The influence and importance of daily weather conditions in the supply of chloride, sulphate and other ions to fresh waters from atmospheric precipitation

Abstract

This paper deals with chemical analyses for hydrogen, sodium, potassium, calcium, chloride, sulphate and nitrate ions in daily precipitation samples collected in the English Lake District, and examines the correlations of these analyses with ( a ) one another, ( b ) the amount of precipitation, ( c ) wind direction, ( d ) wind velocity, and ( e ) temperature. Highly significant correlations are found between estimates for H, K, Ca, NO ₃ and SO ₄ ions, and atmospheric pollution appears to supply much of the sulphate in Lake District rain, since it comes largely as sulphuric acid from the south-east. The correlation between Na and Cl is also extremely close, with the proportion between the two ions very similar to that in sea water. Since these two ions are supplied mainly from the south-west, in amounts which are greatly increased by high winds, sea spray may be inferred as the principal source. Nevertheless, evidence is presented for some small supply from atmospheric pollution, and it is furthermore suggested that mixed droplets of sea salt and sulphuric acid may, upon abstraction of moisture by freezing or evaporation, release gaseous hydrogen chloride to the air and so alter the balance of Na and Cl in precipitation. The supply of Na and Cl is also considerably higher in cold than in warm weather, a phenomenon possibly connected with stormier conditions and greater evaporation over the Atlantic Ocean. The atmospheric contribution of ions to natural waters via precipitation is shown to be of great importance, in particular to bog pools, to upland tarns on hard rocks, and to the soil solution in highly organic mor humus layers overlying heavily leached hillside soils. Sodium and chloride in most surface waters appear to be almost wholly supplied by sea spray, while calcium and magnesium bicarbonates are the main products of soil weathering. A net loss of rain acids is evident in all soil waters, although some may be more acid than normal rain water owing to considerable concentration by evaporation. The soil waters are relatively rich in potassium, presumably supplied from decaying vegetation.