much attention in recent years, both in North America (Likens et al. 1972, Likens and Bormann 1974, Cogbill and Likens 1974, Likens 1972) and in Europe (Bolin et al, 1971, Oden 1968, Brosset 1973).¹ The acidity of rainfall is not only of importance in itself, because of its known or suspected effects on biological systems (Chapter 5), but can give indirect information—otherwise lacking-about the acidity of the suspended sulfates.

The occurrence of sulfates in rain and snow must be interpreted in the context of the sulfur cycle in nature (Kellogg et al, 1972, Rodhe 1973, Robinson and Robbins 1968, Eriksson 1963, Bolin et al. 1971). Sulfur is introduced into the atmosphere by three principal routes: in spray from ocean waves (primarily in the form of neutral sulfates); by decomposition of biological materials (primarily in the form of hydrogen sulfide); and by combustion of fossil fuels (primarily in the form of sulfur dioxide). After transport and oxidation, it is returned to earth by four principal routes:

1. absorption of gaseous sulfur dioxide by the soil or vegetation;

2. deposition of sulfur dioxide in rain or snow;

3. deposition of sulfates (including sulfuric acid) in rain or snow;

4. dry deposition of particles containing sulfates.

Studies of the sulfur cycle (Kellogg et al. 1972, Rodhe 1972, Robinson and Robbins 1968, Eriksson 1963, Bolin et al. 1971) suggest that (a) and (c)—direct absorption of sulfur dioxide and deposition of sulfates in precipitation—are the most important routes of deposition. Concentrations of sulfur dioxide in precipitation appear to be generally low in comparison with those of sulfates (Miller and de Pena 1972, Dana et al. 1973), and suspended sulfates are generally on small particles which are deposited very slowly (Garland 1974).