Acid Rain - What Is Acid Rain?

Acid rain is the common name for acidic deposits that fall to Earth from the atmosphere. The term was coined in 1872 by English chemist Robert Angus Smith to describe the acidic precipitation in Manchester, England. Today scientists study both wet and dry acidic deposits. Although there are natural sources of acid in the atmosphere, acid rain is primarily caused by emissions of sulfur dioxide (SO₂) and nitrous oxide (N₂O) from electric utilities burning fossil fuels, especially coal. These chemicals are converted to sulfuric acid and nitric acid in the atmosphere and can be carried by the winds for many miles from where the original emissions took place. (See Figure 7.1.)

Wet deposition occurs when the acid falls in rain, snow, or ice. Dry deposition is caused by very tiny particles (or particulates) in combustion emissions. They may stay dry as they fall or pollute cloud water and precipitation. Moist deposition occurs when the acid is trapped in cloud or fog droplets. This is most common at high altitudes and in coastal areas. Whatever its form, acid rain can create dangerously high levels of acidic impurities in water, soil, and plants.

Measuring Acid Rain

The acidity of any solution is measured on a potential hydrogen (pH) scale numbered from zero to 14, with a pH value of seven considered neutral. Values higher than seven are considered more alkaline or basic (the pH of baking soda is eight); values lower than seven are considered acidic (the pH of lemon juice is two). The pH scale is a logarithmic measure. This means that every pH change of one is a ten-fold change in acid content. Therefore, a decrease from pH seven to pH six is a ten-fold increase in acidity; a drop from pH seven to pH five is a 100-fold increase in acidity; and a drop from pH seven to pH four is a 1,000-fold increase. (See Figure 7.2.)

Pure, distilled water has a neutral pH of seven. Normal rainfall has a pH value of about 5.6. It is slightly acidic because it accumulates naturally occurring sulfur oxides (SO₅) and nitrogen oxides (NO₅) as it passes through the atmosphere. Acid rain has a pH of less than 5.6.

Figure 7.3 shows the average rainfall pH measured during 2002 at various field laboratories around the country by the National Atmospheric Deposition Program, a cooperative project between many state and federal government agencies and private entities. Rainfall was most acidic in the Mid-Atlantic states, particularly New York, Pennsylvania, Maryland, Ohio, West Virginia, and in portions of western Virginia, North Carolina, eastern Tennessee, and Kentucky. Unfortunately, the areas with lowest rainfall pH contain some of the country's most sensitive natural resources—the Appalachian Mountains, Adirondack Mountains, Chesapeake Bay, and Great Smoky Mountains National Park.