Abstract
ABSTRACTThe 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) (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.
Highlights
Precipitation is an important mechanism for scavenging of atmospheric pollutants, both particles and gases (Migliavacca et al, 2005)
The objectives of this study are to (i) characterize the chemical composition of rainwater collected during different rainy seasons over Indo-Gangetic Plains (IGP) region, (ii) investigate the spatial variability of wet deposition flux of measured ions in rainwater in this polluted region, and (iii) provide a better understanding of the sources of SO42– and NO3– as well as other measured ions that contribute to the rainwater contents at a scale of the season and year
In some cases, the volume weighted mean (VWM) pH values were lower than the threshold value for acid rainwater
Summary
Precipitation is an important mechanism for scavenging of atmospheric pollutants, both particles and gases (Migliavacca et al, 2005) These species affect the chemistry of rainwater and reflect the air quality in the region where the precipitation samples were collected (Al-Khashman, 2005, 2009). Decreasing pH values in rainwater is the result of major inorganic acids such as sulfuric, nitric, and organic acids including carboxylic acids, carbonic acids, acetic acid, formic acid etc., resulting from complex homogeneous and heterogeneous phase reactions in the atmosphere (Rocha et al, 2003; Niu et al, 2014) The precursors of these the first three acids are emissions of oxides of sulfur and nitrogen, carboxylic compounds and hydrocarbons (Migliavacca et al, 2005). North America and Europe have been adversely affected by acidic deposition (Rodhe et al, 2002)
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