Averages of annual wet deposition data are often used as an indicator of acid amounts in the atmosphere. From the view point of statistics, the average is a meaningful estimator only for identically distributed data with specific types of probability distribution. Wet deposition data usually carry seasonal trends. To learn about year-to-year concentration changes, a more accurate summary information accommodating the trend is thus necessary. This paper suggests a quantity (index) describing the annual wet deposition of SO_4 and formulas for its calculation. The formulas are in some sense optimal in case the data can be considered independent, lognormally distributed and have a time dependent trend. Generalization of these assumptions is also discussed. The index is derived from specific features of the average applied to data with lognormal distribution. The theory is utilized to analyze a set of SO_4 precipitation concentrations observed in the Ontario region.
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