The principal means of removal of sulfur oxides from the atmosphere include absorption of gaseous sulfur dioxide by the ground or by vegetation, and deposition of sulfates in rain and snow (see Chapter 7). Surveys conducted in the northeastern United States suggest that roughly 33 percent of the sulfur oxides are eventually returned to earth as sulfates in precipitation (see Table 7–2). Figure 6–1 pictures the way sulfur oxides are transported after emission, transformed into sulfates, and ultimately returned to ground.
Because sulfur dioxide is absorbed fairly rapidly by the ground, emissions from stacks are probably more important than low level emissions as a source of sulfate aerosols downwind (see Chapters 6, 7). For the same reason, ambient concentrations of sulfur dioxide measured at ground level are determined primarily by sources nearby and a short distance upwind; in contrast, ambient concentrations of sulfates are determined by sources further upwind (see Chapter 6). Accordingly, ambient concentrations of sulfur dioxide are generally greater than those of sulfates at urban stations, but are lower at some rural stations (see Chapter 6). Measurements of suspended sulfate aerosols (see Figure 6–4) and of sulfates in precipitation indicate that high levels of sulfate are dispersed very widely throughout the northeastern United States and eastern Canada (see Figure 7–1). The pattern of deposition suggests transport over distances of several hundred km downwind from the principal source areas (see Chapter 7, and Figure 7–1).
The acidity of the suspended sulfate aerosols has not been measured directly, but can be determined indirectly by measuring the acidity of precipitation (see Chapter 7). Acid precipitation is a regional phenomenon in the northeastern United States and eastern Canada, and its distribution covers roughly the same area as experiences the highest sulfate levels (see Figures 7–2, 7–4). The fraction of sulfates falling out as acid sulfates in precipitation is in some areas as much as 80 percent; its regional average is about 24 percent (see Table 7–3). About three-quarters