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Unit 11: Atmospheric Pollution // Section 7: Acid Deposition

Acid rain was first identified in the 19th century, when English pharmacist Robert Angus Smith measured high acidity levels in rain falling over industrial regions of England and much lower levels in less-polluted areas near the coast. However, this pattern did not receive sustained attention until biologists began to notice sharp declines in fish populations in lakes in Norway, the northeastern United States, and Canada in the 1950s and 1960s. In each case researchers found that acid precipitation was altering lake chemistry. These findings spurred research into the causes of acid rain.

Pure water has a pH value of 7 (neutral), but rainwater falling in the atmosphere always contains impurities. The atmosphere contains natural acids including CO2 (a weak acid); nitric acid produced naturally from NOx emitted by lightning, fires, and soils; and sulfuric acid produced by the oxidation of sulfur gases from volcanoes and the biosphere. It also contains natural bases, including ammonia (NH3) emitted by the biosphere and calcium carbonate (CaCO3) from suspended soil dust. CO2 alone at natural levels (280 parts per million volume) would result in a rain pH of 5.7. Taken together, natural contaminants produce natural rain with pH values ranging from about 5 to 7 (recall that the pH scale is logarithmic, so one pH unit represents a factor of 10 difference in acid H+ concentration).

Acid rain refers to precipitation with pH values below 5, which generally happens only when large amounts of manmade pollution are added to the atmosphere. As Figure 10 shows, acid deposition takes place throughout the eastern United States and is particularly severe in the industrial Midwest due to its concentration of coal-burning power plants. Tall power plant stacks built to protect local air quality inject SO2 and NOx at high altitude where winds are strong, allowing acid rain to extend more than a thousand miles downwind and into Canada.

Hydrogen ion concentration

Figure 10. Hydrogen ion concentration
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Source: Courtesy National Atmospheric Deposition Program/National Trends Network,

The main components of acid rain worldwide are sulfuric acid and nitric acid. As discussed above in Section 3, these acids form when SO2 and NOx are oxidized in the atmosphere. Sulfuric and nitric acids dissolve in cloudwater and dissociate to release H+:

HNO3 (aq) → NO3 - + H+
H2SO4(aq) → SO4 2- + 2H+

Human activity also releases large amounts of ammonia to the atmosphere, mainly from agriculture, and this ammonia can act as a base in the atmosphere to neutralize acid rain by converting H+ to the ammonium ion (NH4 +). However, the benefit of this neutralization is illusory because NH4 + releases its H+ once it is deposited and consumed by the biosphere. The relatively high pH of precipitation in the western United States is due in part to ammonia from agriculture and in part to suspended calcium carbonate (limestone) dust.

Acid rain has little effect on the environment in most of the world because it is quickly neutralized by naturally present bases after it falls. For example, the ocean contains a large supply of carbonate ions (CO3 2-), and many land regions have alkaline soils and rocks such as limestone. But in areas with little neutralizing capacity acid rain causes serious damage to plants, soils, streams, and lakes. In North America, the northeastern United States and eastern Canada are especially sensitive to acid rain because they have thin soils and granitic bedrock, which cannot neutralize acidity.

High acidity in lakes and rivers corrodes fishes' organic gill material and attacks their calcium carbonate skeletons. Figure 11 shows the acidity levels at which common freshwater organisms can live and reproduce successfully. Acid deposition also dissolves toxic metals such as aluminum in soil sediments, which can poison plants and animals that take the metals up. And acid rain increases leaching of nutrients from forest soils, which weakens plants and reduces their ability to weather other stresses such as droughts, air pollution, or bug infestation.

Acid tolerance ranges of common freshwater organisms

Figure 11. Acid tolerance ranges of common freshwater organisms
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Source: Courtesy United States Environmental Protection Agency.

In addition to making ecosystems more acidic, deposition of nitrate and ammonia fertilizes ecosystems by providing nitrogen, which can be directly taken up by living organisms. Nitrogen pollution in rivers and streams is carried to the sea, where it contributes to algal blooms that deplete dissolved oxygen in coastal waters. As discussed in Unit 8, "Water Resources," nutrient overloading has created dead zones in coastal regions around the globe, such as the Gulf of Mexico and the Chesapeake Bay. The main sources of nutrient pollution are agricultural runoff and atmospheric deposition.

Acid rain levels have decreased and acid rain impacts have stabilized in the United States since SO2 and NOx pollution controls were tightened in 1990 (see Section 12, "Major Laws and Treaties," below). However, acid deposition is in large part a cumulative problem, as the acid-neutralizing capacity of soils is gradually eroded in response to acid input, and eventual exhaustion of this acid-neutralizing capacity is a trigger for dramatic ecosystem impacts. Continued decrease in acid input is therefore critical. Figure 12 compares nutrient cycling in a pristine Chilean forest and in a forest impacted by acid deposition.

Impact of acid rain on forest nutrient cycles

Figure 12. Impact of acid rain on forest nutrient cycles
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Source: © Martin Kennedy, University of California-Riverside.

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