Spatial and long-term analysis of rainwater chemistry over the conterminous United States

A comprehensive analysis of the chemical composition of precipitation was performed on rainwater samples collected between 1978 and 2017 over the conterminous US. A total of 86470 data records downloaded from the National Atmospheric Deposition Program were statistically analyzed and assessed in terms of precipitation chemistry. The ion abundance followed the Cl⁻ > Na⁺ > SO₄²⁻ > Ca²⁺ > H⁺ > NH₄⁺ > NO₃⁻ > Mg²⁺ > HCO₃⁻ > K⁺ downward trend, showing that chloride and sodium were the most dominant among anions and cations. Ca²⁺, SO₄^2- and NH₄⁺ concentrations were notable in desert areas or in regions with significant anthropogenic activity. Frequency analysis of pH values showed that the 87.90% of the pH is acidic, exhibiting values under 5.6. According to the acidifying and neutralization potential, rainwater pH is mostly alkaline in the Western region, presenting acidic values in highly industrialized areas, in the Central and Eastern Regions. Fractional acidity showed that in the majority of the studied sampling sites 61% of the acidity in precipitation is neutralized, due to the presence of the main neutralizing agents (NH₄⁺, Ca²⁺, Na⁺), fact sustained by the neutralization factor values. The relationship between acidic and alkaline components was thoroughly examined by ionic ratios and the ammonium availability index. Wet deposition rates of major ions confirmed the dominance of acidic species over neutralizing ones, as well as the significant imprint of regional climate and heavily industrialized areas on the precipitation chemistry. The complex major ion source apportionment, including marine and crustal enrichment factors, sea salt and non-sea salt fractions, Spearman's rank correlation analysis and Principal Component Analysis, showed that anthropogenic influences are the most significant, including coal-fired power plants, oil refineries, major industries and agricultural activities. Crustal and marine sources also presented a prominent imprint on the rainwater chemistry of the conterminous US.