In 1990, Congress created a cap-and-trade programme to cut acid-rain pollution from power plants; over decades, SO₂ emissions fell by more than 95%

Acid rain was one of the most significant environmental issues in the United States in the late twentieth century. Sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from fossil-fuel power plants travelled through the atmosphere and later fell as acid rain.

Raccontata datimesofindia.indiatimes.com