Today, many industrial countries are working to reduce emissions of greenhouse gases. One proposal is to establish a system requiring companies that create greenhouse gases to pay to emit carbon dioxide above a specified level. This payment could take several forms, including (1) purchasing the rights to pollute from a company with carbon dioxide emissions below the specified level; (2) purchasing forests, which absorb carbon dioxide, and keeping them from being developed; or (3) paying to upgrade a plant in a developing country, thus lowering that plant��s carbon dioxide emissions.

B. Acid Rain [24]

Another environmental problem is acid rain, which forms from sulfur contained in coal. As coal burns, the sulfur combines with oxygen in the air to form sulfur dioxide. As sulfur dioxide is released into the atmosphere, this compound reacts with atmospheric moisture [25], forming sulfuric acid. This acidic moisture eventually falls back to Earth in the form of precipitation [26] known as acid rain. Environmental studies indicate that acid rain damages crops and forests as well as streams, lakes, and rivers.

The U.S. Clean Air Act, implemented in 1970 and revised in 1970 and 1990, is the federal law regulating air pollution in the United States. This legislation has significantly reduced emissions of sulfur oxides, known as acid gases. For example, the Clean Air Act requires facilities such as coal-burning power plants to burn low-sulfur coal. High-grade coals (coals with a higher heating value) generally contain more sulfur than low-grade coals such as lignite and subbituminous coal. Therefore, certain processes have been developed to remove sulfur-bearing compounds from high-grade coal prior to burning. The Clean Air Act also requires use of pollution-trapping equipment such as air scrubbers [27] (devices installed inside plant smokestacks [28] to remove sulfur dioxide from coal emissions). In addition, revisions to the Clean Air Act in 1990 established a system that allows coal-burning power plants to buy and sell sulfur emission permits with one another. This system tries to establish a financial incentive [29] to lower sulfur emissions by rewarding [30] power plants that reduce emissions below federal levels. Power plants that cut their sulfur emissions below the permitted levels can sell permits to burn coal to companies that exceed federal levels. Companies that reduce emissions reap financial rewards while polluters must pay an extra cost to pollute.

C. Fly Ash [31]

The burning of coal releases ashes known as fly ash into the atmosphere. Fly ash contains toxic metals [32] such as arsenic [33] and cadmium [34]. In the United States the Clean Air Act requires that fly ash be removed from coal emissions. As a result, antipollution devices such as air scrubbers, baghouses [35], and electrostatic precipitators are used to t

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