Natural Causes of Acid Rain

Acid rain is precipitation with a pH of less than 5.6. Its principal constituents are nitric and sulfuric acids formed by chemical reactions of nitrogen and sulfur oxides with water vapor in the atmosphere. Although associated mostly with industrial, vehicle and power generation emissions, these and other gases that react with water to make acid rain are also released by natural phenomena such as volcanic eruptions and biological processes.

Atmospheric Composition

The pH of rainwater in unpolluted regions is 5.6, making it a weakly acidic solution. A neutral solution has a pH of 7. Rainwater acidity is caused by the presence of carbon dioxide, nitric oxide and sulfur dioxide in the atmosphere. Carbon dioxide concentrations in the atmosphere are 335 parts per million compared with 0.01 ppm for nitric oxide and up to 0.01 ppm for sulfur dioxide. Carbon dioxide reacts with water vapor to produce carbonic acid. However, in pristine environments the natural rainwater pH may become more acidic, ranging between 4.5 and 5.6, because of the variability of naturally occurring sulfur and nitrogen oxides in the atmosphere.

Volcanic Eruptions

Volcanic eruptions release water vapor and gases such as carbon dioxide and monoxide, sulfur dioxide, hydrogen chloride, hydrogen sulfide, hydrogen fluoride, ammonia, methane and silicon tetrafluoride. These gases react with water vapor to precipitate as acid rain. Hydrochloric and hydrofluoric acids, together with other fluoride and chloride compounds, which build up in large concentrations in the vicinity of the eruption area, are poisonous to plants and animals, cause skin and eye irritations and can damage machinery. Sulfur dioxide from a volcano, together with aerosols, can rise 19 to 32 kilometers (12 to 20 miles) into the atmosphere and cause a global cooling effect by blocking solar radiation.

Lightning Strikes

Lightning is one of the major sources of localized atmospheric nitrogen, sulfur and carbon oxides. Lightning bolts reach temperatures of about 30,000 degrees Celsius (54,000 degrees Fahrenheit) and can transmit a current of up to 20,000 amps. This is powerful enough to cause nitrogen and oxygen in the atmosphere to react and create nitrogen oxide and nitrogen dioxide. The latter dissolves in water vapor to precipitate as weak nitric acid. Lightning also triggers wildfires that emit carbon and sulfur oxides into the atmosphere that in turn dissolve in water vapor and precipitate as carbonic and sulfuric acids.

Emissions from Vegetation

Terrestrial vegetation produces carbon dioxide as part of the transpiration process and when rotting on the ground. Aquatic plants and bacteria that exist in anaerobic -- oxygen-free -- conditions emit methane. Tropical and temperate forests emit isoprene, a biogenic volatile organic compound. Isoprene is oxidized in the troposphere and reacts with nitrogen oxides to produce ozone, organic nitrates and organic acids such as methacrylic and acrylic acids. These acids contribute to the acidification of rainwater.