Acid rain is precipitation containing an unusually high concentration of hydrogen ions, making it significantly more acidic than normal rain. This phenomenon, more accurately called acid deposition, includes all forms of precipitation such as rain, snow, fog, and dry particles or dust that settle out of the atmosphere. Acidity is measured using the pH scale, a logarithmic scale where lower numbers indicate a higher degree of acidity. While water is neutral, airborne chemicals cause precipitation to become acidic.
How Acid Rain Forms
The formation of acid rain begins with the emission of specific gaseous compounds into the atmosphere, primarily sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)). These precursor pollutants are largely generated by the combustion of fossil fuels, such as coal and oil, in industrial settings like electric power generators and vehicle exhaust.
Once released, these gases are transported by wind and air currents, often traveling long distances from their source. In the atmosphere, sulfur dioxide and nitrogen oxides undergo complex chemical reactions with water, oxygen, and other substances. This process oxidizes the pollutants, transforming them into strong acids: sulfuric acid (\(\text{H}_2\text{SO}_4\)) and nitric acid (\(\text{HNO}_3\)).
These newly formed acids return to the Earth’s surface through two main mechanisms, known as acid deposition. Wet deposition occurs when the acids are dissolved in rain, snow, or fog. Dry deposition involves acidic particles and gases settling onto surfaces like buildings, vegetation, and water bodies without moisture.
Determining Acidity Levels
Acidity is quantified using the pH scale, which ranges from 0 to 14, with a pH of 7 representing a neutral substance. Substances below pH 7 are acidic, and those above 7 are basic. Normal, unpolluted rain is naturally slightly acidic, possessing a pH of approximately 5.6.
This natural acidity is due to atmospheric carbon dioxide dissolving into the water to form a weak carbonic acid. Precipitation is considered acid rain when its pH falls below this 5.6 threshold; typical acid rain registers pH values between 4.0 and 5.0.
Because the pH scale is logarithmic, a drop of one full unit represents a tenfold increase in acidity. For example, rain with a pH of 4.0 is ten times more acidic than rain at pH 5.0.
Consequences for the Environment and Human Structures
The ecological effects of acid rain are most apparent in sensitive aquatic environments, such as lakes and streams. The influx of acid lowers the water’s pH, which can be toxic to fish and other organisms, particularly during vulnerable life stages like egg hatching. Low pH levels also cause aluminum to leach from soil particles into the water, where high concentrations can be fatal to fish by clogging their gills.
Acid deposition also harms terrestrial environments by stripping essential nutrients, such as calcium and magnesium, from the soil. Nutrient-poor soil weakens trees and plants, making them more susceptible to disease, insects, and cold weather. Acidic fog and clouds at high elevations can also directly damage tree foliage.
Acid rain causes measurable damage to human structures and infrastructure. Buildings, statues, and monuments made from materials like limestone and marble are vulnerable because they contain calcium carbonate. The acids react with the calcium carbonate, causing the stone to dissolve, which leads to roughened surfaces and the loss of carved details. Acid rain also accelerates the corrosion of metals, including steel, bronze, and copper, compromising the integrity of bridges and other infrastructure.