What Is Acid Deposition? Causes, Types, and Environmental Impact
Acid rain is given as a popular expression for the more scientific term - acid deposition that refers to the several ways where acidity can move from the atmosphere to the surface of Earth. Acid deposition includes acidic rain and other forms of acidic wet deposition - such as sleet, hail, snow, and fog (or cloud water). Also, acid deposition includes the dry deposition of gases, acidic particles, which may affect landscapes during dry periods. Hence, acid deposition is capable of affecting landscapes and living things, which reside within them even when precipitation is not taking place.
About Acidity
Acidity is the measure of hydrogen ions (H+) concentration in a solution. The pH scale measures whether a solution is either acidic or basic. The substances are considered acidic below a pH value of 7, and every unit of pH below 7 has 10 times more H+, or 10 times more acidic, than the unit above it. For example, rainwater with a pH of 4.0 has a concentration of 100 micro equivalents of H+ per litre, whereas rainwater with a pH value of 5.0 contains a concentration of 10 micro equivalents of H+ per one litre.
Nitrogen Cycle
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Normal rainwater is defined as weakly acidic due to the absorption of carbon dioxide (CO2) from the atmosphere, which is a process that produces carbonic acid and forms organic acids that are generated from biological activity. Additionally, volcanic activity may form nitric acid (HNO3), hydrochloric acid (HCl), and sulfuric acid (H2SO4) based on the emissions associated with particular volcanoes.
The generation of nitrogen oxides from atmospheric molecular nitrogen (N2) conversion by lightning and the organic nitrogen conversion by wildfires are two other natural causes of acidification. The geographic range of any given natural source of acidification, on the other hand, is very limited, and in most instances, it only lowers the pH value of precipitation to around 5.2.
Sulfur Cycle
Sedimentary rocks that emit hydrogen sulphide gas, as well as human sources such as fossil-fuel combustion and smelters, all of which release sulphur dioxide into the atmosphere, are mentioned below as major sulfur-producing sources.
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Anthropogenic activities, specifically the burning of fossil fuels (oil, coal, natural gas) and the smelting of metal ores, are the primary causes of acid deposition. In the US, electric utilities produce around 70% of SO2 and around 20% of NOx emissions. Fossil fuels, which are burned by vehicles, account for around 60% of NOx emissions in the US. Sulfuric and nitric acids are generated in the atmosphere and react with water, when SO2 and NOx, respectively.
The acid rain chemical reaction is given below.
The simplest reactions of acid rain chemical formula or the formation of acid rain chemical equation are given as:
SO2 + H2O → H2SO4 ↔ H+ + HSO4 ↔ 2H+ + SO42
NO2 + H2O → HNO3 ↔ H+ + NO3
By the equation of acid rain given above,
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Wet deposition products are created by these specific reactions in the aqueous phase (for example, in cloud water). They can produce dry, acidic deposition in the gaseous phase. Also, acid formation can take place on particles in the atmosphere.
Where the fossil fuel consumption is large and the emission controls are not in place to reduce NOx and SO2 emissions, the acid deposition will take place in areas downwind of the emission sources, often ranging from hundreds to thousands of km away. In that type of area the pH of precipitation may average 4.0 to 4.5 annually, and the pH of individual rain events may sometimes drop below 3.0.
Ecological Effects of Acid Deposition
Effects on Lakes and Rivers
In the late 1960s and early 1970s, when the regional effects of acid deposition were first noted in parts of western Europe and eastern North America, in the chemistry, the changes of both the lakes and rivers, often in a few remote areas, were linked to declines in the health of aquatic species such as crayfish, the resident fish, including the clam populations.
Excess amounts of acid deposition in the sensitive areas caused tens of thousands of streams and lakes in both Europe and North America to become very more acidic than they had been in the previous decades. Acid-sensitive areas are the ones, which are predisposed to acidification due to either the soil regions having a low buffering capacity or a low acid-neutralizing capacity (ANC).
Effects on Forested and Mountainous Regions
Forests in southern Scandinavia, central Europe, and eastern North America displayed troubling signs of forest dieback and tree mortality in the 1970s and 1980s. In a survey conducted in 1993, 27 European countries have revealed that air pollution damage or mortality in 23% of the 100,000 trees surveyed.
Also, it is likely that the dieback was the result of several factors, including the acid deposition (for example, soil acidification and buffering capacity loss), exposure to the ground-level ozone, possible excess fertilization from the nitrogen compound deposition (such as ammonium, nitrates, ammonia compounds), and general stress, which is caused by a combination of these factors.
Once a tree is in a weakened condition, it is very likely to succumb to other environmental stressors such as insect infestation, drought, and pathogen infections. Often, the forest dieback areas were found to be associated with regions having low buffering capacity where damage to the aquatic ecosystems because of acid deposition was also taking place.
The below figure shows the spruce trees, which are damaged by acid rain in Karkonosze National Park, Poland.
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FAQs on Acid Deposition: Complete Study Guide
1. What is acid deposition?
Acid deposition is a broad term that refers to any form of precipitation with acidic components, such as sulfuric acid (H₂SO₄) or nitric acid (HNO₃), that falls to the ground from the atmosphere. It can occur in two forms: wet deposition (acid rain, fog, and snow) and dry deposition (acidic gases and particles).
2. How is acid rain chemically different from normal rain?
Normal rain is naturally slightly acidic, with a pH of about 5.6, because carbon dioxide (CO₂) in the atmosphere dissolves in it to form weak carbonic acid. In contrast, acid rain is significantly more acidic, with a pH typically between 4.2 and 4.4. This increased acidity is due to the presence of strong acids like sulfuric acid and nitric acid, which are formed from industrial pollutants.
3. What are the primary pollutants that cause acid deposition?
The primary pollutants responsible for acid deposition are sulfur dioxide (SO₂) and nitrogen oxides (NOx). The main sources of these pollutants include:
- Sulfur Dioxide (SO₂): Primarily from the burning of fossil fuels like coal in power plants and industrial smelters.
- Nitrogen Oxides (NOx): Primarily from the exhaust of vehicles and emissions from industrial furnaces.
4. What are the key chemical reactions that form the acids in acid rain?
The formation of acid rain involves atmospheric chemical reactions where primary pollutants are converted into acids. The two main reaction pathways are:
- Formation of Sulfuric Acid: Sulfur dioxide (SO₂) reacts with oxygen and water in the atmosphere to form sulfuric acid (H₂SO₄). This process can be catalysed by dust particles or other pollutants.
- Formation of Nitric Acid: Nitrogen oxides (NOx) react with water to form nitric acid (HNO₃).
5. What is the difference between wet and dry acid deposition?
The key difference lies in the form in which the acidic pollutants reach the Earth's surface.
- Wet Deposition: This is what is commonly known as acid rain. It refers to acidic rain, fog, and snow. In this process, sulfuric and nitric acids are formed in the atmosphere and fall to the ground with precipitation.
- Dry Deposition: This occurs when acidic pollutants, such as SO₂ gas and nitrate or sulfate particles, settle on surfaces directly from the atmosphere without precipitation. These particles can then react with water on surfaces to form acids.
6. What are the major environmental effects of acid deposition?
Acid deposition has several harmful effects on the environment:
- Aquatic Ecosystems: It lowers the pH of lakes and streams, making them toxic for fish, amphibians, and other aquatic organisms.
- Forests: It damages tree leaves and bark, leaches essential nutrients like calcium and magnesium from the soil, and releases toxic aluminium, which can harm tree roots.
- Soil: It alters soil chemistry, making it less fertile and harming soil microorganisms.
7. How does acid deposition damage historical monuments and buildings?
Many historical monuments and buildings are made of marble or limestone, which are primarily composed of calcium carbonate (CaCO₃). The sulfuric acid in acid rain reacts with calcium carbonate in a chemical reaction that forms calcium sulfate (CaSO₄), which is soluble in water. This process causes the stone to crumble and wash away over time, leading to significant damage and decay. The discolouration of the Taj Mahal is a well-known example of this effect.
8. Can acid rain directly harm humans?
Walking in acid rain or swimming in an acidic lake is not directly harmful to human skin. However, the pollutants that cause acid rain—sulfur dioxide (SO₂) and nitrogen oxides (NOx)—are very harmful when inhaled. These pollutants can form fine sulfate and nitrate particles that travel deep into the lungs, causing or worsening respiratory diseases such as asthma and bronchitis. The harm to humans is therefore indirect, arising from the presence of these pollutants in the air we breathe.
9. Why is 'acid deposition' a more accurate term than 'acid rain'?
The term 'acid deposition' is more accurate because it is more comprehensive. 'Acid rain' only refers to the wet form of acidic pollutants falling with precipitation. However, a significant amount of acidic pollution also falls from the atmosphere in the absence of rain, through dry gases and particles. This is known as dry deposition. The term 'acid deposition' correctly includes both wet and dry forms, providing a complete picture of the environmental problem.
10. What are some effective ways to control or reduce acid deposition?
Reducing acid deposition primarily involves controlling the emission of its primary pollutants, SO₂ and NOx. Effective strategies include:
- Using low-sulfur coal or washing coal to remove some of the sulfur.
- Installing scrubbers in the smokestacks of power plants to remove SO₂ from emissions.
- Using catalytic converters in vehicles to reduce NOx emissions.
- Switching to cleaner energy sources, such as solar, wind, and nuclear power, which do not produce these pollutants.
