Deposition of Air‐borne Acidifiers in the Western Environment

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Abstract
Regional deposition chemistry in the western USA is reviewed and analyzed to assess the potential for acid deposition effects in the rural West. Annual average and seasonal data from NADP/NTN sites over the period 1978 to 1984 are analyzed. These data meet specific data completeness criteria to ensure a representative wet deposition data set. Acidity levels in western precipitation are typically less than one‐fifth of their values in the East. The conventional method used to calculate annual average pH values does not account for net alkalinity present in some samples. This method overestimates acidity in the West by as much as 58% on an annual basis. Concentrations and depositions of SO2−4 and NO−3 are generally one‐tenth to one‐half of eastern values. Sulfate deposition is well below that associated with adverse effects on the environment. Sulfate concentrations are highly correlated with Ca2+ concentrations and only weakly correlated with H+ concentrations; in the East the converse is true. This result suggests the importance of suspended soil dust as a mediating factor in precipitation acidity. Dry and wet deposition in the mountainous terrain containing potentially susceptible ecosystems is estimated to be roughly equal in magnitude. Deposition from interception of cloud or fog droplets is estimated to contribute significantly to total deposition only in mountains along the Pacific Coast. Regional source‐receptor relationships in the West are poorly defined due to inadequate meteorological and deposition data. Areas in which our understanding of the acid deposition phenomenon in the West is inadequate are described. Specific objectives for acid deposition research in the West are identified and prioritized.

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