Acid rain

Acid rain is rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions (low pH). Most water, including drinking water, has a neutral pH that exists between 6.5 and 8.5, but acid rain has a pH level lower than this and ranges from 4–5 on average. The more acidic the acid rain is, the lower its pH is. Acid rain can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce sulfuric and nitric acids.

Acid rain has been shown to have adverse impacts on forests, freshwaters, soils, microbes, insects and aquatic life-forms. In ecosystems, persistent acid rain negatively affects tree bark, leaving trees more susceptible to environmental stressors such as drought, fire, solar radiation, frost and pest infestation. Acid rain is also capable of detrimenting soil composition by stripping it of nutrients such as calcium and magnesium which play a critical role in plant growth and maintaining healthy soil. In terms of human infrastructure, acid rain also causes the peeling of paint, corrosion of steel structures, and weathering of stone buildings and statues as well as having impacts on human health.

Some governments, including those in Europe and North America, have made efforts since the 1970s to reduce the release of sulfur dioxide and nitrogen oxide into the atmosphere through air pollution regulations such as the US Clean Air Act and the EU Ambient Air Quality Directive.