Acid Rain and Acid Deposition

Acid rain is rain that is unnaturally acidic. This results from reactions with pollutive, acid-forming gases, such as sulfur dioxide and nitric oxides. Lakes, forests, soils, and human structures in the eastern part of the United States and southeastern Canada have been damaged by acid rain and deposition of sulfuric and nitric acid aerosol on terrestrial objects.

Definition and Cause

Acid rain is rain that is unnaturally acidic. Typically, this is because rainwater has reacted with acid-forming pollutive gases. The acidity of rain is measured in pH units, which are the negative logarithmic values of the concentrations of hydrogen ions in solution. Pure water, which is neutral, has a pH of 7, reflecting the natural concentration of hydrogen ions in pure water of 1 × 10^-7 moles per liter; any solution with a pH greater than 7 is basic, or alkaline, and any solution with a pH less than 7 is acidic. The lower the pH, the more hydrogen ions there are and the more acidic the solution is. The natural acidity of rain is determined by its reaction with carbon dioxide gas in the atmosphere, a reaction that produces carbonic acid. Carbonic acid partly dissociates to produce hydrogen ions and bicarbonate ions. As a result of this reaction, pure rain should be moderately acidic, with a pH of 5.7. Any rain with a pH less than 5.7 is called “acid rain” and has reacted with acidic atmospheric gases other than carbon dioxide. Reaction of water with sulfur dioxide, for example, produces sulfuric acid in rain, and reaction of water with nitrogen dioxide produces nitric acid in rain. In some cases, acid rain has been observed to have a pH value as low as 2.4, which is as acidic as vinegar.

In addition to acid rain, there is “dry deposition,” which occurs without rain and deposits acidic nitrate and sulfate particles and sulfuric and nitric acid aerosols from the atmosphere. The acidic particles are trapped by vegetation or settle out, and the gases are taken up by vegetation. “Acid deposition” usually refers to dry deposition of acids.

Acid rain was first recognized in Scandinavia in the early 1950s. It was discovered that acid rain (with a pH from 4 to 5) came from winter air masses that were carrying pollution into Scandinavia from industrial areas in Central and Western Europe. Rain became more acidic over the next twenty years, and the area of Europe receiving acidic rains increased. By the mid-1970s, most of northwestern Europe was receiving acid rain with a pH of less than 4.6. As a result of the discovery of acid rain in Europe, scientists began measuring the acidity of North American rain. Initially, around 1960, acid rain was concentrated in a bull’s-eye-shaped area over New York, Pennsylvania, and New England. By 1980, however, most of the United States east of the Mississippi River and southeastern Canada was receiving acid rain (pH less than 5.0), and the central bull’s-eye was receiving very acidic rain, having a pH less than 4.2. The greatest increase in acidity of rain was in the southeastern United States.

The primary cause of acidity in US and European rains is sulfuric acid, which comes from pollutive sulfur dioxide gas produced by the burning of sulfur-containing fossil fuels, particularly coal, but also oil and gas. In the United States, much of the sulfur dioxide gas is produced in the industrial area of the Midwest. However, sulfur dioxide gas—and the resulting sulfuric acid—can be transported for a distance of 800 kilometers to the northeast by the prevailing winds in the atmosphere before precipitating as acid rain in the northeastern United States and southeastern Canada. To reduce the acidity of rain in the East, then, the emissions of sulfur dioxide gas in the Midwest would have to be reduced. Another source of sulfur dioxide gas is smelters that process ores, such as that in Sudbury, Ontario, located north of Lake Huron in Canada, which is one of the largest sources of sulfur emissions in the world. This smelter’s high exhaust stack spreads sulfuric acid aerosols over an extensive area hundreds of kilometers downwind. The original intent of building high exhaust stacks was to reduce local air pollution, but the net effect has been to spread the pollution over much larger areas. Acid rains are even found in Alaska, where sulfuric acid particles have been transported from the contiguous United States.

Nitric acid is a secondary cause of acid rain (contributing about 30 percent of the acidity), but it is one that is increasing. Nitric acid comes from the nitrogen oxide gases, nitrogen dioxide, and nitric oxide, which are produced by the burning of fossil fuels. In contrast to sulfur dioxide, 40 percent of the pollutive nitrogen oxide gas comes from vehicles and most of the remainder from power and heating. The production of nitrogen oxides therefore tends to be concentrated in urban areas. Nitric acid is an important component of acid rain in Los Angeles, for example, because air pollution from vehicle exhaust tends to become trapped in this area.

Some acid rain results from natural causes. Reduced sulfur gases, such as hydrogen sulfide and dimethyl sulfide, are produced by organic matter decay and converted to sulfur dioxide and sulfuric acid in the atmosphere. This process results in naturally acid rain. Volcanoes are another natural source of sulfur dioxide gas. Nevertheless, about 75 percent of the sulfur dioxide gas produced in the United States comes from the burning of fossil fuels. Naturally acid rain (with a pH less than 5.5) is uncommon, falling chiefly in remote areas such as the Amazon basin and some oceanic areas.

There are natural factors that work to reduce or neutralize the acidity of rain in certain areas. Windblown dust, particularly that containing limestone particles, tends to make rains in arid areas of the western United States less acidic by reacting with the acid to produce a rainwater solution with a pH of 6 or more. In addition, the presence of ammonia gas produced in agricultural areas by animal waste, fertilizers, and the decomposition of organic matter will reduce or counteract the acidity of rain on a local scale.

Effects of Acid Rain and Acid Deposition

The detrimental effects of dry deposition and acid rain include the corrosion and chemical erosion of structures and buildings made of susceptible materials, changes in soil characteristics, increases in the acidity of lakes, and other biological effects, particularly in high-altitude forests. The corrosive effects of airborne acids are particularly obvious on limestone, a rock composed of calcium carbonate, which reacts easily with acid rain. In many New England cemeteries, tombstones made of marble, a form of limestone, have been badly corroded, although older tombstones made of slate, which is less affected by acid rain, are intact. Limestone components of buildings and other structures are similarly corroded.

The effect of acid rain on soils depends on their composition. Alkaline soils that contain limestone can neutralize acid rain. Even in soils that do not contain limestone, several processes operate to neutralize acid rain, though such processes invariably alter the chemical nature of the soil. Cation exchange occurs, whereby hydrogen ions from the rainwater are exchanged for metal ions, such as calcium or magnesium, on the surface of clays and other minerals. This exchange removes excess hydrogen ions from soil solutions, rendering them less acidic. Another neutralization process involves the release of soil aluminum into solution and the accompanying uptake of hydrogen ions. This process occurs by dissolution of aluminum bound to clays and organic compounds. Frozen soils and sandy soils containing mostly quartz, which does not react with acid rain, have little ability to neutralize acid rain.

Lakes in certain areas have become acidic (with a pH less than 5) from the deposition of acid rain. Lakes in granitic terrain are most affected by acid rain because the surrounding bedrock has little or no ability to react with or neutralize the excess hydrogen ions in the water. Areas with many acid lakes include the Adirondack Mountains in New York, the Pocono Mountains in Pennsylvania, the Upper Peninsula of Michigan, Ontario, Nova Scotia, and Scandinavia. Generally, the deposition of highly acidic rain having a pH less than 4.6 over a long period of time is required. The effect is enhanced in bodies of water that are maintained by watershed drainage rather than by freshwater springs. Lake waters that tend to become acidified initially have little ability to neutralize acid rain because they are low in carbonate and bicarbonate ions, which come predominantly from limestone. Such lakes are described as being poorly buffered. (Buffering is the resistance to changes in pH upon the addition of acid or base.) The soil in the drainage area surrounding acid lakes does not neutralize acid rain adequately before it reaches the lake because of a lack of limestone and clay minerals or because the soil cover is thin or lacking altogether. In addition, some lakes, although not usually acidic, may have periods of elevated acidity due to the runoff of snowmelt, which collects acid precipitation stored in the snow. This runoff gives a sudden large pulse of highly acidic water to the lake. In certain areas, such as Florida, acid lakes result partly due to causes other than acid rain, such as the presence of organic acids produced by the decay of vegetation in poorly drained areas and nitric acid formed from nitrate-based fertilizer runoff. The gradual acidification of lakes results in the death of fish populations because of reproductive failure, as well as other changes in the organisms living in the lake. A reduction in the number of species occurs at all levels of the food chain. In some cases, snowmelt acidity has been identified as the cause of a massive, instantaneous fish kill in lakes.

Rivers are also known to become acidic. Eastern US rivers show high concentrations of sulfate and a low pH in cases where the soil cannot neutralize the acid rain it receives. Certain acid rivers are caused by acidic drainage from mine dumps rather than by acid rain. Acid rivers rich in organic matter are found in the eastern United States coastal plain and in the Amazon basin. These rivers have naturally high concentrations of dissolved organic acids.

Acid rain and acid deposition are implicated in the decline and death of certain forests, particularly evergreen forests at high elevations. These forests receive very acidic precipitation from the accumulation of clouds at the mountaintops. It is thought that acid rain does not actually kill the forests, but rather provides a stress that causes them to become less resistant and die from other causes. The actual stress provided by acid rain is still being studied. Possible stresses include loss of nutrients from soil and leaves through leaching, destruction of beneficial soil microorganisms, and increased susceptibility to frost damage during winter.

Efforts have been made to reduce the acidity of rain, particularly by controlling sulfur emissions. Power plants have been required to reduce the sulfur content of coal that they burn, thus lowering the amount of sulfur dioxide that is produced. Sulfate concentrations in rain in the northeastern United States have been reduced by this method. Nitrogen oxide emissions from cars have also been reduced through stricter emissions controls and improved engineering design for more efficient combustion. In some cases, acid lakes have been treated with limestone to temporarily neutralize their acidity, but the only permanent solution is a reduction in the acidity of the rain that they receive.

Study of Acid Rain and Acid Deposition

The acidity of rain can be measured directly by an electronic device called a pH meter. A pair of electrodes is inserted into a solution, and the electrical potential, or voltage, is measured between them. This voltage is directly related to the concentration of hydrogen ions in the solution—that is, to the acidity of the solution. To monitor and measure the acidity of rainwater, networks have been constructed to collect rain samples over large geographical areas. The acidity of rainwater over the course of the entire year must be measured because pH varies between rainfalls, both seasonally and according to whether the air masses that produce the rain have passed over significant sources of pollution. The pH of rainwater and other forms of precipitation is also measured over a period of years. In addition to the concentration of hydrogen ions, the concentration of other ions, such as sulfate from sulfuric acid and nitrate from nitric acid, is measured in the rainwater samples. Such measurements give evidence of the source of the acidity—that is, which proportion is attributable to sulfuric acid and which to nitric acid. The pH levels of samples collected over a large geographical area are plotted on a map, and contours are drawn through equal values of the pH. Such maps show which areas are receiving the most acid rain. The amount of sulfate and nitrate being deposited by rain is also plotted separately. Meteorologists also use information about a storm system’s path as it moves across the country. Such atmospheric systems transport pollutive gases from one area to another. Combining deposition patterns on maps with information about the path followed by a storm shows where the gas residues in rainwater may be coming from and suggests sources of the acidity.

Computers have been used to predict where acid rain will fall and how acidic it will be, given the sources and amounts of sulfur and nitrogen emissions, particularly from power plants and smelters, and the weather patterns. Predictions of this type require a detailed knowledge of the atmospheric chemistry by which sulfur dioxide is converted to sulfuric acid and the oxides of nitrogen are converted to nitric acid. This type of modeling is necessary to predict how much reduction in the acidity of rain in a distant area will result from a given reduction in a power plant sulfur source, for example.

The effects of acidity on soils have been the subject of study for many years. Laboratory experiments can demonstrate how soil clays and other minerals react to acid rain, including which chemical species are taken up and which are released. In addition, soil solutions and minerals are collected and analyzed from actual field areas affected by acid rain. Ideally, such an analysis should be carried out over a period of time to determine whether any changes in the soil solution chemistry are occurring. From a knowledge of the soil chemistry, it is possible to predict how long a soil can receive acid rain before it loses particular nutrients or the ability to neutralize the excess acidity.

Measuring the acidity and chemical composition of lakes in various areas over long periods and sampling their fish populations and other biota enables scientists to see increases in lake acidity and to correlate the increases with changes in the populations of affected species. In some areas, lakes have been artificially acidified so that the changes in their chemistry and biological populations can be observed. Apparently, acidic lake water inhibits reproduction in fish and other creatures, in addition to destroying the organisms that they use for food. Computer models of acid rain falling on susceptible drainage areas of lakes are made in order to predict how the drainage area reacts to acid rain and how much reduction in acid rain would be necessary to lower the acidity of the lake to the point where it would support fish. In badly affected areas such as the Adirondacks, it may be necessary to reduce the acidity by half.

To study forest decline, surveys of present forest conditions are compared with historical records for the same areas. For example, in high-elevation areas in New England and the Adirondacks, more than half of the red spruce died between 1965 and 1990. Tree rings, which record annual growth, show reduced growth in certain forests. It is known that acid rain causes changes in the soil, such as the release of aluminum, which is toxic to root tissues and so prevents the uptake of essential nutrients. In addition, acid rain causes the loss of certain nutrients from the soil, such as sodium, calcium, and magnesium. Another effect of acid rain is the reduction of the numbers of microorganisms in the soil. Yet, because acid rain from nitric acid contains nitrogen, a plant nutrient, it may fertilize the soil if there is a deficiency of soil nitrogen. One problem in studying forests receiving acid rain is determining which of the many changes occurring are contributing most to forest damage. It is often difficult to distinguish between the stresses of acid rain and other stresses, such as those caused by drought, cold, and insects. Field studies in this area may involve artificial acidification of forest environments to determine which mechanisms are important.

Principal Terms

acid deposition: the depositing of acidic materials on the ground surface through the action of precipitation

acid rain: rain composed of water having a lower-than-normal pH due to having dissolved and reacted with airborne contaminants to produce acidic materials

alkaline: having a pH greater than 7 due to a lower concentration of hydrogen (H⁺) ions than are in neutral water

bicarbonate: a negatively charged ion, as HCO₃^-, that effectively neutralizes excess hydrogen ions in natural waters, reducing acidity

cap and trade legislation: legislation that places limits on the emission of acid-producing materials, such as sulfur dioxide, while allowing emitters of excess amounts to purchase and utilize the unused allowances of those whose emissions are below the legislated limit

limestone: a rock containing calcium carbonate that reacts readily with acid rain and tends to neutralize it, being chemically eroded in the process

neutralization: the adjustment of the concentration of hydrogen ions in solution to achieve neutral pH

nitric acid: an acid formed in rain from nitric oxide gases in the air

nitric oxide gases: gases formed by a combination of nitrogen and oxygen, particularly nitrogen dioxide and nitric oxide

pH: a measure of the hydrogen ion concentration, which determines the acidity of a solution; the lower the pH, the greater the concentration of hydrogen ions and the more acidic the solution

sulfur dioxide: a gas whose molecules consist of one sulfur atom and two oxygen atoms, formed by the combustion of sulfur in the presence of oxygen

sulfuric acid: an acid formed as the primary component of acid rain by reaction of sulfur dioxide gas with liquid water in the atmosphere

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