Chemical composition of rainfall and wet deposition over northern Britain

Abstract

Results are presented for the chemical composition of precipitation sampled monthly at 16 sites in northern Britain for 1981. Distribution maps of concentration of H +, NO 3 − and non-marine SO 4 2− show gradients increasing from the north-west to the south and east (H + from 10 to 40 μeq ℓ −1, NO 3 − from 10 to 30 μeq ol −1, non-marine SO 4 2− from 15 to 60 μeq ℓ −1) but the distribution of NH 4 + was not simple. Deposition of all ions was greatest in the mountainous areas of the south and west, where rainfall amounts were largest. Concentrations of NH 4 + and NO 3 − were approximately the same in 1980and 1981, but H + and non-marine SO 4 2− concentrations were significantly smaller in 1981 than in 1980. Longer term trends (1978–1982) were site-dependent, with large variability between years. Acidity decreased at all sites over this period, but no conclusions can be drawn on the causes of the observed decrease. Seasonal variations in 1981 were also strongly site-dependent; pronounced annual cycles in concentration were observed at southeastern sites for H +, NH 4 +, NO 3 − and non-marine SO 4 2−, with maxima in late spring. Sea-derived material (Na + and Cl −) showed pronounced cycles in concentration at all sites, with maxima in autumn/winter, annual cycles in deposition depending upon the annual cycle in precipitation amount. Deposition of H +, NH 4 +, NO 3 − and non-marine SO 4 2− was greatest in the autumn at northwest sites and in the spring at southeast sites.