the shade-tobacco industry. On the other hand, the production of spinach declined sharply, in part because the plants available were so susceptible.
Aside from the importance of such work in its own right, the investigations with plants may provide information about the subtle biochemical interactions of pollutants that would be much more difficult to discover initially with animals, but once known or suspected, could be effectively sought.
The interaction between sulfur dioxide and the photochemical oxidants supports the view that under various circumstances, interactions within the biological system negate analysis based on a model of “one agent, one effect.” Furthermore, the two (or more) agents are likely in this case to come from two different sources—sulfur dioxide from an electric utilities plant, photochemical oxidants from vehicular traffic. The apportionment of responsibility, therefore, may likewise involve interactions.
This section is concerned with the effects of energy systems on precipitation, climate, and water supply. As dealt with here, these tend to be very large-scale problems, affecting regions of the nation or even the entire world.
The formation of acids in the atmosphere from combustion-generated sulfur dioxide (SO2) and nitrogen dioxide (NO2) acidifies rain and snow.161–163 It was estimated that about 60 percent of the effect 10 years ago was due to acid-sulfate aerosols, and 40 percent to acid-nitrate aerosols. During the past 10 years, however, the nitric acid moiety has become relatively more important, presumably because of the increased use of low-sulfur fuels. Best available control technology (cf. chapter 4) will continue for some years to reduce sulfur oxide emissions relative to those of nitrogen oxides.
The distribution pattern of acid rain, determined by meteorological conditions, may extend for many hundreds of miles from the source, as first dramatically demonstrated in Sweden and Norway, where the effects were attributed to emissions from central Europe and the United Kingdom. In the United States, the Northeast is the focal area, and the pH of precipitation now averages annually about 4–4.3, with values as low as 2–3 observed at particular locations during storms. To the west and northwest, the pH gradient rises (to fall eventually in certain restricted areas), and in the desert regions, the pH averages about 7 (neutrality). The