What Acid Rain Is and Ways to Restore the Damage It Causes

Acid rain is detrimental to the environment, but its effects can be reversed.
Christopher McFadden

Acid rain can have serious implications for the environment -- especially aquatic environments and soils. While action has been taken to clean up many of the emissions that cause it, the damage has already been done to many natural environments around the world.

However, there is light at the end of the tunnel. It turns out there are some methods that can be employed to help reduce and reverse the effects of acid rain on the natural world. 

What is acid rain and what causes it?

As you may remember from your high school days, acid rain is formed when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and
transported by wind and air currents. These pollutants react with water molecules in the atmosphere, as well as oxygen and other chemicals to form sulfuric and nitric acids.

affects of acid rain on soil
Source: alegria2014/Pixabay

These acids then mix with more water and other materials in the atmosphere, before falling to the ground as acid rain. 


The phenomenon was first identified back in the 1800s, during the height of the Industrial Revolution. Robert Angus Smith, a Scottish chemist working in Manchester, England, made the connection between acid rain and atmospheric pollutants. 

A small amount of sulfuric dioxide and nitrous oxides are natural components of the environment and come from sources like volcanoes, electrical discharge from lightning, etc. However, the much larger amounts of acid rain found today comes from human industrial activities -- most notably burning fossil fuels.

At present, the most common sources of these oxides are: 

  • Burning of fossil fuels to generate electricity.  Two-thirds of SO2 and one-fourth of NOX in the atmosphere come from electric power generators.
  • Vehicles and heavy equipment exhaust.
  • Manufacturing, oil refineries, and other industries.

One major problem with these pollutants is that they can be carried over very long distances before forming acid rain. This means that countries can suffer the consequences of industrial activities in far off places, not just locally. 

What different forms of acid rain are there?

Acid rain can form, or be deposited, in two different ways: 

  • Wet deposition
  • Dry deposition

Wet deposition is what we most commonly refer to as acid rain. This is where sulfuric and nitric acids formed in the atmosphere fall to the ground carried by rain, snow, fog, or hail.

Dry deposition, on the other hand, consists of deposits from the atmosphere in the absence of moisture. 

"The acidic particles and gases may deposit to surfaces (water bodies, vegetation, buildings) quickly or may react during atmospheric transport to form larger particles that can be harmful to human health. When the accumulated acids are washed off a surface by the next rain, this acidic water flows over and through the ground, and can harm plants and wildlife, such as insects and fish." - EPA.

The amount to which either dry or wet deposition occurs is dictated by the amount of rainfall of an affected area. Deserts, for example, tend to show more dry deposition compared to somewhere that experiences several centimeters of rain per annum. 

what is acid rain
Source: TP_/Flickr

What is the pH of acid rain?

The term acid rain is an interesting one as regular rain, on average, is actually slightly acidic too. Typically, clean rain has a pH of between 5 and 5.6

As you are probably aware, the pH scale ranges from 0 to 14 and measures the relative acidity or alkalinity of an aqueous solution determined by the hydrogen ion content (H+). The scale was invented by a Danish scientist, Søren Sørensen, in 1909. 

It is a logarithmic scale and each pH unit represents a ten-fold increase in acidity. 

For reference, distilled pure water has a pH of 7 and the acid within a battery can have a pH of 0. On the other end of the pH scale, bleach has a pH of about 12.6 and liquid drain cleaner up to pH 14.

The reason regular rain is slightly acidic is because of dissolved carbon dioxide that forms carbonic acid. Acid rain, on the other hand, tends to have a pH of between 4.2 and 4.4.

This decrease in pH between clean rain and acid rain means that the latter can be considerably more acidic.

acid rain pH
Source: OpenStax College/Wikimedia Commons

On some occasions, the pH of acid rain has been recorded as low as 3 -- similar to the pH of vinegar. An even lower figure was once recorded in 1982 when the pH of fog on the West Coast of the U.S. measured a pH of 1.8!

Why is acid rain harmful to the environment?

Acid rain can be incredibly detrimental to the natural environment. Ecologically speaking, acid rain is far more devastating in aquatic environments like streams, lakes, and marshes.

Acid rain can, and will, dramatically change the average pH of these environments, potentially killing many species of fish and other aquatic organisms that are adapted to a higher pH.

Many aquatic organisms have something called a "critical pH level" in which they can survive. For example, snails tend to suffer badly in pHs lower than pH 6, mayflies about pH 5.5, and frogs somewhere in the region of pH 4

For fish, low pH levels can also prevent their eggs from hatching. All of these effects tend to dramatically reduce the biodiversity of these ecosystems. 

When acid rain penetrates and infiltrates the soil, it can leach poisonous metals like aluminum, cadmium, and mercury from soil and clay particles, which then tend to flow into streams and lakes. For example, the more acidic the rain, the more aluminum is released, compounding pollution problems.

Acid rain also removes a large number of calcium cations from the soil -- which is an important mineral for the local ecology. Significant loss of it can damage, and even kill, trees, plants, and crops. 

Aluminum has long been recognized as very toxic for freshwater organisms, and can badly affect terrestrial ecosystems too. In aquatic environments, aluminum is a particularly potent toxin for gill-breathing organisms like fish and invertebrates.

Exposure to large doses of aluminum causes plasma- and hemolymph (an equivalent of blood in invertebrates) problems and can eventually lead to osmoregulatory (the regulation of fluid and electrolytes in organisms) failure in affected animals. For fish, in particular, aluminum reduces the efficiency of their gills and can lead to the death of the gills cells.  

Aluminum can also accumulate in freshwater invertebrates. This has a knock effect for any mammalian and avian predators.

On land, aluminum, washed into the soil as a result of acid rain, can adversely affect the fine root systems of plants. As in some aquatic animals, the presence of aluminum in sufficient concentrations affects the systems that are important for the uptake of vital nutrients. 

acid rain trees
Source: Forest Service NW/Twitter

"At high elevations, acidic fog and clouds might strip nutrients from trees’ foliage, leaving them with brown or dead leaves and needles. The trees are then less able to absorb sunlight, which makes them weak and less able to withstand freezing temperatures." - EPA. 

Aluminum also accumulates in plants, like some invertebrates, affecting the entire food chain in turn. 

Acid rain can also damage, and eventually, kill plants directly. Apart from acidification of the soil, acid rain can also cause drying of waxy leaf cuticles that have evolved in some plants to prevent water loss. 

This ultimately results in excessive water loss from the plant to the atmosphere. Affected plants will dehydrate and perish. Plants experiencing this will usually show yellowing between the veins of their leaves. 

Increased acidification of the plant's internal tissues can also result in the dissolution of important minerals, fatally weakening it.

The acidification of soil also dramatically impacts soil microbial biodiversity. Some microbes cannot tolerate low pH and are consequently killed. 

Acid rain can also be very detrimental for shallow, coastal waters. Ocean acidification can prevent marine invertebrates from effectively creating calcified exoskeletons. 

acid rain corals
SourceKhaled bin Sultan Living Oceans Foundation

Corals are particularly sensitive to lower pH levels, where their calcium carbonate skeletons can dissolve. Any effect on the lower members of the ocean food chain will also have a knock-on effect on other higher marine animals. 

In summary, the 3 main effects of acid rain on the environment are (courtesy of Washington University): 

  • Freshwater habitats become so acidic that animals cannot live in them anymore.
  • Degradation of many soil minerals produces metal ions that are then washed away in the runoff, causing several effects:
    • The release of toxic ions, such as Al3+, into the water supply.
    • The loss of important minerals, such as Ca2+, from the soil, killing trees, and damaging crops.
  • Atmospheric pollutants are easily moved by wind currents, so acid-rain effects are felt far from where pollutants are generated.

Is acid rain harmful to humans?

Apart from the serious damage acid rain can cause to the environment it is also harmful to buildings, historical monuments, and statues, especially those made from limestone and marble.

It can also affect human health.

While not directly, per se, the particulate matter in the air that forms acid rain can contribute to heart and lung problems if inhaled, especially by those with asthma and bronchitis. NOx can also lead to the creation of ozone at ground level that, when also inhaled, can promote severe lung problems such as chronic pneumonia and emphysema.

Acid rain at higher altitudes can also lead to the formation of thick acidic fog that affects visibility and irritates the eyes and nose. 

Are there any positive effects of acid rain?

As it turns out, there are actually some interesting positive effects of acid rain. For example, it has been found that acid rain can help reduce the natural production of methane -- a more potent greenhouse gas than carbon dioxide.

This has been especially noted in wetland areas. The sulfur content of acid rain has been shown to limit the activity of methane-producing microbes found in such environments. 

How does acid rain affect the soil?

Soil is one of the most fundamental foundations of all land-based life. Any significant damage to it will dramatically impact entire ecosystems on land.

When the chemical and nutritional value of soils are depleted, entire ecosystems can collapse. This is why it is critically important to eliminate or at least reduce as much as possible the effects of acid rain on soil. 

acid rain and soil
Source: Chet_W/iStock

We have already mentioned some major effects on the soil from acid rain above, but in some cases, forests, streams, and lakes that experience acid rain are able to buffer the effects. Buffering is the ability of an ecosystem to tolerate an increase in acidification from acid rain.

This is entirely dependant on a number of factors. The two major ones being the thickness and composition of the soil and the type of bedrock underneath it.

For example, areas with thick soils rich in calcium, limestone, or marble, are better able to neutralize the acid in the rainwater. This is because limestone and marble are more alkaline (basic) and produce higher pHs when dissolved in water.

In places where the underlying geology, and by extension, the chemistry of the soil, is unable to buffer the effects of acid rain, soil acidification can be devastating. It strips away vital minerals from the soil that can kill existing plants and also threatens the future of forest productivity. 

Acidic soils tend to result in slower growth for plants and trees if they are not killed outright.

"In the Green Mountains of Vermont and the White Mountains of New Hampshire in the United States 50% of the red spruce have died in the past 25 years. There has also been noted a reduced amount of growth in existing trees as measured by the size of growth rings of the trees in these areas." - airquality.org.uk.

soil acid rain
Source: Gustav Bergman/Flickr

How can areas damaged by acid rain be restored?

As we have already seen, the damage caused by acid rain can be very significant for the environment -- especially soil and aquatic environments. While nature does have a great capacity to heal itself, sometimes it is necessary for humans to take action.

Here are some ways that humans can help restore the damage caused by acid rain.

1. Powdered limestone can be added to acidified waterways

acid rain liming
Source: environment.no

One method of artificially restoring the damage caused by acid rain to lakes and rivers is to introduce powdered limestone. Called "liming", the calcium carbonate, and other alkaline components of limestone, help to neutralize the pH of affected waters.

While this is a fairly simple solution, it is not the cheapest method. It is also only ever a temporary solution and must be continued at intervals until the acid rain stops.

It has successfully, been used in places like Norway and Sweden to help restore affected lakes and riverways. Another major liming project was also undertaken in Wales, UK, where 12,000 km, or so, of the waterways had become acidified. 

That project took place in 2003 on the river Wye (a major waterway that runs from Mid-Wales to the Severn Estuary) and actually led to the return of salmon to these areas. These fish hadn't been seen in the river since the mid-1980s. 

2. Calcium pellet "blanket bombing" has also been used to restore acidified soil

acid rain calcium pellets
SourceMariel Carr/Wikimedia Commons

Calcium-based solutions can also be used to help restore acid rain damaged soils. For example, in 1999, 40 tons of dry calcium pellets were spread over a 29-acre watershed at Hubbard Brook, New Hampshire, the U.S. The pellets were spread by helicopter over several days.

The pellets were specially designed to slowly work their way into the watershed over many years and neutralize the acidification of the soil. Researchers monitored the forest over a decade and a half to compare the area with adjacent watersheds that did not receive the same treatment. 

“The treatment increased the forest’s resilience to major disturbances,” said one of the research team. “The trees in the calcium-treated watershed were able to recover faster from a severe ice storm that hit the region in 1998.”

3. Regulate industries to control emissions

acid rain regulations
Source: Pexels

It sounds obvious, but one of the most effective ways to help restore the areas damaged by acid rain is to cut the emission of sulfur and nitrous oxides from the most heavily-polluting industries. By stopping the problem at the source, it allows nature to recover on its own.

This can be achieved through a mixture of washing coal, burning only low-sulfur coals, or installing devices called "scrubbers" to flues and chimneys. Also called flue-gas desulfurization (FGD), this typically works to chemically eliminate SO2 from the gases leaving smokestacks.

They are incredibly effective and can remove as much as 95% of sulfur dioxide from emissions gases. Of course, power plants can also be converted from the use of coal to low sulfur fuels like natural gas, or alternative forms of energy.

For vehicles, the introduction of the catalytic converter to exhaust systems provide pivotal in lowering NOx emissions from cars. 

This has been particularly effective in places like the U.S. and Canada, where government regulations where introduced more than 25 years ago to force industries to clean up their act. Most notably, the Clean Air Act of 1970 and the Canada-United States Air Quality Agreement of 1991.

Some studies from 2015 have shown that after a slow start, acidified soils are now showing accelerated recovery across a broad swathe of Western Ontario and Maine

4. Switching to alternative energy sources also helps

acid rain renewables
Source: Kuebi/Wikipedia Commons

Another strategy employed to stop acid rain and help restore the damage caused has been the widespread introduction of alternative energy sources to generate electricity. Wind, geothermal, solar, hydropower, and nuclear power chief among them.

By removing completely the need to use fossil fuels, these alternative energy sources effectively eliminate the emissions of sulfur dioxide and NOx pollutants into the air. And of course, the same is true for the switch to electric vehicles.

And that's a wrap.

The effects of acid rain on the natural and built environments can be incredibly serious if left unabated. Thankfully a mixture of important legislation and some innovative mitigation strategies have proven effective at reducing, and even reversing, some of the most serious damage.

As countries continue their push for cleaning up their emissions and accelerating their adoption of alternative energy sources, acid rain from human sources will hopefully become a thing of the past.

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