Acid Rain

What is Acid Rain?
First identified in 1872 in Sweden and studied in the U.S. beginning in the 1950s, acid rain is precipitation in the form of rain, snow, hail, dew, or fog that transports sulfur and nitrogen compounds from the high atmosphere to the ground. Sulfur dioxide (SO2) and nitrogen oxides (NO, NO2) are bi-products from burning fuels in electric utilities and from other industrial and natural sources. These chemicals react with water, oxygen, carbon dioxide, and sunlight in the atmosphere to form sulfuric and nitric acids. The acids reach the ground and change the chemistry within the environment.
The acidity of any solution is determined on the pH scale of 0 to 14. A pH level of 0 to 7 is considered acidic; 7 is neural; and a level above 7 is alkaline. As the pH number decreases, acidity increases. Unopened bottled distilled water has a pH of 7, so it is neutral. In comparison, household ammonia is an alkaline with a pH of 11.5. Milk is slightly acidic with a 6.5 pH, and soft drinks, which contain phosphoric acid, have a 3.1 pH.
Although the pH scale may seem straightforward, determining the pH of “normal” rain is much more complex. When distilled water is exposed to air, an interaction with carbon dioxide increases acidity through the formation of carbonic acid, H2CO3, and the pH level falls. Many scientists agree that the normal pH of rain is a slightly acidic 5.6 because of perpetual chemical interactions in the air.
What’s more, rain pH levels can vary significantly over short distances and in a short amount of time, even during the same rainfall. Seasons, climate, and a host of other factors can also influence the acidity of rain.
Rain and snow are not the only processes that deposit sulfur and nitrogen acids from the atmosphere to the ground. These compounds are also present in gases and dry particles, which are more difficult to measure. Like wet deposition, the occurrence of “dry deposition” of acids varies in different areas, depending on distance from the emission source and climatic conditions.