The Acid Rain Program, established under the Clean Air Act Amendments of 1990, created a nationwide cap-and-trade system to reduce sulfur dioxide and nitrogen oxide emissions from power plants.

From 1989–1991 through 2020–2022, wet sulfate deposition fell by more than 70%, and critical-load exceedances in monitored lakes and streams dropped by 81%, signaling broad environmental recovery.

Continuous emissions monitoring provided real-time data to verify compliance and sustain program credibility over decades.

Under the allowance system, one allowance equals one ton of SO2; facilities could bank or trade unused allowances, enabling cost-effective, flexible reductions where they were most practical.

Phase I in 1995 targeted hundreds of coal-fired units in the eastern and midwestern United States, while Phase II in 2000 expanded coverage to more than 2,000 generating units, including smaller plants powered by coal, oil, and natural gas.

Nitrogen oxide regulations achieved substantial reductions through emission rate limits, even though they did not adopt a cap-and-trade framework.

Today the program remains in effect and operates alongside newer measures like the Cross-State Air Pollution Rule, continuing to cut emissions through a mix of monitoring, caps, and trading.

Results show over 95% reductions in annual SO2 and about 89% reductions in annual NOx emissions from power plants, improving air quality and public health.

The program prioritized emission outcomes over prescriptive technology mandates, using a cap with trading to meet limits and allow flexible compliance.