Precipitation with a pH below 5.6, caused primarily by emissions of sulfur dioxide and nitrogen oxides from fossil fuel combustion. Acid rain damages aquatic ecosystems, forests, infrastructure, and drinking water supplies.
Acid rain (acid deposition) refers to precipitation—including rain, snow, fog, and dry particulate deposition—with a pH significantly below the natural background of approximately 5.6 (the pH of water in equilibrium with atmospheric CO₂). The primary chemical precursors are sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) emitted by coal-fired power plants, industrial facilities, and vehicles, which react with water vapor in the atmosphere to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Acid rain severely impacts freshwater ecosystems by lowering the pH of lakes and streams, mobilizing toxic aluminum from soils into waterways, and reducing the reproductive success of fish and amphibians. The northeastern United States, Scandinavia, and parts of Eastern Europe experienced severe acidification impacts from the 1960s through 1990s. Regulatory actions, notably the U.S. Clean Air Act Amendments of 1990 and the European Convention on Long-range Transboundary Air Pollution, have dramatically reduced SO₂ emissions and acid deposition in North America and Europe. Many affected water bodies are slowly recovering, though recovery may take decades due to depleted soil buffering capacity. Acid rain remains a significant concern in rapidly industrializing nations, particularly China and India, where coal combustion continues to increase.