Sulphur and nitrogen contributions to the acidity of rain

Abstract

The dominant ions contributing to the acidity of precipitation are usually SO 4 −2, NO 3 − and NH 4 +after allowance for sea salts. In remote areas with low concentrations of SO 2 theoretical estimates suggest that the washout of the gas is unlikely to produce marked changes in the pH of rain although the presence of NH 3 enhances the final sulphate concentration. The contribution of washout of SO 2 gas and sulphate aerosol is unlikely to contribute more than 50% to the final concentration of SO 4 2− in precipitation in remote areas. The chemical composition of precipitation in remote areas shows a correlation between the ions SO 4 2−, NOP 3 − and NH 4 +. Some physical mechanisms such as the variation in composition with rain density clearly contribute to the correlation but it appears that there are chemical mechanisms correlating these ions. A similar correlation is also found in aerosols nearer source areas. The role of NH 3 is straightforward acid neutralisation but the relationship between NO 3 − and SO 4 2− is more difficult to define. It appears that the nitrate is formed after sulphate production.