Acid rains are produced by

1. excess emissions of NO₂ and SO₂ from burning fossil fuels
2. excess production of NH₃ by industry and coal gas
3. excess release of carbon monoxide by incomplete combustion
4. excess formation of CO₂ by combustion and animal respiration

NO₂ and SO₂ after reacting with water in atmosphere produce HNO₃ & H₂SO₄ which falls down as acid rain.