Chemical composition, sources, and deposition fluxes of water-soluble inorganic ions obtained from precipitation chemistry measurements collected at an urban site in northwest China

Abstract

Precipitation samples were collected at an urban site in Xi'an, northwest China during March to November in 2009 and were then analyzed to determine the pH and concentrations of water-soluble inorganic ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, SO₄²⁻, NO₃⁻, Cl⁻, and F⁻) in precipitation. The pH of precipitation ranged from 4.1 to 7.6 for all of the samples with an annual volume-weighted mean of 6.4. While a large portion of the precipitation events were weakly acidic or alkaline, around 30% of the precipitation events in the autumn were strongly acidic. Precipitation events with air masses from the northeast and the southeast were weakly acidic while those with air masses from the northwest and the southwest were alkaline. SO₄²⁻, Ca²⁺, NH₄⁺, and NO₃⁻ were dominant ions in the precipitation, accounting for 37%, 25%, 18%, and 9%, respectively, of the total analyzed ions. Ca²⁺ and NH₄⁺ were found to be the major neutralizers of precipitation acidity; however, the contribution of Mg²⁺, although much lower than those of Ca²⁺ and NH₄⁺, was important, in many cases, in changing the precipitation from weakly acidic to weakly alkaline. Enrichment factor analysis confirmed that SO₄²⁻ and NO₃⁻ were produced from anthropogenic sources, Ca²⁺, K⁺, and 80% Mg²⁺ were from crustal sources, and Na⁺, Cl⁻, and ∼20% of Mg²⁺ were from marine sources. The annual wet depositions were estimated to be 3.5 t km⁻² per year for sulfur; 2.3 t km⁻² per year for nitrogen, of which 0.8 t km⁻² per year was oxidized nitrogen and 1.5 t km⁻² per year was reduced nitrogen; and 3.0 t km⁻² per year for Ca²⁺.