Ionic composition of rain during sixteen convective showers

Abstract

Rain from 16 summer convective showers over 2 years in the southwestern United States was sampled with a frequency of about twice per min. Concentrations of major components, NH + 4, NO − 3, H +, SO 2− 4 and Ca 2+ were determined for all rains (464 samples); those of Mg 2+, Na +, K + and Zn 2+ were found for eight of the rains (232 samples). The aim was to determine, via statistical analysis, sources, associations and incorporation processes of the components. The concentrations of components showed considerable changes during the showers. Coefficients of correlation between all components except H + were positive. However, correlations between elements known to have a common source (Ca 2+, Mg 2+, Na +, K + and Zn 2+) were not significantly different from those between components not having a common immediate source. At least for the clean conditions of the southwest U.S., the statistical method cannot be used to infer sources. Concentration changes were primarily determined by precipitation processes. All components showed a negative rainfall-rate dependence, with H + having the weakest. This has been attributed primarily to spatial variations in cloud liquid water content during the rainout process. All components except H + showed a negative correlation between the logarithm of concentration and accumulated rainfall (such as would occur ideally through washout); the correlation was strongest for Ca 2+; for H + the correlation was larger and positive. Thus in the arid southwestern United States, rain pH typically decreases during a shower, apparently as a result of preferential washout of calcareous soil particles. Concentrations of NH + 4 and NO − 4 were typically 3 × 10 −5 moles I. −1, those of SO 2− 4, and total soilderived metallic cations 2 × 10 −5: moles I. −1. If it is assumed that NH + 4 and H + are in equilibrium with atmospheric NH 3, an average ammonia concentration of 13 pptv (1 pptv = 10 −12 atmos) is deduced for these shower downdrafts. This ammonia concentration should not be considered representative of the ambient atmosphere.