Even perfectly clean rain is slightly acidic, but acid rain is far worse
All rain carries a little carbonic acid from dissolved carbon dioxide, so clean rain sits just below neutral. Acid rain, loaded with sulphur and nitrogen pollution, typically falls at pH 4 to 5, and industrial fog has been measured below 2.4. A Manchester chemist named the problem in 1872.
Sulphur dioxide and nitrogen oxides react with atmospheric water to form sulphuric and nitric acids. Most of these gases come from human activity, though lightning makes nitrogen oxides and volcanoes release sulphur dioxide. As long ago as the 17th century, the diarist John Evelyn noticed London's smoky air eating into the Arundel marbles. In 1852 Robert Angus Smith linked acidic rain to pollution in Manchester, and he coined the phrase two decades later.
Scandinavians saw the problem cross borders. Waldemar Christofer Brøgger first recognised pollution drifting from Britain to Norway, Swedish scientists reported in 1955 that Swedish rain was more acidic than carbon dioxide alone could explain, and the soil scientist Svante Odén raised the alarm in newspapers and a landmark paper of 1968. Brynjulf Ottar studied long-range transport systematically in the 1970s. Systematic research began in Europe in the 1960s, North America in the 1970s and India in the late 1980s.
In the United States the key evidence came from Hubbard Brook in New Hampshire's White Mountains, where Gene Likens and colleagues measured rain at pH 4.03 in 1972; coverage in The New York Times spread public concern. Follow-up studies showed acid rain reacting with aluminium in soil and speeding weathering. In 1980 researchers deliberately acidified Norris Brook and watched species diversity fall and food webs simplify.
The damage is broad. Persistent acid rain weakens tree bark, leaving forests vulnerable to drought, frost, fire and pests, and strips soils of calcium and magnesium that plants need. It harms freshwater life, peels paint, corrodes steel and erodes stone buildings and statues. From the 1970s governments in Europe and North America cut emissions through laws like the US Clean Air Act and EU air-quality rules. America's National Acid Precipitation Assessment Program ran an 18-year study of effects on waters, land and historic monuments. These efforts, built on decades of research, have had measurable success.
Source: Acid rain
