What is the principal regulatory mechanism established by the Clean Air Act to control acid rain?

The principal regulatory mechanism established by the Clean Air Act to control acid rain is the Title IV Acid Rain Program, which uses a cap-and-trade system to reduce emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from power plants. Acid rain is caused by the release of SO2 and NOx into the atmosphere, primarily from the burning of fossil fuels, especially coal, by power plants. These pollutants react with water, oxygen, and other chemicals in the atmosphere to form sulfuric acid and nitric acid, which then fall to the earth as acid rain. The Title IV Acid Rain Program, established in 1990 amendments to the Clean Air Act, set a cap on the total amount of SO2 that power plants could emit nationwide. The program allocated allowances to power plants, authorizing them to emit a certain amount of SO2. Power plants that reduced their emissions below their allocated level could then sell their excess allowances to other power plants that were exceeding their limits. This cap-and-trade system created a market incentive for power plants to reduce their SO2 emissions. The program also set limits on NOx emissions from power plants, although these limits were primarily achieved through command-and-control regulations, rather than a cap-and-trade system. The Acid Rain Program has been highly successful in reducing SO2 and NOx emissions and significantly reducing the impacts of acid rain in the United States. The program has led to a substantial decrease in acid deposition in sensitive ecosystems, such as the Adirondack Mountains and the Appalachian Mountains, and has improved the health of lakes and forests affected by acid rain.