Acid Rain Program

The United States' Acid Rain Program, established in the 1990s, is an environmental policy designed to efficiently reduce air pollution, specifically sulfur dioxide (SO₂) and nitrogen oxides (NO_x), which are primary contributors to acid rain. These pollutants are mainly released from the combustion of fossil fuels by power plants, vehicles, and industrial facilities. Acid rain is precipitation with a pH of 5.2 or below, and it can harm ecosystems by lowering the pH of surface waters, reducing biodiversity, weakening trees, and depleting soil nutrients.123

The Acid Rain Program utilized an emissions trading system, a cap-and-trade mechanism, to achieve significant reductions in SO₂ emissions. This approach set a limit on total emissions and allowed polluters to buy and sell emission allowances, creating a market price for pollution and incentivizing cost-effective reductions. Amendments to the Clean Air Act in 1990 mandated a halving of SO₂ emissions within two decades. By 2010, power plants in the program had reduced their SO₂ emissions to well below the set cap, and acid rain in North America was largely brought under control.1