The negative effects of air pollution have been known since at least the time of the ancient Greeks, when laws protected the quality of air, as indicated by its odor (Jacobson 2002).

Since the Industrial Revolution, air pollution has mainly been associated with urban industrial centers, power plants, and oil and gas refineries. These stationary sources of atmospheric pollutants mainly affect the areas immediately adjacent to them and are usually considered regional rather than global problems.

Fossil fuel combustion, agriculture, and urban and suburban development have influenced fluxes of N and S to an even greater degree than fluxes of C. Emissions of N and S into the atmosphere have resulted in two related environmental issues: acid precipitation and N deposition. Emissions of N and S are only a subset of the multiple types of air pollution, but they are among the most far-reaching. Sites affected by acid precipitation and N deposition now include national parks and wilderness areas (FIGURE 25.17).

FIGURE 25.17 Air Quality Monitoring in the Sierra National Forest Airsamplesare collected regularly to monitor temporal changes in air chemistry and particulates. Air quality in national parks and wilderness areas, such as the Sierra National Forest, has been compromised by emissions of pollutants, including NO_x and sulfate aerosols. These pollutants not only lower

visibility, but also pose a health hazard to the organisms that come into contact with them, including humans. View larger image