Stability of the pH and the contents of ammonium and nitrate in precipitation samples

The changes in the pH and the contents of NH₄⁺, NO₃^? and 10 other components including trace elements were studied in precipitation samples for a period of up to 300 days. The concentration of free H⁺ ions is affected after deposition by several processes, the most important being bioconsumption of NH₄⁺ leading to an increase in the H⁺ ion level depending on the length of the sampling interval, the time of year and the way of storing the samples prior to their analysis. It is proposed that, for the purpose of comparing the acidities of precipitation waters and their effects on acidification of the environment, the corrected H⁺ ion concentration (H_c⁺) be calculated from the pH value and the concentrations of NH₄⁺ and NO₃^? (H_c⁺ = H⁺+NH₄⁺?NO₃^?). The H_c⁺ value depends little on the bioconsumption of NH₄⁺ and NO₃^? after deposition and thus is not very sensitive to the way of storing the samples, the sampling interval and the time elapsed between deposition and the sample analysis. Dissolution of Fe, Al, Si and some trace elements from the particles scavenged during precipitation was slow in the studied samples; it is advantageous to digest the sample in a microwave oven prior to determination of these elements.