Acid Rain and Acid Deposition

Oceanography in the 21st Century - An Online Textbook

Acid rain is a common name for the deposition of acidic material from the atmosphere either as:

Wet deposition of acid in precipitation (rain, snow, or fog); or
Dry deposition of acidic material on dust, smoke, or other aerosols (small, microscopic particles in the air).

Acid rain was a major problem in Europe and the USA in the last few decades of the 20th century. Strong emission control laws have greatly reduced the problem in these areas. However, acid rain continues to be a major problem in some developing countries, especially China.

Here I will use acid rain to cover both types of deposition.

Sources of Acid rain

The acidic materials come from sulfur dioxide (SO₂), ammonia (NH₃), nitrogen oxides (NOx) and acidic particles emitted into the atmosphere by burning of fossil fuels in power plants and cars. In the United States, roughly 2/3 of all SO₂ and 1/4 of all NOx come from burning of fossil fuels, especially coal, in electric power plants.

Left: Coal-fired power plants emit large quantities of acid pollutants into the atmosphere, although the volume of pollutants has been decreasing as scrubbers that remove pollutants from exhaust gases have become more widely used. The image shows exhaust from the American Electric Power's Gen. James M. Gavin Plant in Chesire in Gallia County, in the Ohio Valley. It is one of the largest coal-fired power plants in Ohio. Most of the visible exhaust is condensed vapor, but the brownish haze includes acids.
From Ohio.Com of the Akron Beacon Journal, article Ohio EPA cites area for soot problems.
Right: Scrubber at base of Georgia Power's Bowen Plant removes 95% of the sulfur dioxide in the plant's exhaust gas. Click on the image to bring up a diagram of how a scrubber works from Scrubber Freshens Smokestack by Wade Rawlins, Staff Writer for the News Observer.
From Rome-News Tribune.

Acid rain occurs when these gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds. The result is a mild solution of sulfuric acid and nitric acid. When sulfur dioxide and nitrogen oxides are released from power plants and other sources, prevailing winds blow these compounds across state and national borders, sometimes over hundreds of miles.
– What is Acid Rain, EPA.

Most acid rain falls downwind of power plants. In the USA, many are located in the mid-west, and acid rain is common there and throughout the east coast. As power plant emissions are increasingly regulated, the amount of acid rainfall has decreased. Total annual emissions of SO₂ in the USA dropped from 28.8 × 10⁶ metric tons in 1978 to 17.8 × 10⁶ metric tons in 1998.

Acid rain deposition in the USA from 1983 through 1997. From: Driscoll (2001).

Acidity of Rain

Acidity of precipitation is measured in pH units, where

pH ≈ –log[H⁺]

where H⁺ is the dissolved hydrogen ion concentration in a weak solution in water. The lower the pH the more acidic the precipitation, the higher the pH the more basic the precipitation. Pure water water has a pH of 7.0, and pure rain has a ph of 5.6 because carbon dioxide dissolved in water forms a weak acid, carbonic acid, H₂CO₃.

H₂O + CO₂ –> H₂CO₃

pH scale from Environmental Protection Agency, pH Scale.

The pH of precipitation from very polluted air can be less than 2 in extreme cases. Mostly, the pH of precipitation ranges from 4.4 to 5.8.

US map of pH of rainwater measured at field sites in 2006
Acidity of precipitation measured by the National Atmospheric Deposition Program in 2006. Notice that precipitation is most acidic downwind of the large concentration of power plants in the Ohio Valley.

Effect on Soils

Neutralization:
The effect of acid rain on the environment depends greatly on the ability of soils to neutralize the acid. Limestone and other rocks and soils containing calcium carbonate are most effective. Acids react with calcium carbonate to produce neutral compounds and carbon dioxide. For example, sulfuric acid and hydrochloric acid react with calcium carbonate:

CaCO₃ + H₂SO₄ –> CaSO₄ + H₂CO₃

H₂CO₃ –> CO₂ gas + H₂O

2HCl + CaCO₃ -> CO₂ + CaCl₂ + H₂O

The extent to which soils can neutralize acid rain depends on several factors: type of soil, thickness, weather, and water flow patterns. If the ground is frozen, as in the winter, soil process cannot work, the acid is not neutralized. If the soil is mainly quartz, such as sand, it is resistant to weathering and no bases are present to neutralize acid. If the soil has very little base such as limestone, the acid is neutralized only slightly or with the passage of time, not at all.

In the slightly acidic soils in typical evergreen forests in the Northeastern U.S., Canada, and Europe, two other effects can neutralize the acid rain. The acid can be immobilized as the soil or vegetation retains the sulfate and nitrate ions (from sulfuric and nitric acids). Very deep soils have a large capacity to retain sulfate and nitrate ions.
From Virtual ChemBook, Elmhurst College: Acid Rain – Soil Interactions.

Mature forest soils are also able to neutralize the acids in rain. Such soils are acidic and rich in humic acids. Krug (1983) reports that mature soils in New England or Norwegian forests have a pH of 3.8, and they may contain as much acid as would fall in 1000 years of rain at 1m/year with a pH of 4.3. These soils are highly resistant to acid rain.

In contrast, thin alpine soils lacking carbonates and humic acid and overlying granite rocks are not able to neutralize acid rain. The same is true for disturbed forests where forest fires and logging have reduced the organic material in the soil, which also make the soil more susceptible to damage by acid rain.

Leaching of Nutrients
Acids leach nutrients from the soil. They are carried deeper into the ground or into streams, depriving plants of essential elements calcium, potassium, magnesium and trace metals. Normally, metals are attached to clay particles produced by weathering of rocks. The H⁺ anion of the acid replaces the metal ions in the clay in a process called ion exchange. The metals occur in groundwater as sulphates and nitrates. Two H⁺ anions displace one Ca²⁺or Mg²⁺ anion or two K⁺ anions.

Release of Aluminum
Aluminum in soils and rock is very insoluble if the pH is greater than 5. More acidic solutions dissolve aluminum from the soil, and the aluminum is carried into streams and lakes by runoff and groundwater.

Effect on Vegetation

Bavaria, Germany, high-altitude forest damaged by acid rain. Photo by Spitzbergler. From AccuWeather. More photos of Bavarian forest by Spitzbergler.

In some regions, especially regions where granite is close to the surface and where soils have been degraded by logging and forest fires, the soil has little ability to neutralize the acid. In these regions, acid deposition depletes the available plant-nutrient cations Ca²⁺, Mg²⁺, and K⁺, it increases the leaching of aluminum, and it increases the amount of sulfur and nitrogen in the soil. All lead to weakening of trees, leading to their death by bark beetle infestations and disease.

Some of the most dramatic effects on forests have been observed in Europe. In 1983, a survey in West Germany showed that 34% of the country's total forest is damaged by air pollution. This included about one half of the famous Black Forest. Switzerland has recorded damage to 14 % of her forest trees...

Vogelmann, measured the reproductive capacity of the forest by counting the total trees in an area. Red spruce dropped from 6,000 trees to 1,000 trees, a decline of 80 % from 1965 to 1983. Very few pine cones and young trees were found. Sugar maple tree counts dropped 84 % and beech tree counts dropped 63% over the same time period...

Acid rain or acid cloud droplets that fall on the leaves and needles of trees leaches the nutrients from them. Calcium, magnesium, and potassium ions may be removed from the leaves faster than the roots can resupply them. Acid rain in combination with ozone may damage the waxy coating on leaves and needles. This may weaken, damage them, and provide opportunities for diseases to enter the tree.
From Virtual ChemBook Elmhurst College Charles E. Ophardt Acid Rain Effects on Forests.

Effect on Aquatic Animals

When nutrients and metals, including heavy metals and aluminum, are leached from the soil, they are carried by runoff and groundwater into streams and lakes where they kill aquatic life. Aluminum dissolved by acid rain is highly toxic to many aquatic animals, especially young animals including eggs and larvae.

Not all species of animals are equally sensitive to acids and aluminum. Some fish species (such as (brown bullhead, yellow perch, golden shiner, brook trout, and white sucker) are tolerant of water with pH < 6, while others (such as Atlantic salmon, tiger trout, redbreast sunfish, bluegill, tiger musky, walleye, and alewife) cannot tolerate such waters. Most fish are killed if pH falls below 5.2 (Driskoll, 2001).

In areas where soils have little capacity to buffer acids in water, acidic precipitation can be a problem because the infiltrating acidic water can increase the solubility of metals, which results in the flushing of high concentrations of dissolved metals into surface water. Increased concentrations of naturally occurring metals such as aluminum may be toxic to aquatic organisms. Studies of watersheds have indicated that the length of subsurface flow paths has an effect on the degree to which acidic water is buffered by flow through the subsurface. For example, studies of watersheds in England have indicated that acidity was higher in streams during storms when more of the sub-surface flow moved through the soil rather than through the deeper flow paths. Moreover, in a study of the effects of acid precipitation on lakes in the Adirondack Mountains of New York, the length of time that water was in contact with deep subsurface materials was the most important factor affecting acidity because contact time determined the amount of buffering that could take place.
– US Geological Survey. Circular 1139, Ground Water and Surface Water A Single Resource. Effects of Atmospheric Deposition on the Quality of Ground Water and Surface Water.