EGEE 102
Energy Conservation for Environmental Protection

Acid Deposition


Sunlight increases the rate of most of the SO2 and NO reactions. The result is a mild solution of sulfuric acid and nitric acid. "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:
    • the acidity of the water;
    • the chemistry and buffering capacity of the soils involved;
    • 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.
    • Acidic particles and gases are blown by the wind 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.

Process of Acid Deposition

Prevailing winds blow the compounds that cause both wet and dry acid deposition across state and national borders, and sometimes over hundreds of miles. Please watch the 1:22 presentation below to learn more about the process of acid deposition.

Acid Deposition
Click Here for Transcript of Acid Deposition video

In this diagram, we are seeing how the acid deposition occurs. When the sources emit pollutants such as SO2, NOx, mercury, and volatile organic compounds, primarily SO2 and NOx, which are acidic gases, are deposited in two ways. One is dry deposition, the other one is wet deposition. The SO2 and NOx when they deposit back either gaseous pollutants or as particulates, it's called dry deposition. When these pollutants dissolve in water, cloud water, and then deposit, it's called wet precipitation. Or that is what we call acid rain. The dry, gaseous pollutants or particulate matter can sometimes get dissolved in water and come down again as wet precipitation. Receptors are the species that receive this acid rain and get affected. These receptors can be materials that we care about, or aquatic life, human beings, or lakes and streams.