Acid rain is rain or other forms of precipitation that is unusually acidic, meaning it has higher amounts of hydrogen ions, which makes its pH level lower than normal. Most water, such as drinking water, has a neutral pH between 6.5 and 8.5. Acid rain has a pH level lower than this, usually between 4 and 5. The more acidic the rain is, the lower its pH becomes. Acid rain can harm plants, fish, and other living things, as well as damage buildings and structures. It is caused by emissions of sulfur dioxide and nitrogen oxide, which combine with water in the atmosphere to form acids.

Acid rain can harm forests, freshwater sources, soil, microorganisms, insects, and aquatic life. In ecosystems, long-term acid rain weakens tree bark, making plants more vulnerable to stress like drought, extreme temperatures, and pests. It also harms soil by removing important nutrients such as calcium and magnesium, which help plants grow and keep soil healthy. For human-made structures, acid rain can cause paint to peel, damage steel bridges, and wear away stone buildings and statues. It may also affect human health.

Since the 1970s, governments in Europe and North America have worked to reduce the release of sulfur dioxide and nitrogen oxide into the air through pollution laws. These efforts have helped because research on acid rain began in the 1960s, and information about its dangers became widely known. The main sources of sulfur and nitrogen compounds that cause acid rain are human-related, but nitrogen oxides can also form naturally during lightning strikes, and sulfur dioxide can come from volcanic eruptions.

Definition

"Acid rain" is rain that has a pH lower than 5. "Clean" or unpolluted rain has a pH higher than 5 but still lower than 7 because of the acidity caused by carbon dioxide in the air. This happens through chemical reactions.

Many natural and human-made sources contribute to the acidity of rain. For example, nitric acid is created when lightning, which is a type of electric discharge in the atmosphere, interacts with air. Common human-made sources include sulfur dioxide and nitrogen oxide. These substances react with water (just like carbon dioxide does) to form solutions with a pH lower than 5. In some industrial areas, rain and fog water have been found to have pH levels as low as 2.4 or even lower.

History

Acid rain was first studied in a planned way in Europe during the 1960s, in the United States and Canada during the 1970s, and in India in the late 1980s. In the 17th century, John Evelyn noticed that polluted air in cities damaged limestone and marble, such as the Arundel marbles. Since the Industrial Revolution, the release of sulfur dioxide and nitrogen oxides into the air has increased. In 1852, Robert Angus Smith showed the link between acid rain and pollution in Manchester, England. Smith first used the term "acid rain" in 1872.

In the late 1960s, scientists began studying acid rain more widely. Early research focused on local effects. Waldemar Christofer Brøgger was the first to recognize that pollution could travel long distances from the United Kingdom to Norway. Brynjulf Ottar studied this problem in the 1970s. Ottar’s work was influenced by Svante Odén, a Swedish scientist who raised awareness about acid rain in Europe through newspapers and a major paper in 1968.

The first report about acid rain in the United States came from chemical evidence collected in Hubbard Brook Valley. Public interest in acid rain in the US grew in the 1970s after The New York Times shared these findings. In 1972, scientists like Gene Likens discovered that rain in the White Mountains of New Hampshire was acidic, with a pH of 4.03 at Hubbard Brook. The Hubbard Brook Ecosystem Study later examined how acid rain affected the environment. Soil minerals helped reduce the acid in stream water. Research showed that acid rain reacting with aluminum increased soil erosion. Experiments on Norris Brook in New Hampshire found that higher acidity in streams reduced species diversity, made some species more dominant, and simplified food webs.

In 1972, the US Congress passed the Acid Deposition Act. This law started an 18-year program called the National Acidic Precipitation Assessment Program (NAPAP). NAPAP expanded monitoring sites to measure acid rain levels and track long-term changes. It also studied dry deposition and how acid rain affected freshwater, land, historical buildings, and monuments. NAPAP funded research on atmospheric processes and ways to control pollution.

From the start, people with different opinions tried to influence NAPAP’s work to support their own ideas or criticize others. For the US scientific community, NAPAP taught important lessons about managing environmental research and sharing results with scientists, managers, and the public.

In 1981, the National Academy of Sciences studied acid rain controversies. President Ronald Reagan ignored acid rain concerns until he visited Canada and saw pollution drifting from US smokestacks. Reagan agreed to support Canada’s pollution controls. In 1982, Reagan asked William Nierenberg to join the National Science Board. Nierenberg chose scientists like Gene Likens to help write a report on acid rain. In 1983, the scientists concluded that acid rain was a real problem and needed solutions. The White House suggested changes to the report, but the scientists rejected these and sent the report to Nierenberg. In May 1983, the House of Representatives voted against laws to reduce sulfur emissions. Some said Nierenberg delayed the report, but he denied this, explaining it was not ready to publish after the vote.

In 1991, NAPAP reported that 5% of lakes in New England were acidic, with sulfates being the main issue. They found that 2% of lakes could no longer support Brook Trout, and 6% were unsuitable for many minnow species. Later reports showed changes in soil and water ecosystems, nitrogen saturation, soil nutrient loss, acid events, haze, and damage to historical monuments.

In 1990, the US Congress updated the Clean Air Act. Title IV created a cap and trade system to control sulfur dioxide and nitrogen oxide emissions, which harm air quality. Title IV aimed to reduce sulfur dioxide emissions from power plants by about 50%, reaching 8.7 million tons by 1995. Phase I included 110 large plants, such as Merrimack in New England. Phase II, starting in 2000, affected most power plants nationwide.

During the 1990s, research continued. In 2005, the EPA introduced the Clean Air Interstate Rule (CAIR), which limited sulfur dioxide and nitrogen oxide emissions in the eastern US. CAIR reduced sulfur dioxide emissions by over 70% and nitrogen oxide emissions by over 60% compared to 2003 levels.

The cap and trade program helped reduce sulfur dioxide emissions by 40% since the 1990s. Acid rain levels dropped 65% since 1976. In Europe, conventional regulations reduced sulfur dioxide emissions by over 70% during the same time.

In 2007, sulfur dioxide emissions reached 8.9 million tons, meeting the program’s goal ahead of the 2010 deadline. The EPA estimated that by 2010, the cost of following the program would be $1 billion to $2 billion annually, much less than predicted. By 2010, sulfur dioxide emissions had fallen to 5.1 million tons due to the Clean Air Interstate Rule.

The term "citizen science" was first used in January 1989 during a campaign by the Audubon Society.

Emissions of chemicals leading to acidification

The most important gas that causes acidification is sulfur dioxide. Emissions of nitrogen oxides, which change into nitric acid, are becoming more important because rules now limit sulfur compound emissions more strictly. Each year, 70 teragrams of sulfur in the form of sulfur dioxide come from burning fossil fuels and industry, 2.8 teragrams from wildfires, and 7 to 8 teragrams from volcanoes.

The main natural source of acid-producing gases is volcanic emissions. For example, fumaroles at the Laguna Caliente crater of Poás Volcano release large amounts of acid rain and fog, which can have a pH as low as 2. This level of acidity removes vegetation and causes eye and lung irritation for nearby people. Acid-producing gases also come from biological processes in land, wetlands, and oceans. The main biological source of sulfur compounds is dimethyl sulfide.

Nitric acid in rainwater provides nitrogen for plants and is created by electrical activity in the atmosphere, such as lightning.

Acid deposits have been found in glacial ice that is thousands of years old in remote areas of the world.

The main cause of acid rain is sulfur and nitrogen compounds from human activities, such as electricity generation, animal farming, factories, and vehicles. Power plants, which use electric generators, produce about a quarter of nitrogen oxides and two-thirds of sulfur dioxide in the atmosphere. Industrial acid rain is a major problem in China, Russia, and areas downwind from them. These regions often burn coal that contains sulfur to create heat and electricity.

The problem of acid rain has grown with population and industrial expansion and has spread to more areas. Using tall smokestacks to reduce local pollution has increased the spread of acid rain by releasing gases into the atmosphere, allowing them to travel farther and cause damage over large areas. Often, pollution settles far from where it was released, with mountainous regions receiving the most deposition because of higher rainfall. An example is the low pH of rain in Scandinavia. pH levels below 7 are considered acidic, and acid rain has a pH of about 4. This level is harmful for humans to consume. When low pH levels occur in certain areas, they harm both the environment and human health. In humans, low pH levels can cause hair loss, low urinary pH, severe mineral imbalances, constipation, and chronic conditions such as Fibromyalgia and Basal Carcinoma.

Chemical process

When fuels are burned and certain ores are smelted, sulfur dioxide and nitric oxides are produced. These gases are then changed into sulfuric acid and nitric acid.

In the gas form, sulfur dioxide is oxidized to form sulfuric acid.

Nitrogen dioxide reacts with hydroxyl radicals to create nitric acid.

The detailed processes depend on the presence of water and small amounts of iron and manganese.

Several oxidants, besides oxygen (O2), can cause these reactions. These include ozone, hydrogen peroxide, and oxygen.

Acid deposition

Wet deposition of acids happens when rain, snow, or other forms of precipitation carry acids from the air down to Earth's surface. This can occur because acids form in raindrops or because precipitation removes acids that are already present in clouds or below clouds. Both gases and tiny particles in the air are important for this process.

Acid deposition also happens through dry deposition when there is no precipitation. This type of deposition can account for up to 20 to 60% of all acid deposition. It occurs when particles and gases in the air settle on the ground, plants, or other surfaces.

Adverse effects

Acid rain can cause harm to forests, freshwaters, and soils. It can kill insects and aquatic life, damage buildings, and affect human health. Sulfuric acid and nitric acid in acid rain can change aquatic ecosystems by lowering water pH, increasing aluminum levels, and altering chemical processes. Water with a pH below 5 can prevent fish eggs from hatching and harm adult fish. As lakes and rivers become more acidic, fewer species of plants and animals can survive. Acid rain has removed insect life and some fish, like brook trout, from areas such as the Adirondack Mountains in the United States.

The impact of acid rain on lakes and rivers depends on the surrounding land. The United States Environmental Protection Agency (EPA) reports that acid rain caused acidity in 75% of acidic lakes and 50% of acidic streams surveyed. Lakes in areas with silicate rocks are more acidic than those in limestone areas because limestone can neutralize acid.

Acid rain harms soil by killing microbes that cannot survive in low pH conditions. The acid can change the shape of enzymes in these microbes, making them unable to function. Acid rain also releases harmful toxins like aluminum and removes important nutrients such as magnesium from the soil.

When acid rain removes minerals like calcium and magnesium from soil, it harms plants and animals that depend on them. For example, sugar maple trees are sensitive to these changes. Acid rain can damage plants in two ways: minor effects, where plants may survive for a time but eventually die due to mineral loss, and major effects, where damage happens more quickly. Acid rain can also dry out the waxy layer on leaves, causing plants to lose water and die. Changes in soil pH can reduce helpful microbes, making it harder for plants to access nutrients. Signs of soil acidification in plants include yellowing between leaf veins and reduced ability to photosynthesize.

Acid rain can harm plants even in high-altitude forests, where clouds and fog are more acidic than rain. Some plants, like those on Jinyun Mountain in China, can adapt to acid rain. Mild acid rain can improve plant growth, but very acidic rain (pH below 3.5) causes harm. Acid rain damages leaves by reducing chlorophyll, altering leaf cells, and thinning the protective cuticle. This weakens plants, reduces canopy cover, and makes them more vulnerable to disease.

Acid rain can kill trees by leaching aluminum from soil, which harms both plants and animals. It also removes nutrients needed for tree growth. At higher altitudes, acidic fog and clouds can damage tree leaves, reducing their ability to absorb sunlight and survive cold weather.

In agriculture, lime and fertilizers can replace nutrients lost from acid rain, but this is not possible in wild areas. Acid rain can harm crops by reducing soil nutrients and causing plant death. In oceans, acid rain lowers pH, making it harder for marine life to build shells and skeletons. Coral reefs are especially vulnerable because their limestone skeletons dissolve in acidic water. Excess nitrogen from acid rain can also cause harmful algae blooms, reducing oxygen in the water and creating "dead zones."

Acid rain can harm human health when people breathe in pollutants like nitrogen dioxide. Long-term exposure may cause breathing problems, and acid rain can also affect health through contaminated food and water.

Affected areas

Acid rain has greatly affected many places around the world, such as most of eastern Europe starting in Poland and moving north to Scandinavia, the eastern part of the United States, and southeastern Canada. Other places that are affected include the southeastern coast of China and Taiwan.

Prevention methods

Many coal-fired power plants use a process called flue-gas desulfurization (FGD) to remove sulfur-containing gases from the smoke they release. In a typical coal-fired power plant, FGD systems can remove 95% or more of sulfur dioxide (SO₂) from the flue gases. One example of FGD is the wet scrubber, which is widely used. A wet scrubber works by using a tower with a fan that pulls hot gases from a power plant into the tower. Lime or limestone in liquid form is added to the tower, where it mixes with the gases and reacts with sulfur dioxide. The calcium carbonate in limestone changes into calcium sulfate, a neutral substance that is removed from the scrubber. This process turns sulfur pollution into industrial sulfates.

In some areas, these sulfates are sold to chemical companies as gypsum if they are very pure. In other places, they are sent to landfills. Acid rain can have long-lasting effects because changes in water pH can cause harmful chemicals to continue leaching into clean water sources. This can harm insects, fish, and make it harder to restore natural life in affected areas.

Another method to reduce sulfur emissions is fluidized bed combustion, which burns fuel in a way that limits sulfur release. Vehicle emissions control systems also help reduce nitrogen oxide emissions from cars and trucks.

International agreements to address air pollution have been made by many countries for years. In 1979, European nations met to agree on principles from the UNECE Convention to fight long-range transboundary air pollution. The 1985 Helsinki Protocol aimed to reduce sulfur emissions under the same convention. These efforts have led to measurable results, such as a 40% drop in particulate matter in North America. The success of these agreements inspired further international efforts, such as the 1991 Air Quality Agreement between Canada and the United States. Most European countries and Canada signed these treaties. After 1999, the Long-Range Transboundary Air Pollution Convention paused until 27 countries met to further reduce acid rain effects. In 2000, Asian countries began discussing acid rain prevention, leading to the creation of the Acid Deposition Monitoring Network in East Asia (EANET) in 2001. EANET includes countries such as Cambodia, China, Indonesia, Japan, and others as of 2023.

Under this system, polluting facilities receive or can buy permits to release specific amounts of pollutants. If they install pollution control equipment and reduce emissions, they can sell unused permits to other facilities. This gives companies financial reasons to invest in pollution control.

The first emissions trading system was created in the United States with the Clean Air Act Amendments of 1990. The Acid Rain Program, established by this law, aimed to reduce sulfur dioxide and nitrogen oxide emissions, which cause acid rain. By using both rules and market-based methods, the program seeks to cut pollution at the lowest cost to society.