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Acid Precipitation

Acid Precipitation

Seeing as all living organisms require water to survive, you would expect all rainfall to positively impact ecosystems and the environment. However, small changes in pH can cause precipitation to become acidic and have devastating impacts on soils, trees, aquatic ecosystems, and even man-made structures. Acidic conditions can stem from gaseous emissions too. Today we will look at the causes, effects, and solutions of acid precipitation.

Acid Precipitation Definition

Acid precipitation is rainfall with a pH value of 5.2 or less. This is acidic enough to damage soils, trees, and buildings.

Remember, anything with a pH above 7 is alkaline, whereas anything below 7 is acidic.

Normal rainfall will have a pH of roughly 5.6. To put this into context, bottled water has a pH of 6.5-7.5, and seawater has a pH of 8.2. So why would a difference of >1 pH make such a big difference? This is because for every unit you go down on the pH scale, acidity is multiplied by 10, so there is actually a substantial difference between solutions very close to each other on the pH scale. Here is a table showing the logarithmic pH scale:

Acid rain can have a pH as low as 3!

Examples of acid precipitation

In environmental science, acid precipitation is not a stand-alone concept. Acid deposition is an umbrella term for any changes in acidity from atmospheric conditions. Acid deposition doesn't just refer to the liquid phase either, the term encompasses rain, snow, hail, gases, dust, and fog. Thus, there are two types of acid deposition that you need to come to terms with:

  • Wet deposition: this is when sulfuric or nitric acid (formed from reacting nitrous oxides and sulfuric dioxide in the atmosphere) falls within rain, snow, hail, or sleet.
  • Dry deposition is when acidic gaseous particles or particulates stick to surfaces and are washed from these surfaces by precipitation.

Here are some different examples of acid rain:

  • Nitric acid rain is formed from the reaction between water and various nitrous oxides.
  • Sulfuric acid rain is formed from the reaction between water and sulphur dioxide.
  • Carbonic acid rain is formed from the reaction between water and carbon dioxide (carbonic acid is much weaker than nitric and sulfuric).

Acid Precipitation Formation

Hydrochloric, sulfuric, and nitric acid released from volcano activity can fall immediately as acid rain. From human activity, on the other hand, acid rains form when nitrous oxide and sulphur dioxide emissions react with atmospheric water to form nitric acid and sulfuric acid. These reactions will often occur in the aqueous phase but can also occur in the gaseous phase when no water is present. Here are the chemical equations for the formation of nitric acid and sulfuric acid:

SO2 + H2O H2SO4 H+ + HSO4 2H+ + SO42

NO2 + H2O HNO3 H+ + NO3

The H+ in the reaction signifies the acidity of the solution.

Acid Precipitation Causes

Acid precipitation and deposition can be caused by a variety of reasons, these include the natural release of gases, fossil fuel combustion, and agriculture. Let's go into more detail:

Natural Causes

Acid deposition is often regarded as being solely down to human activity, but gas emissions can release dangerous substances into the air too:

  • Volcanic activity is the major contributor to natural acid deposition around the world. This is because volcanoes are constantly releasing dangerous gases into the atmosphere, and even release acids such as hydrochloric, nitric, and sulfuric directly into the atmosphere.
  • Forest fires, large decomposition events, and other biological processes release carbon dioxide into the atmosphere, which has the potential to form carbonic acid.
  • Lightning strikes fix nitrogen in the atmosphere and can form nitrous oxides, which then react with water to form acid rain.

Human Causes

Human activity is the largest driver of gaseous emissions that cause acid deposition and has been increasing exponentially since the Industrial Revolution. Sulphur dioxide and nitrous oxides are by-products of a variety of man-made processes, ranging from electricity to power your homes to fossil fuels being burnt in vehicle engines. Let's have a look at some human activities that are causing acid precipitation:

Industrial Processes

Coal-burning power plants have been built around the globe in the last century to generate energy in the form of electricity, but the combustion of coal releases dangerous gaseous by-products. These include sulphur dioxide and nitrous oxides which will travel downwind of the plants and from acid rain. Nuclear power plants will also release sulphur dioxide as a by-product. The increasing requirement for electricity in modern times means it will be a struggle to limit the use of these plants.

Agriculture

The overuse of fertilisers and other agrochemicals containing nitrogen compounds can eventually result in acid rain. Nutrient-rich soils will leach into nearby waters and cause evaporating water to contain dangerous amounts of nitrate ions, which may react to form nitrous oxides and nitric acid during cloud formation.

Fossil Fuels

Transportation, heavy machinery, and mass production use fossil fuels for energy and release huge amounts of carbon dioxide. Atmospheric carbon dioxide not only acts as a greenhouse gas warming the planet but can also form carbonic acid and result in acid rain. The combustion of fossil fuels can release nitrous oxides and sulphur dioxide as by-products as well.

Human-induced Climate Change

The combustion of fossil fuels, deforestation, and agriculture all release greenhouse gases (such as carbon dioxide and methane) into the environment. These greenhouse gases will contribute to the greenhouse effect and warm the planet. Rising temperatures will increase the frequency and severity of precipitation patterns, meaning that acid rain will fall more regularly.

Effects of Acid Precipitation

Acid rain and deposition have damaging effects on soils, trees, terrestrial and aquatic ecosystems. They can also erode man-made buildings and statues. Let's go over some of these effects:

Effects on Soils

Acid deposition increases the hydrogen ion content of the soils, which in turn decreases the availability of important ions such as calcium, magnesium, and potassium, which are essential for the growth of plants and trees. This is done by leaching of these ions into different soils or water. Increasing acidity also increases the chance of aluminium mobilization, resulting in the transition of aluminium hydroxides to poisonous aluminium ions. Dissolving nitrous oxides can be acted on by denitrifying bacteria causing an increase in the nitrogen content of the soil.

Leaching is the runoff of the nutrient content of soils by precipitation.

Limestone availability (calcium carbonate) in soils acts as a buffer to increased acidity. Type of soil, weather conditions, and water flow affects the ability of soil to neutralise acids.

Effects on Terrestrial Ecosystems

The instant impacts of acid rain on ecosystems include the physical damage of any sheltering structures, such as the weakening of tree leaves and corrosion of rocky areas, as well as coastal erosion. Both these effects reduce habitat sizes and can cause ecosystems to become crowded or certain species to migrate to safer regions. The effects of acid deposition on the soil will have lasting consequences for the ecosystem. Magnesium ions are essential in photosynthesis while calcium ions are necessary for cell formation, so without access to these nutrients producer populations will begin to break down. Herbivorous consumers will therefore have fewer food resources. Exposure to aluminium ions stunts root growth in plants and reduces the ability of microorganisms to release nutrients into the soil.

Species migration is dangerous for the functionality of ecosystems because new competitors and predators will disturb ecological balances.

Although you may associate the majority of extinctions at the end of the Jurassic period (65 million years ago) with asteroid impact, the resulting expulsion of sulphur trioxide and mass downpouring of acid rain was a major factor in mass extinctions. The relentlessness of the following acid deposition would have not allowed ecosystems to recover and drove many terrestrial and aquatic species to extinction.

Acid deposition does not only corrode rocky habitats but also man-made structures like buildings and statues.

Effect on Aquatic Ecosystems

Acid deposition and leaching of soils which have been affected have harmful effects on aquatic ecosystems. Many marine organisms become stressed in acidic environments, especially those that require carbonates to manufacture their exoskeletons (acidification reduces carbonate availability). The leaching of nutrient-rich soils from acid rain can cause anoxia in aquatic environments. The addition of nitrogen oxides increases the nitrogen content of the soil, when this soil is leached a process called eutrophication occurs. Increased nitrogen availability in waters causes algal populations to increase rapidly and form algal blooms on the surface. When these blooms are broken down by anaerobic bacteria, the available oxygen is used up and the ecosystem is suffocated.

Anoxia is the absence of oxygen.

Amazingly, acid precipitation has been found to reduce methane emissions from peat bogs! Many archaic decomposers thrive in the low-nutrient, low-oxygen environments of peat bogs and release huge amounts of methane into the atmosphere. Methane is a highly potent greenhouse gas, so contributes massively to the greenhouse effect causing global warming and climate change. However, scientists have found an alternative bacteria which outcompetes the archaic bacteria in high-sulfate environments (acid rain reacts to form sulphates). Decomposition from these new bacteria has been found to release up to 40% less methane.

Peat is a muddy deposit found in bogs and is composed of dead, organic material (mostly plant material).

Acid precipitation - Key takeaways

  • Acid deposition is the increase in acidity of an area either by wet deposition (acidic precipitation) or dry deposition (gaseous particles attaching to surfaces).
  • Nitric acid rain is formed from the reaction between water and various nitrous oxides.
  • Sulfuric acid rain is formed from the reaction between water and sulphur dioxide.
  • Carbonic acid rain is formed from the reaction between water and carbon dioxide (carbonic acid is much weaker than nitric and sulfuric).
  • Climate change contributes to acid rain as rising temperatures will increase the frequency and severity of precipitation patterns, meaning that acid rain will fall more regularly.
  • Volcanic activity, combustion of fossil fuels, and climate change increase the sulfur dioxide and nitrogen oxide content of the atmosphere and the frequency of acid deposition.
  • Acid deposition has devastating effects on soils (reduces ion content, mobilises aluminium), terrestrial ecosystems (breaks down microorganism and producer populations), and aquatic ecosystems (causes anoxia via eutrophication of nearby waters).

Frequently Asked Questions about Acid Precipitation

Acid precipitation is rainfall with a pH of 5.2 or under. This means that it is acidic enough to cause damage.


Natural causes include volcanic activity, decomposition, forest fires, and lightning strikes, while human causes include sulphur dioxide and nitrous dioxide emissions from fossil fuel combustion, coal power plants, and agriculture.

Acid precipitation is formed after the release of nitrous oxides and sulphur dioxide into the atmosphere. These gaseous molecules will react with water in clouds to form nitric acid and sulfuric acid and then fall as acid rain.

Nitric acid, sulfuric acid, and carbonic acid can all fall as acid precipitation.

Natural causes include volcanic activity, decomposition, forest fires, and lightning strikes, while human causes include sulphur dioxide and nitrous dioxide emissions from fossil fuel combustion, coal power plants, and agriculture.

Final Acid Precipitation Quiz

Question

What is acid precipitation?

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Answer

Rainfall which has a pH of less than 5.2.

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Question

What is wet deposition?

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Answer

Where nitrous oxides and sulfur dioxide forms acid rain and fall as precipitation.

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Question

What is dry deposition?

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Answer

When acidic gaseous particles stick to surfaces and are washed away by rainfall.

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Question

How many times more acidic is a solution of pH 4 compared to one of pH 6?

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Answer

100 times.

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Question

What acid does nitrous oxide form?

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Answer

Nitric acid.

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Question

What acid does sulfur dioxide form?

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Answer

Sulfuric acid.

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Question

What are the natural causes of acidic deposition?

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Answer

Emissions from volcanic activity, nitrogen fixation from lightning strikes, the release of carbon dioxide from decomposition, and forest fires.

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Question

What fossil fuel is burnt in power plants?

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Answer

Coal.

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Question

What gas do nuclear power plants release as a by-product?

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Answer

Sulphur dioxide.

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Question

How does agriculture cause acid rain?

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Answer

Heavy machinery burns fossil fuels and releases nitrogen oxides and sulphur dioxide as by-products.

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Question

How can the leaching of nutrient-rich soils cause further acid rain?

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Answer

Nitrogen in evaporating water can react to form nitrous oxide and nitric acid.

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Question

How does climate change increase acid rain frequency?

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Answer

Increasing temperatures increase the frequency and severity of precipitation.

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Question

What are the effects of acid rain on soil?

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Answer

Reduces magnesium and calcium ion content and mobilises aluminium.

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Question

What are the effects of acid rain on ecosystems?

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Answer

Breaks down producer and microorganism populations.

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Question

What are the effects of acid rain on aquatic ecosystems?

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Answer

Leaching of nitrogen-rich soils causes eutrophication of nearby waters which results in anoxia.

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