Study of acid deposition in Switzerland: Temporal variation in the ionic composition of wet precipitation at rural sites during 1983–1984

A study of the chemical characteristics of wet precipitation was carried out in central Switzerland. Wet-only samples were collected weekly at three sites during 1983 and 1984. The results were examined for spatial and temporal variations in the concentrations of major cations and anions and also compared with similar results from two other Swiss stations, one in the eastern part of the country and one at an alpine location. At the sites not under the direct influence of urban emissions, concentrations of sulphate, nitrate and chloride were very similar at altitudes of 515 and 950 m above sea level. Volume-weighted average concentrations observed were lower than at most other European sites used for comparison. Between 950 and 3500 m above sea level, concentrations decreased, but much less in the case of sulphate than in the case of nitrate. At sites surrounded by agriculture, most nitrate and sulphate was associated with ammonium, and not with hydrogen, ions. Anion acids were enriched in precipitation collected downwind of urban areas. The temporal variation in the concentration of sulphate and nitrate did not depend on location. It was determined by precipitation volume and seasonal factors; for instance, the increase in air temperature in spring, which was associated with an increase in sulphate. The most acidic events occurred during spring and summer months. They were characterised by an increase in the nitric acid content. Possible relationships between the variation in precipitation acidity and meteorological conditions are discussed.