Acid Rain - What Is Acid Rain?

Sulfur dioxide and nitrogen oxides are gases that occur naturally in the Earth's atmosphere. These gases react with water, oxygen, and other chemicals in the atmosphere to form various acidic compounds, including mild sulfuric acid and nitric acid. In nature, the combination of rain and these oxides is part of a natural balance that nourishes plants and aquatic life. However, when human activity increases the amount of acid-forming chemicals in the air, the results can be harmful to humans and the environment.

Acid rain is the common name applied to any form of precipitation that contains a greater than normal amount of acid. It would be more accurate to call it "acid deposition." It occurs in two ways: wet and dry. Acidic fog, hail, rain, sleet, and dew are wet deposition. Dry deposition consists of acidic aerosols, particles, and gases. About half of the acid in the atmosphere falls to earth as dry deposition.

Dry deposition introduces acidic particles and gases into water in two ways: direct deposition onto the water body surface and indirect deposition. One example of indirect deposition is snowpack melt. In cold parts of the country, pollutants are concentrated in the upper layers of the snowpack because of wet deposition during snowfall, and dry deposition during periods of clear weather. During the spring snowmelt, runoff containing large amounts of acidic particles accumulated over the winter can flow into a lake or river, causing acid shock to aquatic inhabitants. (See Figure 9.1.)

Origin of the Term "Acid Rain"

The concept of acid rain originated in 1872 when the term was first used by Robert Angus Smith, an English chemist, to describe acidic precipitation in and around the city of Manchester, England. Subsequent scientific research on acid rain was sporadic and largely focused on local problems until the late 1960s, when Scandinavian scientists began more systematic studies. Acid precipitation was not identified in North America until 1972, when scientists found that precipitation was acidic in eastern North America, especially in the northeastern United States and eastern Canada. The 1975 First International Symposium on Acid Precipitation and the Forest Ecosystem (in Columbus, Ohio) helped scientists define the acid rain problem and initiated further research.

Formation of Acid Rain

One of the main components of acid rain is sulfur dioxide. When sulfur dioxide reaches the atmosphere, it oxidizes to first form a sulfate ion. It becomes sulfuric acid when it joins with hydrogen atoms in the air and falls back to earth. The most oxidation occurs in clouds, especially in heavily polluted air where other compounds such as ammonia and ozone help catalyze (accelerate) the reaction, converting more sulfur dioxide to sulfuric acid. Not all of the sulfur dioxide, however, is converted. In fact, a substantial amount of sulfur dioxide can float up into the atmosphere, be transported to another location, and return to earth unconverted.

Nitric oxide and nitric dioxide are the other major components of acid rain. Like sulfur dioxide, these nitrogen oxides rise into the atmosphere and are oxidized in clouds to form nitric acid. These reactions are also catalyzed in heavily polluted clouds where iron, manganese, ammonia, and hydrogen peroxide are present.

Figure 9.2 illustrates how sulfur and nitrogen oxides, as well as hydrocarbons, are carried into the air to become acid deposition. Gases and particulate matter are carried into the atmosphere where they mix with moisture and other pollutants to form dry and wet acid deposition. Wet deposition returns to earth as precipitation, which enters the water body directly, percolates through the soil, or becomes runoff to nearby water FIGURE 9.1
What causes acid rain? SOURCE: "What Is Acid Rain and What Causes It?" in Clean Air Markets—Environmental Issues: Acid Rain, U.S. Environmental Protection Agency, January 2005, http://www.epa.gov/airmarkets/acidrain/ (accessed April 14, 2005) bodies. Dry deposition builds up over time on all dry surfaces to be transported to water bodies in runoff during periods of precipitation, or falls directly onto a water surface.

Measuring Acid Rain

The acidity of any solution is measured on a pH (potential hydrogen) scale, numbered from zero to fourteen, with a pH value of seven considered neutral. Values higher than seven are considered more alkaline or basic (the pH of baking soda, a mild alkali, is eight); values that are lower than seven are considered acidic (the pH of lemon juice, an acid, is two). Pure, distilled water has a pH level of seven. The pH scale is a logarithmic measure. This means that every pH drop of one is a tenfold increase in acid content. Therefore, a decrease from pH six to pH five is a tenfold increase in acidity; a drop from pH six to pH four is a hundredfold increase in acidity; and a drop from pH six to pH three is a thousandfold increase. (See Figure 9.3.)

Normal rainfall has a pH value of 5.65. It is not pure because it accumulates naturally occurring sulfur oxides and nitrogen oxides as it passes through the atmosphere. In comparison, acid rain has a pH of about four. "Normal" pH for freshwater streams and lakes in the United States is about six. The introduction over time of large volumes of acid deposition into unbuffered water bodies (mostly freshwater systems) can increase natural acidity as much as a hundredfold. Buffers are substances in the soils or water that offer resistance to changes in pH. When levels of alkaline chemicals that neutralize the acid rain are low in the soil or water, acid deposition directly affects the surface water pH.