Chemical characteristics of rainwater associated with cyclones, typhoons and stationary fronts: the record at a coastal site in southwestern Japan from 1996–2003

Abstract

The ion concentration in the rainwater at a coastal site in southwestern Japan during the period 1996–2003 was investigated according to the rain types of cyclones (Cy), stationary fronts (SF) and typhoons (Ty). The eight-year mean concentrations of Na⁺, Cl⁻, nss-SO₄²⁻, NO₃⁻, NH₄⁺ and nss-Ca²⁺ in the Cy rainwater were 68.6±33.1, 80.9±40.9, 32.1±8.8, 14.4±3.7, 15.3±3.5 and 8.3±3.4 μeq/L, respectively. In contrast, the respective concentrations of these ions in the SF rainwater were 24.9±6.2, 30.2±6.9, 18.2±7.5, 8.1±3.1, 9.9±4.2 and 4.4±2.7 μeq/L. In the Ty rainwater (11 cases in total), the concentrations of Na⁺ and Cl⁻ were 227.7±518.3 μeq/L and 275.6±619.4 μeq/L, respectively, together constituting 79.2% of the ion content. Backward trajectories and statistical analyses revealed that differences in ion concentration between the Cy and SF rainwater were due to the different air parcels origins and chemical processes. The yearly mean ion concentrations in the total rainwater were dominated by the Cy and SF rain. A simple estimation according to the rainfall variation by SF showed that the variation could cause a significant change in the ion concentrations. Considering the distinctiveness of the rain types is thus essential for an accurate evaluation of ions in the rainwater, a meaningful inter-comparison between regions, and a more confident projection of future scenarios of rainwater chemistry.