Sulphates and sulphuric acid in the atmosphere in the years 1971–1976 in The Netherlands

Abstract

In 1971 an orientating study was started concerning the ground-level concentrations of sulphate and sulphuric acid relative to sulphur dioxide. In order to achieve a more selective method for the determination of sulphuric acid, the separation of sulphuric acid from sulphates by isopropanol was used. After separation sulphuric acid and sulphate are determined spectrophotometrically with barium chloranilate. Air samples were taken with air samplers using 0.8 μm pore cellulose triacetate filters. In the winter 1971–1972 at Arnhem the ratio sulphate/total sulphur correlated significantly with the relative humidity and decreased with increasing SO 2 concentration. The mean value of the ratio was 12.5% at a mean SO 2 concentration of 71 μg m −3. The mean ratio H 2SO 4/total sulphur was 0.34%. In summer 1972, with a mean ratio sulphate/total sulphur of 36.8% at a mean SO 2 level of 8 μg m −3, there was a correlation between sulphate and NH 3 on days with a relative humidity >80%; this suggests an influence on the oxidation rate by NH 3. The amounts of NH 4 +, SO 4 2−, and NO 3 − give rise to the supposition that the greater part of sulphate and nitrate is present as ammonium salts. The measurements were continued at Arnhem and Rotterdam (Rijnmond area) in 1973, 1975 and 1976. In these years there was a strong decrease in SO 2 concentration with a less stronger decrease in sulphate concentration. This together with the uniform distribution of sulphates in The Netherlands indicates that the sulphate level is caused mainly by long range transport. There was no increased reactivity in the formation of sulphates in the industrial Rijnmond area as compared to the eastern part of The Netherlands. Increased H 2SO 4 concentrations occurred as a result of transport of aerosols over sea by western winds. In summer 1976 there were significant correlations between both sulphate and nitrate with ozone at all the measuring stations, from which the existence of photochemical reactions over large areas can be concluded.