The variability of pH in convective storms

Abstract

The rainwater pH was measured in a total of 22 storms which occurred in 1972 and 1974 in the METROMEX (METROpolitan Meteorological EXperiment) rainwater sampling network. The network consists of 81 collectors in an area of 1800 kM² over and east of St. Louis, Missouri.

The data set is composed of dry fallout samples as well as precipitation samples which have been analyzed independently. An analysis of the frequency distribution of these data shows the rainfall samples form a bimodal distribution of pH with relative frequency maxima at pH 7.0 and 4.0 while the dry samples are unimodal with the maximum frequency at pH 7.0.

The areal distribution of precipitation weighted mean pH indicates a variability unrelated to total precipitation. However, the derived deposition of H+ is more similar to the areal rainfall pattern than to the pH. The dH is defined which incorporates both the rainfall and pH which is useful for climatological trend studies of acid rainfall.

Examples of two individual storm events illustrate an inexplicable area-wide variation from nearly an all acid rain (pH ⩽ 4.5) to an all alkaline rain (pH ⩾ 5.5). These case studies indicate some of the meteorological as well as chemical problems which must be considered when attempting to characterize convective storm rainfall pH.