Acid Rain

What is Acid Rain and What Causes It?

"Acid rain" is a broad term used to describe several ways that acids fall out of the atmosphere. A more precise term is acid deposition, which has two parts: wet and dry.

Wet deposition refers to acidic rain, fog, and snow. As this acidic water flows over and through the ground, it affects a variety of plants and animals. The strength of the effects depend on many factors, including how acidic the water is, the chemistry and buffering capacity of the soils involved, and the types of fish, trees, and other living things that rely on the water.

Dry deposition refers to acidic gases and particles. About half of the acidity in the atmosphere falls back to earth through dry deposition. The wind blows these acidic particles and gases onto buildings, cars, homes, and trees. Dry deposited gases and particles can also be washed from trees and other surfaces by rainstorms. When that happens, the runoff water adds those acids to the acid rain, making the combination more acidic than the falling rain alone.

Acid rain occurs when gases like sulphur dioxide (SO₂) and nitrogen oxides (NOx) react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds. Sunlight increases the rate of most of these reactions. The result is a mild solution of sulphuric acid and nitric acid.

Effects of Acid Rain

Acid rain causes acidification of lakes and streams and contributes to damage of trees at high elevations (for example, red spruce trees above 2,000 feet) and many sensitive forest soils. In addition, acid rain accelerates the decay of building materials and paints, including irreplaceable buildings, statues, and sculptures that are part of our nation's cultural heritage. Prior to falling to the earth, SO₂ and NOx gases and their particulate matter derivatives, sulphates and nitrates, contribute to visibility degradation and harm public health.

How Acid Rain Forms

Pollutants from a source of pollution drift up to the atmosphere. Once in the atmosphere, the pollutants combine with moisture to form acid rain. The acid rain is collected in a cloud and will be released when it rains. The pollutants used in this example are the two pollutants that cause acid rain: oxides of sulfur and oxides of nitrogen.