Acid Deposition: Definition and Basics
Acid Deposition (commonly known as Acid Rain) means precipitation that has acidic components, i.e. an unusually low pH level (4.2~4.4).
- Term coined in 1852 by Robert Angus Smith, a Scottish scientist who conducted rainwater experiments in industrial regions throughout Scotland and England
Acidity and pH
Acidity is defined by measuring the hydrogen ion (H+) concentration in a chemical solution. This can be determined through the following equation:
pH = –log([H+]) where [H+] is the concentration of H+
- Solutions with pH < 7: Acidic
- Solutions with pH = 7: Neutral
- Solutions with pH > 7: Basic
*Normal rain is already slightly acidic (pH of 5.6) because atmospheric CO2 is absorbed in the precipitation, which form carbonic acid in the following reaction:
- H2O + CO2 ↔ H2CO3
Causes/Pollutants of Acid Rain
The two main causes of acid deposition: sulfur oxides (SO2 or SO3) and nitrogen oxides (NOx)
When these pollutants are emitted into the atmosphere, they react with substances such as water and oxygen to form sulfuric, sulfurous, nitric, and nitrous acids, which then fall to the ground with the precipitation.
Here are the simplest reactions:
- SO2 + H2O → H2SO4 ↔ H+ + HSO4 ↔ 2H+ + SO42
- NO2 + H2O → HNO3 ↔ H+ + NO3
Natural Causes of Acid Deposition
Rotting vegetation and volcanic activity emit chemicals that cause to acid deposition.
However, the major cause of acid deposition is human activity—burning fossil fuels emits large quantities of SO2 and NOx into the atmosphere.
- Generating electricity
- Motor vehicles
- Factories and manufacturing industries
All of these activities require the combustion of fossil fuels (coal, oil, and natural gas), meaning that this heavily contributes to acid deposition: in the United States, electric power generators account for ⅔ of SO2 and ¼ of NOx, and motor vehicles account for almost 60% of NOx in the atmosphere.
Forms of Acid Rain
Acid Deposition occurs in two main forms:
Rain, snow, fog, sleet, hail containing sulfur oxides or nitrogen oxides
Dry materials being deposited to the Earth’s surface in dry periods (without precipitation)
Environmental Effects of Acid Rain
Harm to Fish and Wildlife
Acid deposition flows through soil and into lakes and streams. This causes lakes and streams to become more acidic, which has been linked to declining populations of aquatic organisms.
- pH < 5 can kill adult fish
- pH < 5.5 can kill mayflies
Harm to Plants and Trees
Acid rain causes soil acidification, which damages soil biology (e.g. some microbes cannot survive low pH).
Acid rain also leaches essential nutrients from the soil, such as calcium and magnesium, harming the growth of plants and trees.
→ This makes plants and trees more susceptible to cold temperatures, insects, and disease.
Damaging of Manmade Structures
Common materials used to make structures like buildings and statues include marble and limestone.
- Sulfur dioxide in the rain reacts with the calcium compounds in these substances to dissolve them
- This damages the surfaces of such structures and causes them to deteriorate more quickly:
- CaCO3 (s) + H2SO4 (aq) ↔ CaSO4 (s) + CO2 (g) + H2O (l)
Acid deposition also causes some metals (iron, steel, copper, bronze) to corrode more quickly.
Acid deposition contains nitrogen, contributing to nitrogen pollution in some ecosystems. This can also have an adverse impact on aquatic ecosystems, potentially causing a decrease in fish populations.
Because fossil fuel combustion is the main source of acid deposition pollutants, there must be limitations and standards placed on the release of these pollutants.
Some examples include:
- 1991 Air Quality Agreement between the United States and Canada
- 1985 Helsinki Protocol on the Reduction of Sulphur Emissions between European nations
- 1990 Clean Air Act in United States
- Helped to decrease SO2 emissions 88% between 1990–2017
“Acid Rain.” Wikipedia, Wikimedia Foundation, 6 June 2020, en.wikipedia.org/wiki/Acid_rain.
“Effects of Acid Rain.” EPA, Environmental Protection Agency, 4 May 2020, http://www.epa.gov/acidrain/effects-acid-rain.
Likens, Gene E., and Thomas J. Butler. “Acid Rain.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 19 Mar. 2019, http://www.britannica.com/science/acid-rain.
Nunez, Christina. “Acid Rain, Explained.” National Geographic, 21 Feb. 2018, http://www.nationalgeographic.com/environment/global-warming/acid-rain/.
“What Is Acid Rain?” EPA, Environmental Protection Agency, 12 May 2020, http://www.epa.gov/acidrain/what-acid-rain.
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