO42–, 25.2 kg ha–1 y–1 and NO3: 18.3 kg ha–1 y–1). The mean ratio of H+/(NO3 + SO42–) 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.


Rainwater chemistry Ion balance Positive matrix factorization Specific conductivity Acid rain

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