Acid Rain Effects, Causes and Types of Acid Deposition

What is acid rain?

Acid rain, also known as acid deposition or acid precipitation, is any kind of precipitation that is unusually acidic due to the emission of nitrogen oxides and sulfur dioxide. This acidic precipitation could fall to the ground from the atmosphere as rain, sleet, snow, hail, fog, or even as dust, dry acidic particles, and gases. It includes the various forms at which acidity can move to the surface of the earth from the atmosphere.

This precipitation tends to have high levels of hydrogen ions (low pH). Water has a neutral pH that ranges between 6.5-8.5 but the acid rain pH is lower and ranges averagely from 4-5. Therefore, the lower the pH value of the precipitation, the more acidic it is. Acid rain could be sulfuric acid or nitric acid with a pH level of about 5.2 or below.

Acid rain, also known as acid deposition or acid precipitation, is any kind of precipitation that is unusually acidic due to the emission of nitrogen oxides and sulfur dioxide

 Acid deposition fall to the earth’s surface from the atmosphere as rain, sleet, snow, hail, fog, or even as dry acidic particles, and gases

There are two kinds of acid deposition- wet and dry deposition. The combustion of fossil fuels is the main cause of acid rain because sulfur dioxide and nitrogen oxides are emitted into the atmosphere during combustion. Even though acid rain has been reduced significantly in some areas, it is still recognized as an environmental issue. It affects a large part of the globe like some areas in western Europe, western North America, Asia, South America, Australia, and Africa. Major industrial and agricultural regions are usually affected.

Acid precipitation is one of the adverse effects of air pollution on the ecosystem. It causes the pH of surface water to reduce and this, in turn, lowers biodiversity. Also, it can weaken trees and make them susceptible to other stressors like extreme cold, drought, and pests. Even plant nutrients like magnesium and calcium are depleted from the soil as a result of acid rain. One of the effects of acid deposition is the corrosion of exposed surfaces and deterioration of limestone as well as marble monuments and buildings. It can cause the corrosion of steel structures and cause paint to peel. The weathering of stone buildings and statues can also be caused by acid rain.

Plants, forests, freshwaters, aquatic animals, soil, and infrastructure are all affected by acid deposition. How acid rain is formed is attributed to air pollution that involves the emission of sulfur dioxide and nitrogen oxide. Hence, acid rain is caused by the presence of SO2 and NOx in the atmosphere. These gases react with the water molecules in the atmosphere to form acid. Human activities are to blame for the emission of sulfur and nitrogen compounds into the atmosphere. However, these compounds can still be emitted naturally by volcanic eruptions (producing sulfur dioxide) and lightning strikes (producing nitrogen oxides).

How is acid rain formed?

Acid rain contains carbonic acid H2CO3, sulfuric acid H2SO4, and nitric acid HNO3. How acid rain is formed is mainly from the reactions of nitrogen oxide and sulfur dioxide with water and oxygen in the atmosphere. This reaction forms sulphuric acid and nitric acid.

The decomposition of vegetation and erupting volcanoes contributes to the emission of sulfur dioxide into the atmosphere. However, the major source of SO2 and NO2 in the atmosphere is anthropogenic which includes factories, coal-burning power plants, and automobiles. Acid rain formation commences when fossil fuel is burned which is subsequently followed by emissions of oxides of nitrogen (NOx), sulfur trioxide (SO3), and sulfur dioxide (SO2) into the atmosphere.

These air pollutants then react with oxygen water and other substances in the atmosphere to form sulfuric, sulfurous, and nitric acid depositions. Wind plays a role by spreading this acid precipitation throughout the atmosphere and across long distances. Once, it reaches earth, it can sink into the soil, flow across the earth’s surface in runoffs, and enter water systems.

Acid rain chemical formula

Carbonic acid (H2CO3)

H2O (l) + CO2 (g) ⇌ H2CO3 (aq)

Water + Carbondioxide → Carbonic acid

The reaction of water and carbon dioxide in the air gives a weak acid (carbonic acid) that makes the pH of unpolluted rain no lesser than 5.7. However, unpolluted rain can contain other chemicals that affect its pH like in the case of lightning in the atmosphere which releases electric discharge that produce nitric acid. For acid precipitation to be an environmental issue, other additional acids other than H2CO3 has to be included.

Sulfuric acid (H2SO4)

In gas phase

SO2 + OH → HOSO2

As seen in the equation above, Sulfur dioxide reacts with the hydroxyl radical and is oxidized, which is followed by:

HOSO2 + O2 → HO2 + SO3

Then, SO3 (sulfur trioxide) in the presence of water is converted rapidly to sulfuric acid:

SO3 (g) + H2O (l) → H2SO4 (aq)

Sulfur trioxide + Water → Sulfuric acid

In cloud droplet

The loss rate of SO2 is faster in the presence of clouds than in the gas phase. This is a result of the reactions in the liquid water droplets.

Sulfur dioxides dissolve in water and hydrolyze in a series of equilibrium reactions:

SO2 (g) + H2O ⇌ SO2H2O
SO2H2O ⇌ H+ + HSO3−
HSO3− ⇌ H+ + SO32−

Nitric acid ( HNO3)

NO2 + OH → HNO3

NO2 reacts with the hydroxyl radical (OH) to form nitric acid rain formula (HNO3).

How is acid rain measured?

The acidity of a solution is the measure of the concentration of hydrogen ions (H+) in the solution. Alkalinity and acidity are measured using a pH scale. On a pH scale, 7.0 is neutral and any measure lower than 7 is acidic, whereas a pH value greater than 7 is basic (alkaline). Usually, the pH of normal rain is about 5.6 which is slightly acidic. This is because carbon dioxide dissolves into it to form a weak carbonic acid. The acid rain pH, on the other hand, is between 4.2 and 4.4.

Wet deposition is easier to measure compared to measuring dry deposition which is difficult and expensive. The wet deposition measurement is carried out by the National Atmospheric Deposition Program’s (NADP) National Trends Network. Research scientists, ecologists, policymakers, and modelers depend on NADP/NTN for the measurements of wet deposition.

The Clean Air Status and Trends Network (CASTNET) provides the dry deposition estimates for nitrogen and sulfur pollutants. CASTNET measures the air concentrations at more than 90 locations. Moreso, some water bodies turn acidic when the acid deposition is washed into lakes and streams. Hence, at over 280 sites, the surface water chemistries of water bodies are measured and monitored by the Long Term Monitoring (LTM) Network to give valuable information on the aquatic ecosystem health. LTM Network helps to evaluate how the water bodies respond to the changes in acid deposition and acid-causing emissions.

Types of Acid Deposition

  1. Wet acid deposition
  2. Dry acid deposition

Wet deposition

The wet acid deposition is what we think mostly of as acid rain. Wet deposition of acid is any form of precipitation such as rain, fog, hail, snow, etc that delivers the acid from the atmosphere to the surface of the earth. The nitric acids and sulfuric acids formed in the atmosphere mix up with rain, snow, hail, and fog and falls to the earth’s surface.

Wet deposition is a result of the deposition of acids produced in the raindrops. It can also occur due to the precipitation removing acids in or below clouds. Furthermore, the wet removal of aerosols and gases is essential for wet deposition.

Dry deposition

The dry acid deposition is the acidic particles and gases that are deposited without moisture from the atmosphere to surfaces of vegetation, water bodies, and buildings. Dry deposition accounts for about 20-60% of the total acid deposition. The dry acid deposition may deposit quickly to the earth’s surface or during atmospheric transport, may react to form larger particles that can be detrimental to human health.

Evidently, the amount of acid deposition that falls to the earth’s surface through dry deposition depends on the amount of rainfall in an area. For instance, The ratio of dry deposition to wet deposition in desert areas is higher compared to areas with several inches of rainfall every year. Dry acidic particles and gases can stick to the ground, other surfaces, and plants. Hence, when rain falls, it washes off the accumulated acids on the surfaces. This acidic water flows through the ground and over it, thereby, causing harm to plants, insects, fishes, and wildlife.

Sources

The sources that cause acid rain include:

Emissions from natural phenomena

The normal rainwater as earlier said is slightly acidic due to the absorption of carbon dioxide in the atmosphere. This carbon dioxide is released into the atmosphere due to biological activities. Nevertheless, there are other natural phenomena that release emissions that cause acid deposition.

  • Volcanoes: Volcanic eruptions, depending on the emissions can produce nitric acid, sulfuric acid, and hydrochloric acid, also see vulcanicity. It is a major natural phenomenon that contributes acid-producing gases to the atmosphere. Take for instance fumaroles from the Laguna Caliente crater of Poas Volcano. This volcano produces high amounts of acid fog and rain that measure an acidic pH of 2. This resulting acid rain and fog clears any area of vegetation and usually causes eye and lung irritation to the inhabitants nearby.
  • Lightning: This can be a source of acidification as it produces nitrogen oxides from conversing atmospheric molecular nitrogen. Electric activities in the atmosphere produce nitric acid.
  • Wildfire: This can cause acid from the conversion of organic nitrogen.

The fact is that the geographic extent of the natural sources of acidification is small compared to anthropogenic sources. Hence, most times, the natural sources only lower the pH of precipitation by not more than 5.2.

Burning of fossil fuels

Human activities that involve the burning of fossil fuels and the smelting of metal ores are the primary causes of acid deposition. Electric power generators generate about two-thirds of SO2 and one-fourth of NOx in the atmosphere. In the U.S, about 70% of SO2 and 20% of NOx emissions come from electric utilities. Then, about 60% of NOx emissions in the U.S come from vehicular fuel combustion.

The NOx and SO2 generated react with water in the atmosphere to form acid. These reactions in the gaseous phase produce acidic dry deposition while in the aqueous phase, it produces wet deposition products. The acid deposition will take place in areas downwind of emission sources as far as fossil fuel combustion is large and there is no emission control in place. In polluted areas, the cloud fog and water may be more acidic than the rain falling over the area.

Nitric acid in the atmosphere causes other environmental problems. For example, eutrophication can result from the high concentration of nitrogen in water bodies from terrestrial runoff and atmospheric acid deposition. A water body or system being eutrophic leads to algae bloom which causes oxygen depletion of the water. Without dissolved oxygen in the water, many aquatic species cannot survive.  Aside from causing acid deposition, NOx emissions and hydrocarbon emissions are primary elements in the formation of a type of air pollution called photochemical smog.

Manufacturing, oil refineries, and other industries

Sulfur and nitrogen from human sources are the primary cause of acid rain. Electricity generation, animal agricultural activities, and factories are examples of such sources. In China and Russia, industrial acid rain is a substantial problem including in areas downwind from these countries. To generate heat and electricity, these areas all burn sulfur-containing coal, hence emitting sulfur dioxide.

The problem of acid in rain has become more widespread and in order to reduce local pollution, tall smokestacks are being used. This, unfortunately, releases gases into regional atmospheric circulation and has contributed to the spread of acid deposition. The dispersal from these tall smokestacks causes air pollutants to be carried farther which causes the widespread of this ecological problem. Moreso, the acid in rain occurs in areas of considerable distance downwind of the emissions. The mountainous regions due to their higher rainfall tend to get the greatest deposition. The acid in the rain that falls in Scandinavia is a typical example.

Acid rain effects

  • Harms aquatic life and other terrestrial animals
  • Soil acidification
  • Adverse effects on vegetation and forests
  • Lead to ocean acidification
  • Damages buildings and monuments
  • Causes an adverse impact on human health

Harms aquatic life and other terrestrial animals

Acid rain causes adverse effects on aquatic life that cannot tolerate acidity. The lower pH of surface water caused by acid rain damages fishes and other aquatic species. Hence, the acid in water bodies can reduce biodiversity. The majority of fish eggs do not hatch at a pH lower than 5 and this pH level can kill the adults as well.

As a result of acid rain, insect life and some species of fish have been eliminated e.g brook trout in sensitive areas like the Adirondack Mountains of the United States. Of all the lakes and streams surveyed by the United States Environmental Protection Agency (EPA), the acidity in 75% of acidic lakes and 50% of acidic streams was caused by acid precipitation.

Soil acidification

One of the effects of acid rain is soil acidification. Acid rain can cause severe damage to soil biology and chemistry. Due to soil acidification,  some microbes that can’t tolerate low pH levels are killed because acidity denatures their enzymes. The essential nutrients and minerals in the soil such as magnesium can be leached away by the hydronium ions of acid deposition. Therefore, the chemistry of the soil changes when the base cations like magnesium and calcium are leached by acid deposition. Also, the hydronium ions of acid rain can mobilize toxins like aluminum. This can affect sensitive plant species like sugar maple.

Soil acidification causes minor or major effects. Plants that are affected by soil acidification tend to have leaves that are yellow in between their veins. When the pH of the soil is normal, the leaves of the plants are usually green and healthy. The plant having yellowing between the veins on their leaves shows that the pH of the soil is acidic and the plant is unhealthy. This acid rain effects on plants make plants suffer from not being able to carry out photosynthesis. This is because the chloroplast organelles of the plant cell can be destroyed as a result of the plant drying out due to the acid in water. Since the plant cannot photosynthesize, it cannot produce nutrients for it to survive on or give out oxygen for other organisms (aerobic) to use. This will definitely affect organisms on earth and the existence of plants.

Adverse effects on vegetation and forests

Forests and other vegetation suffer from the adverse effect of acid rain. Acid water can lower the plant’s natural pH which can cause the death of plants. Essential plant nutrients can get dissolved or carried away when acid water enters the plant. This results in the death of vegetation due to a lack of minerals. As acid rain falls on the leaves of plants, the waxy leaf cuticle dries out which leads to rapid water loss. Due to the drying of the cuticle, the plant loses excess water to the atmosphere and eventually dies.

High altitude forests that are usually surrounded by fog and clouds are very vulnerable because acidic precipitation (fog and clouds) is more acidic than rain. The acid deposition effect on food crops can be reduced by applying fertilizer and lime in order to replace lost nutrients. Also, limestone can be added in cultivated areas to increase the soil’s ability to keep the pH stable. Take the red spruce plant, for instance, which becomes less cold tolerant when calcium is leached from the needles due to acid precipitation. These trees also exhibit winter injury as a result of the calcium leaching from the soil and can even die.

Ocean acidification

Another effect of acid rain on the environment is ocean acidification which is another form of water pollution. Acid rain causes less harmful effects on oceans on a global scale but a greater impact in shallower waters of coastal waters. Ocean acidification is a condition whereby the ocean’s pH lowers, mostly due to acid in rain. This makes it difficult for coastal species to create the exoskeleton that they need to survive. The limestone skeleton of corals happens to be very sensitive to pH decrease. This is because the calcium carbonate that makes up the skeleton dissolves in acidic solutions.

Acid rain formed from nitric acid causes eutrophication in water bodies. This can result from the high concentration of nitrogen in water bodies from terrestrial runoff and atmospheric acid deposition. A water body or system being eutrophic leads to algae bloom which causes oxygen depletion of the water. Without dissolved oxygen in the water, many aquatic species cannot survive leading to dead zones in the water.

Damages buildings and monuments

Statues, buildings, and historic monuments, that are made of limestone and marble can be damaged by acid rain. Gypsum that flakes off is created as the acid in the rain reacts with the calcium compounds in the stones. These damaging effects are commonly seen on old gravestones. The acid rain causes the inscriptions on gravestones to become completely illegible. It also causes the corrosion of metals.

Causes an adverse impact on human health

Acid rain effects on humans are not direct because skin contact with acid deposition doesn’t cause a health risk. The effects of acid rain on humans are not entirely adverse because the acid in the rain is too dilute to harm humans directly. However, the particulates and air pollutants that form acid rain have adverse effects on humans. These pollutants in the atmosphere cause heart and lung problems, including respiratory issues like asthma and bronchitis.

Acid rain solutions

The major solution to acid rain is to reduce and curb the emission of gases that causes acid in the rain. This is only possible by controlling and regulating the sources of NO2 and SO2. The fact is as long as fossil fuel is in use, the problem of acid rain will continue. A study showed that in China, acid rain may have contributed to the deadly 2009 landslide.

Currently, about 75% of the emissions of sulfur dioxides in china have fallen since 2007 due to the control measures implemented. Setting air quality standards also helped the U.S cut down emissions of NO2 by 50% between 1990 and 2017. Therefore the main acid rain solutions include:

  • The reduction of the use of fossil fuels in the automotive sector and industries
  • Usage of renewable sources of energy will help curb emissions of pollutants
  • Air quality standards should be set. For example, putting pollution limits in place has helped cut down 8% of sulfur dioxide emissions in the U.S, between 1990 and 2017.

FAQs

What are the causes of acid rain?

What causes acid rain is attributed to natural and anthropogenic sources that emit sulfur dioxide and nitrogen oxides into the atmosphere. Most of the emissions of these gases that cause acid deposition come from burning fossil fuels and only a small amount comes from natural sources.

How does acid rain affect the environment?

Acid rain affects the environment in several ways. Terrestrial and aquatic habitats are both affected. In the aquatic ecosystem, acid rain causes ocean acidification and eutrophication, which affects aquatic species that are intolerant to acidity. Plants in the terrestrial habitat suffer as well from soil acidification that is caused by acid rain. Even soil microbes and insect species are harmed by this environmental problem.

What are the 3 effects of acid rain?

Acid rain causes the corrosion of exposed surfaces and deterioration of limestone as well as marble monuments and buildings.
It can cause the corrosion of steel structures and cause paint to peel. The weathering of stone buildings and statues can also be caused by acid rain.
Plants, forests, freshwaters, aquatic animals, soil, and infrastructure are all affected by acid rain.

How do we stop acid rain?

The only solution to stop acid rain is to reduce and curb the emission of gases that causes acid in the rain. This is only possible by controlling and regulating the sources of NO2 and SO2 emissions. The fact is as long as fossil fuel is in use, the problem of acid rain would continue.

Does acid rain burn skin?

Acid rain is not like the very strong acids that can burn one’s skin once in contact and can even destroy metals. Acid rain is weaker and not acidic enough to burn one’s skin. The rain is usually slightly acidic because it mixes with the naturally occurring oxides in the atmosphere.