Error analysis applied to indirect methods for precipitation acidity

Abstract

The historical increase in the acidity of precipitation in northern Europe has been monitored by direct measurements of pH and titrations of acidity. Indirect methods such as conductivity and charge balances as well as chemical source models have been used to confirm the measured acidities. In the absence of historical records of direct measurements of acidity in precipitation in the United States, indirect methods have been applied to available data on the composition of rainwater to quantify the acidity. Unfortunately, the indirect methods are sensitive to small errors in certain pH regions. The effects of random errors in major ion concentrations as well as the systematic errors due to the exclusion of trace metal and organic ions are shown to be largest around pH 5.6 in the charge balance and chemical source strength calculations and largest around pH 6.1 for the conductivity balance method. Estimates of the uncertainty in historical mean values of precipitation ion concentrations are used to calculate upper and lower bounds on the acidity of rainwater in the United States before 1970. The trend of increasing acidity of precipitation in the northeastern United States is still evident after applying error analysis.

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