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How Acid Rain Works

Emissions of sulfur dioxide and nitrogen oxides react with water vapor in the atmosphere to create sulfuric and nitric acids.

If you hike through the Appalachian Mountains, you'll spot stands of dead and weakened trees. If you live in a city, you might notice worn stone buildings, streaks on your car roof or corroded metal railings and statues. You can see the effects of acid rain nearly everywhere you go, but with media and public attention turned to the more ominous prospect of global warming, acid rain has fallen by the wayside. The scourge from the sky almost seems like a 20th-century problem -- an issue dealt with in the 1980s and 1990s by legislation.

Acid rain occurs mostly in the Northern Hemisphere -- the more industrialized, dirtier half of the globe. Winds can sweep up emissions from high smokestacks and carry pollutants far from their original sources, crossing state lines and national borders in the process. Acid rain may not have the complete global range of greenhouse gases, but it is a transboundary, and therefore international, issue.

Acid rain, also known as acid deposition, is caused by emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from power plants, cars and factories. Natural sources like volcanoes, forest fires and lightning strikes also add to the man-made pollution. SO2 and NOx become acids when they enter the atmosphere and react with water vapor. The resulting sulfuric and nitric acids can fall as wet or dry depositions. Wet deposition is precipitation: acid rain, snow, sleet or fog. Dry deposition falls as acidic particulates or gases.

On the next page, we'll learn about the pH of acid rain.

The pH of Acid Rain

The pH scale is a measure of acidity and alkalinity. Acid rain has a pH of 5.0 or less.

Scientists express the acidity of acid rain using the pH scale. The scale defines a solution's acidity, neutrality or alkalinity based on its concentration of hydrogen ions. Acids have a high concentration of hydrogen ions and a low pH. The scale ranges from zero to 14, with pure water at a neutral 7.0. Most water, however, is not exactly pure. Even clean, normal rain has a pH of about 5.6. This is because it reacts with carbon dioxide in the atmosphere and forms mildly acidic carbonic acid before it becomes rain.

Acid rain has a pH of 5.0 or less. Most acid deposition ranges from pH 4.3 to 5.0 -- somewhere between the acidity of orange juice and black coffee. But comparing acid rain to safe, natural acids can be misleading. Even at its weakest, acid rain wrecks ecosystems by stunting sensitive plants and killing delicate aquatic eggs.

Programs that monitor acid rain analyze hydrogen content to determine pH. They also measure atmospheric concentrations of nitric acid, nitrate, sulfur dioxide, sulfate and ammonium. In the United States, the National Atmospheric Deposition Program (NADP) supervises wet deposition while the Clean Air Status and Trends Network (CASTNET) observes dry deposition. Monitoring acid deposition helps determine critical loads, or the amount of pollutants an ecosystem can support before damage. Accurate critical loads help set effective targets for SO2 and NOx reductions.

Now we'll learn about the harmful effects of acid rain on aquatic environments, forests, finishes, building materials and human health.

Surface Waters

Surface waters and their fragile ecosystems are perhaps the most famous victims of acid rain. Most of the precipitation that enters a lake, river, stream or marsh must first pass over and seep through soil. All soil has a buffering capacity, or ability to resist changes in acidity and alkalinity. The soil's buffering capacity determines a water body's acidity. If the capacity is low, or has reached its limit, acid rain can pass through un-neutralized.

Acid deposition weakens trees and pollutes surface waters.
Acid deposition weakens trees and pollutes surface waters.
Photographer: Detlev Voss | Agency:

Most life is comfortable at a near-neutral pH -- stray too far from pH 7.0, and delicate organisms begin to die. Plankton and invertebrates are sensitive to changes in acidity and die first. At pH 5.0, fish eggs degrade and young cannot develop. Adult fish and frogs can sometimes tolerate acidities as low as pH 4.0, but they starve as their weaker food sources die out. When acid rain disrupts the food chain, biodiversity decreases.

­Nitrogen deposition from acid rain also damages coastal waters and estuaries. Nitrogen-rich water supports ­massive algae growth­ and algal blooms. Bacteria decompose the dead algae, flourish themselves and soak up the water's available oxygen. Fish, shellfish, sea grass beds and coral reefs die in the algae-choked, oxygen-depleted waters. Scientists estimate that 10 percent to 45 percent of human-produced nitrogen that winds up in coastal waters comes from atmospheric deposition [Source: Environmental Protection Agency].

Most acidic bodies of water do not look polluted. As decaying organic matter settles, acidified water can appear clear and blue. Some species, like rushes and moss, even thrive in acidic conditions. But the greenery and clear waters belie an unwholesome environment. Diversity drops, and species left without predators often grow disturbingly large.

Acid rain also damages forests, as we'll see in the next section.


The Effects of Acid Rain

Acid rain can eat through stone and metal. It has accelerated the natural weathering process of this scarred stone angel's face.
Photographer: Michael Drager | Agency:

Forests rely on their soil's buffering capacity to protect them from acid rain. Acidic waters draw out soil toxins like aluminum. Trees take in the poisonous substances, and runoff dumps it in lakes, rivers and streams. Acid rain also dissolves helpful minerals and nutrients like calcium, magnesium and potassium before trees can absorb them. Acid rain rarely kills a forest outright but instead stunts its growth through years of soil degradation. Nutrient deprivation and exposure to toxins make trees more likely to topple in storms or die in cold weather.

Even trees in well-buffered soil can weaken in harsh acid fog. High-elevation forests soak in acidic clouds, which strip leaves of nutrients and break down trees' ability to resist cold. The bald peaks of the Appalachian Mountains tell of the poisonous effect of acid rain on high-elevation forests.

Materials and Finishes

Acid rain has the unsettling ability to erase and obliterate stone and metal, the most durable of materials. Old buildings, monuments and tombstones bear the smooth signs of acidic corrosion and deterioration. Acid deposition speeds up natural weathering caused by rain, sun, snow and wind.

Acid rain also mars automotive paint. The auto industry considers acid deposition one type of corrosive environmental fallout, along with tree sap, pollen and bird droppings. Acid markings leave irregular, etched shapes on horizontal surfaces. Repainting is the only way to fix a car finish disfigured by acid rain.

Photographer: Socrates­ | Agency:


Since acid rain can kill aquatic animals, weaken trees and dissolve stone, it seems like it could also scald or burn humans. But it doesn't affect people in the same way as it does fish or plants. Acid rain feels the same as regular rain -- it's even safe to swim in an acidic lake. But the sulfate and nitrate particulates of dry deposition can ­cause asthma, bronchitis and heart problems. The NOx in acid deposition also reacts with volatile organic compounds (VOCs) to form ground-level ozone. Ozone, or smog, aggravates and weakens the respiratory system.


Reducing Acid Rain

Power plants must limit emissions of SO and NOx to meet targets set by the Acid Rain Program.
Photographer: Czbrat | Agency: 2

Acid rain has existed since the first factories of the Industrial Revolution began spitting out toxic emissions. An English scientist, Robert Angus Smith, coined the term "acid rain" in 1872 when he wrote of its corroding touch on buildings and deadly effect on plants. But acid rain did not become a government-monitored environmental problem until more than a century later. Scientists had by then determined that acid rain was a transboundary rather than a local concern. In 1980, the Acid Deposition Act launched a 10-year study on acid rain under the direction of the National Acidic Precipitation Assessment Program (NAPAP) to monitor sites around the country.

In 1990, armed with the NAPAP's study, Congress changed the existing Clean Air Act to include acid rain. The new Title IV amendment of the Clean Air Act called for SO2 and NOx reductions. The Acid Rain Program (ARP) was formed in 1995 to bring Title IV into effect.

The ARP places limits on the power industry to reduce annual emissions of SO2 and NOx. The ARP uses a cap and trade program to cut SO2 emissions. It sets a cap on the total amount of SO2 that power plants in the contiguous United States can produce. After setting a cap, the ARP distributes allowances to power plant units. Units are only allowed to produce as much SO2 as they have credit for. If they reduce emissions faster than the ARP requires, they can bank allowances for future use or sell them to other plants. The final 2010 cap will be 8.95 million tons allowed per year, a remarkable 50 percent less than power plant emissions from 1980 [Source: EPA].

The ARP regulates NOx reductions with a more conventional rate-based regulatory system. The program sets a limit on allowable pounds of NOx per million British thermal units (lb/mmBtu) for every power plant's boiler. Owners either meet target reductions for individual boilers or average the emissions of all units owned and meet a combined target. The ARP aims to reduce NOx to 2 million tons below the projected 2000 level had Title IV not existed [Source: EPA].

Power plants meet their ARP targets by using low sulfur coal, "wet scrubbers" or flue gas desulphurization systems, low NOx burners and other clean coal technologies. They can also trade SO2 credits amongst themselves.

Even with an increased energy demand, the ARP has successfully reduced emissions of SO2 and NOx. But NAPAP suggests that for ecosystems to fully recover, reductions will have to drop an additional 40 percent to 80 percent below the full-force limits of 2010 [Source: EPA].

Cars also emit NOx. Newer designs of catalytic converters help treat exhaust and remove NOx and other pollutants like carbon monoxide and the VOCs that contribute to smog.

Even with remarkable clean coal technologies, catalytic converters and strong caps and regulations, fossil fuels are still a dirty power source. Alternative forms of energy like nuclear, solar and hydropower do not emit the millions of tons of SO2 and NOx­ that upend ecosystems, blight buildings and monuments and weaken people's health.

­To learn more about acid rain, alternative forms of energy and other related topics, check out the links on the next page.

Related HowStuffWorks Articles

More Great Links


  • “Acid Rain.” Encyclopedia of the Atmospheric Environment.
  • “Acid Rain.” Environmental Literacy Council.
  • “Acid Rain Program: 2005 Progress Report.” U.S. Environmental Protection Agency.
  • “A Brief History.” U.S. Environmental Protection Agency.
  • “Clean Air Status and Trends Network (CASTNET).” U.S. Environmental Protection Agency.
  • “Effects of Acid Rain - Forests.” U.S. Environmental Protection Agency.
  • “Effects of Acid Rain - Surface Waters and Aquatic Animals.” U.S. Environmental Protection Agency.
  • Ganguly, Meenhakshi. “At The Taj Mahal, Grime Amid Grandeur.” Time. September 10, 2001.,9171,1000714-1,00.html
  • “Measuring Acid Rain.” U.S. Environmental Protection Agency.
  • “NADP History and Overview.” National Atmospheric Deposition Program.