Acid Rain

Acid rain is the wet or dry deposition of acidic substances and their precursors on the Earth's surface. The ongoing industrialization of society has resulted in the increased release of acidic chemicals into the atmosphere. These chemicals are deposited as acid rain, impacting lakes, forests and human health.

Description

Wet deposition refers to rain, snow, hail, drizzle and other familiar forms of visible precipitation. Dry deposition, mostly invisible, occurs through gravitational settling of large particles, and the uptake of gases and small particles at the Earth's surface. Rain and other precipitation may be defined as acidic or alkaline (basic) depending on the chemical composition. The degree of acidity is usually measured on the pH scale, a logarithmic measure of the concentration of hydrogen ions (H+) in precipitation. A neutral solution has a pH of seven. Acidic solutions have values below seven and basic solutions have values above seven. For each change of one pH unit, the hydrogen ion content changes by a factor of 10. A clean water sample in equilibrium with atmospheric carbon dioxide will have a value of 5.6, and this is often used as a definition of "clean" rain. When values differ from this, it means that other substances, either natural or man-made, are present in the rain.

Current annual measurements of the average pH of precipitation in the northern hemisphere range from about 4.0 to 7.0. The lower, highly acidic values occur primarily over, and immediately downwind, of urban and industrialized areas in North America, Europe and Asia. Higher pH values in precipitation are found over less industrialized regions where the atmosphere contains larger amounts of alkaline dust. The primary cause of low pH in precipitation over northeastern North America is sulphuric acid (H2SO4) from industrial and urban emissions of sulphur dioxide (SO2). Nitric acid (HNO3) generated from emissions of nitrogen oxides (NOx) is a significant contributing factor in this region. In Canada, as in many other countries, the majority of NOx emissions are from transportation. Acid rain’s precursors, SO2 and NOx, can be transported thousands of kilometres through the atmosphere, returning to Earth as dry or wet deposition.

Emissions Over Time

Canadian emissions of SO2 in 2011 were 1.85 million tonnes, down from 2.2 million tonnes in 2006. As a point of comparison, in 2011 SO2 emissions in the US were 6.28 million tonnes, down from 12 million tonnes in 2006. Fuel for electricity and heating, as well as non-ferrous smelters (producing such metals as nickel and copper are the largest sources of SO2 emissions in Canada, followed closely by emissions produced by the oil and gas sector.

In terms of NOx emissions, Canada produced 1.94 million tonnes in 2011, compared to 2.3 million tonnes in 2006. The US produced about the same in 2011 (1.94 million tonnes), compared to 3.4 million tonnes in 2006. The largest sources of NOx emissions in Canada are transportation vehicles (including cars, trains, planes and boats), and the oil and gas industry.

Effects of Acid Rain

When acid rain reaches the Earth's surface, it can cause damage to aquatic ecosystems and buildings. Acid rain and its associated pollutants (SO2, NOx, sulphate particles and ozone) can also damage forests and crops, and there is evidence of adverse human health effects. The degree of effects depends on the acid-reducing capability of the receptor (e.g., vegetation, soils, rock, lakes and streams). In areas where this buffering capacity is low, like the Canadian Shield, acidic deposition over several years has led to increased acidity of rivers and lakes, and to the accelerated leaching of aluminum from soils. This is seen most in the surface waters of southeastern Canada, where acid rain levels are highest. However, SO2 emissions in western Canada have increased to the point that vulnerable lakes in this region may also be threatened.

Aquatic life is dependent on the balanced pH of surface waters. Once the pH falls below approximately 5.5, both the amount and diversity of vegetation, zooplankton, amphibians and fish decreases. The aluminum leached from soils may also be in a form that is toxic to aquatic organisms. Once the average pH of a lake drops to around 4.5, most fish populations are eradicated due to reproductive failure or the disappearance of suitable food sources. Fish populations in thousands of lakes in eastern North America and Scandinavia have declined or disappeared because of water acidification, and hundreds of thousands more are threatened. Rivers are impacted as well. This is seen in the marked decline of ATLANTIC SALMON in the Maritimes and in Scandinavia. Birds and other fish predators may decrease in numbers because of this reduced food supply.

Reductions in North American SO2 emissions could suggest that aquatic ecosystems will soon recover from acidification. However, this is not the case. Only lakes located near smelters with dramatically reduced emissions approach this expectation. Most lakes are only affected by long-range emissions and so far, they show relatively small increases in pH. This delay in the chemical recovery of lakes is due to several geochemical factors related to the storage or release of acids, or bases from the forest soils and wetlands that surround these lakes. Biological recovery in lakes does not necessarily follow chemical recovery. The only extensive evidence of biological recovery is in lakes from the Sudbury/Killarney region of Ontario.

The effects of acid rain and its associated pollutants on forests and agriculture are not as clear-cut, but are potentially serious. These include direct damage to plant foliage, seed germination failure, retardation of growth (particularly at early life stages), deterioration of plant roots associated with the leaching of soil constituents and, possibly, increased plant susceptibility to insects and diseases.

There are several potential effects of acid rain on human health. The lead, copper and other metals from water delivery pipes can leach and contaminate acidified drinking water. Increased concentrations of heavy metals in fish from acidified rivers and lakes can pose a problem for populations consuming significant quantities of these fish.

Control Methods

Methods available to reduce SO2 emissions include: the use of low-sulphur coal and oil; the removal of sulphur from fuel and feeder ore; the use of technologies that remove the SO2 at the source of emission, like flue-gas desulphurization techniques; energy conservation; and the use of alternative energy sources. North American techniques for controlling acid rain precursors focus primarily at reducing near-source air concentrations to levels necessary to avoid immediate and short-term impacts on human health (see Air Pollution). The installation of pollution control devices and the building of taller emission stacks were effective in achieving the goal of improved air quality in North American cities. However, the taller stacks disperse SO2 and NOx emissions over large regions, and the emission standards for the short-term protection of human health are inadequate for the protection of affected regional environments and longer-term human health.

Emissions of SO2 in both Canada and the US decreased through the early 1970s to the present as a result of the increased use of pollution control devices, the use of more low-sulphur fuels and the introduction of some nuclear power plants. These drops in SO2 emissions have reduced acid rain levels and allowed the chemical recovery of some eastern Canadian lakes, thereby demonstrating the potential effectiveness of further control actions. In the absence of new controls, and the expansion of SO2 emission sources (e.g., in western Canada), the cumulative acidification effects on regional environments remain a serious problem. In addition, there has been little reduction in NOx emissions over North America.

Setting Control Targets

In 1983, as a first step in controlling the effects of acid rain on surface waters, Canada adopted a target load of 20 kg of wet sulphate per hectare per year. It was estimated that a reduction of deposition rates to this value would protect moderately sensitive lake ecosystems and could be achieved by reducing North American SO2 emissions by about 50 percent. The eastern Canadian provinces and the federal government signed several federal-provincial agreements in 1987 aiming to reduce emissions by 50 percent by 1994. Since 1990, Canada has used a more precise standard called the "critical load." The critical load is the highest amount of pollutants an ecosystem can tolerate without exhibiting negative ecosystem effects. For lakes located on the Canadian Shield, the critical load is almost always less than the 1983 target load, and it varies spatially depending on the acid sensitivity of the surrounding terrain.

About one-half of the sulphate deposition in eastern Canada comes from SO2 sources in the US. Therefore, control action in the US was needed for Canada to achieve its target loading goal. After years of pressure from Canada, in November 1990 the United States government passed a new Clean Air Act promising to reduce SO2 emissions by 50 percent by 2000. The following year, the two nations signed the Canada-US Air Quality Agreement, which further codified the reductions in S02 and N0x emissions. In 1998, the federal, provincial and territorial ministers of Energy and Environment agreed to "The Canada-Wide Acid Rain Strategy for Post-2000," which has the long-term goal of reducing acid rain to meet the critical load standard. This means that much greater SO2 emission reductions than those presently required by legislation will be needed to promote widespread chemical and later, biological recovery.

In 1985, Canada signed the United Nations Economic Commission for Europe (ECE) Helsinki Protocol to reduce its sulphur compounds (or the export of these compounds to other countries via the atmosphere) by 1993. In 1994, Canada signed the Oslo Protocol to cap sulphur emissions at 1.75 million tonnes in geographic regions contributing to acidification in Canada and the US. This area includes Ontario, Québec, New Brunswick, Nova Scotia and Prince Edward Island. Acid rain is but one manifestation of the increasing effects of human-made chemicals on the composition of the global atmosphere. Other anthropogenic effects associated with growing industrialization include Arctic haze, climate change and the depletion of the stratospheric ozone layer (see Ozone Depletion). These changes in regional and global environments, and their socio-economic impacts, are attracting increasing international attention. 

Small volcanic eruptions could be slowing global warming

WASHINGTON, DC- Small volcanic eruptions might eject more of an atmosphere-cooling gas into Earth's upper atmosphere than previously thought, potentially contributing to the recent slowdown in global warming, according to a new study.

Scientists have long known that volcanoes can cool the atmosphere, mainly by means of sulfur dioxide gas that eruptions expel. Droplets of sulfuric acid that form when the gas combines with oxygen in the upper atmosphere can remain for many months, reflecting sunlight away from Earth and lowering temperatures. However, previous research had suggested that relatively minor eruptions-those in the lower half of a scale used to rate volcano "explosivity"-do not contribute much to this cooling phenomenon.

Now, new ground-, air- and satellite measurements show that small volcanic eruptions that occurred between 2000 and 2013 have deflected almost double the amount of solar radiation previously estimated. By knocking incoming solar energy back out into space, sulfuric acid particles from these recent eruptions could be responsible for decreasing global temperatures by 0.05 to 0.12 degrees Celsius (0.09 to 0.22 degrees Fahrenheit) since 2000, according to the new study accepted to Geophysical Research Letters, a journal of the American Geophysical Union.

These new data could help to explain why increases in global temperatures have slowed over the past 15 years, a period dubbed the 'global warming hiatus,' according to the study's authors.

The warmest year on record is 1998. After that, the steep climb in global temperatures observed over the 20th century appeared to level off. Scientists previously suggested that weak solar activity or heat uptake by the oceans could be responsible for this lull in temperature increases, but only recently have they thought minor volcanic eruptions might be a factor.

Climate projections typically don't include the effect of volcanic eruptions, as these events are nearly impossible to predict, according to Alan Robock, a climatologist at Rutgers University in New Brunswick, N.J., who was not involved in the study. Only large eruptions on the scale of the cataclysmic 1991 Mount Pinatubo eruption in the Philippines, which ejected an estimated 20 million metric tons (44 billion pounds) of sulfur, were thought to impact global climate. But according to David Ridley, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge and lead author of the new study, classic climate models weren't adding up.

"The prediction of global temperature from the [latest] models indicated continuing strong warming post-2000, when in reality the rate of warming has slowed," said Ridley. That meant to him that a piece of the puzzle was missing, and he found it at the intersection of two atmospheric layers, the stratosphere and the troposphere- the lowest layer of the atmosphere, where all weather takes place. Those layers meet between 10 and 15 kilometers (six to nine miles) above the Earth.

Traditionally, scientists have used satellites to measure sulfuric acid droplets and other fine, suspended particles, or aerosols, that erupting volcanoes spew into the stratosphere. But ordinary water-vapor clouds in the troposphere can foil data collection below 15 km, Ridley said. "The satellite data does a great job of monitoring the particles above 15 km, which is fine in the tropics. However, towards the poles we are missing more and more of the particles residing in the lower stratosphere that can reach down to 10 km."

To get around this, the new study combined observations from ground-, air- and space-based instruments to better observe aerosols in the lower portion of the stratosphere.

Four lidar systems measured laser light bouncing off aerosols to estimate the particles' stratospheric concentrations, while a balloon-borne particle counter and satellite datasets provided cross-checks on the lidar measurements. A global network of ground-based sun-photometers, called AERONET, also detected aerosols by measuring the intensity of sunlight reaching the instruments. Together, these observing systems provided a more complete picture of the total amount of aerosols in the stratosphere, according to the study authors.

Including these new observations in a simple climate model, the researchers found that volcanic eruptions reduced the incoming solar power by -0.19 � 0.09 watts of sunlight per square meter of the Earth's surface during the 'global warming hiatus', enough to lower global surface temperatures by 0.05 to 0.12 degrees Celsius (0.09 to 0.22 degrees Fahrenheit). By contrast, other studies have shown that the 1991 Mount Pinatubo eruption warded off about three to five watts per square meter at its peak, but tapered off to background levels in the years following the eruption. The shading from Pinatubo corresponded to a global temperature drop of 0.5 degrees Celsius (0.9 degrees Fahrenheit).

Robock said the new research provides evidence that there may be more aerosols in the atmosphere than previously thought. "This is part of the story about what has been driving climate change for the past 15 years," he said. "It's the best analysis we've had of the effects of a lot of small volcanic eruptions on climate."

Ridley said he hopes the new data will make their way into climate models and help explain some of the inconsistencies that climate scientists have noted between the models and what is being observed.

Robock cautioned, however, that the ground-based AERONET instruments that the researchers used were developed to measure aerosols in the troposphere, not the stratosphere. To build the best climate models, he said, a more robust monitoring system for stratospheric aerosols will need to be developed.

Acid Rain: 10 quick facts on this catastrophic result of air pollution

Here is what it can do to land, humans and animals:

• The sulphur dioxide and nitrogen oxide that creates acid rain can cause diseases such as cancer, asthma and heart disease

• Acid rain has many ecological effects. The worst effect it has is on lakes, streams, wetlands and other aquatic environments

• Acid rain can kill a whole forest

• It can destroy the leaves on the trees by cutting off their light and nutrient supply. It also change the acidity in the soil, making it impossible for trees and other plant life to grow

• Acid rain changes the pH of water and makes the water toxic to the fish and other aquatic animals

• As per reports, entire lakes have been declared dead because of acid rain

• Acid rain has the same approximate pH as vinegar

• Sulphur dioxide, the major contributor to acid rain, is the by-product of industrial products and is produced by burning fossil fuels

• Most acid rains occur due to human activities

• Stone buildings and monuments can also get damaged from acid rain.

Acid Rain Affect Harmful to Animals

Acid rain is precipitation containing nitric and sulfuric acids. While some natural occurrences like volcanoes and rotting vegetation contribute to these acids, it is the human activity of burning fossil fuels that causes a majority of acid rain. When acid rain reaches the surface of the Earth, it can devastate ecological systems by killing populations, eliminating food sources and reducing biodiversity.

Acid Rain and Water Sources

The U.S. Environmental Protection Agency says the effects of acid rain are most obvious in aquatic ecosystems. Water runoff from forests and roads often flows into streams, lakes and marshes, and acid rain also falls directly into these water sources. While some water sources are naturally more acidic, most lakes and streams have a pH between 6 and 8. As of 2012, acid rain caused 75 percent of acidic lakes and 50 percent of acidic streams, the National Surface Water Survey reports. Some water sources now have a pH of less than 5.

Aquatic Life

Acid rain creates conditions that threaten the survival of aquatic life. Arthropods and fish die in water that has a pH of less than 5. The sensitivity of amphibian eggs to acidity contributes to their decline. While normal lakes might be home to nine to 16 species of zooplankton, acidic lakes retain only one to seven species, reports State University of New York professor Thomas Wolosz. Water with low pH also causes gill damage in fish and death to fish embryos. Reproductive failure is the primary way acid rain causes animal extinction in aquatic systems, says Wolosz. Some affected fish have low calcium levels, which affects reproductive physiology, and some females do not even release ova during mating season in acidic lakes. Also, since the level of carbon dioxide rises in acidic water, the level of carbon dioxide in the blood also increases; thus, oxygen consumption goes up and the rate of growth decreases in animal species. Additionally, bones decalcify due to increased carbon dioxide, which causes deformity in animals.

Bird Life

One less obvious effect of acid rain involves bird life. According to a study by Miyoko Chu and Stefan Hames of the Cornell Lab of Ornithology, acid rain is linked to population decline of the wood thrush. Because female birds require more calcium to solidify their eggs, they rely on calcium-rich foods like snails. In areas of acid rain, snail populations disappear, leading to egg defects for the birds. Both the Cornell Lab and Wolosz cited similar occurrences in the Netherlands, and eggshell defects triggered by acid rain might be the No. 1 cause of loss of bird biodiversity in certain regions.

Other Animals

Acid rain indirectly affects other animals, such as mammals, which depend on animals like fish for food sources. The EPA reports that acid rain causes a reduction of population numbers and sometimes eliminates species entirely, which in turn decreases biodiversity. When one part of the food chain is disturbed, it affects the rest of the chain. The loss of biodiversity affects other species that rely on those animals for food sources. For example, when fish populations are depleted in certain lakes, mammals like bears or even humans who eat those fish need to find alternate sources of food; they can no longer survive in their current environment. More directly, according to Nature.com, breathing acid particles causes respiratory problems like asthma, bronchitis and pneumonia in humans.

Got calcium? Mineral key to restoring acid rain-damaged forests

Calcium can do much more than strengthen bones. The mineral is a critical nutrient for healthy tree growth, and new research shows that adding it to the soil helps reverse the decades-long decline of forests ailing from the effects of acid rain.

Helicopter distributes calcium pellets throughout research watershed at Hubbard Brook Experimental Forest. (Photo courtesy Hubbard Brook Research Foundation)

The paper, published today (Thursday, Sept. 19), in the journalEnvironmental Science and Technology (EST) Letters, and led by John Battles, professor of forest ecology at the University of California, Berkeley, also presents strong evidence that acid rain impairs forest health.

The paper reports on 15 years of data from an ongoing field experiment in the Hubbard Brook Experimental Forest in New Hampshire led by study co-author Charles Driscoll Jr., professor of environmental systems engineering at Syracuse University.

“It is generally accepted that acid rain harms trees, but the value of our study is that it proves the causal link between the chronic loss of soil calcium caused by decades of acid rain and its impact on tree growth,” said Battles. “The temporal and spatial scope of the study – 15 years and entire watersheds – is unique and makes the results convincing.”

The researchers reported that trees in the calcium-treated watershed produced 21 percent more wood and 11 percent more leaves than their counterparts in an adjacent control site. The iconic sugar maple – the source of maple syrup – was the tree species that responded most strongly to the restoration of calcium in the soil.

Acid deposition has altered the calcium cycle in watersheds in the Northeastern United States in ways that are similar to changes observed at Hubbard Brook Experimental Forest. The bar graphs in the photos show the differences in calcium (Ca) levels between 1950, bars on the left, and 1995. (Graphic courtesy of Hubbard Brook Research Foundation)

The research site, managed by the U.S. Forest Service, was targeted because of the declining growth rates and unexpected death of trees in the area. Previous measurements of the forest soil showed a 50 percent depletion of calcium.

Acid rain forms when sulfur dioxide and nitrogen oxides – gases produced from the burning of fossil fuels – react with water molecules in the air. The mountainous regions in the Northeast have thin soils that are already acidic, so they have limited ability to withstand the assaults of nutrient-dissolving acid rain. Moreover, watersheds along the eastern corridor of the United States had been exposed to more acid rain because of the greater number of coal-burning power plants in the region.

The Clean Air Act of 1970 significantly reduced sulfur dioxide emissions, but decades of acid rain already had changed the soil chemistry of many sensitive regions, including the White Mountains of New Hampshire and the Adirondacks of New York.

For the Hubbard Brook study, a helicopter spread 40 tons of dry calcium pellets over a 29-acre watershed over several days in October 1999. The calcium was designed to slowly work its way into the watershed over many years.

“This was restoration, not fertilization,” said Battles. “We were only replacing what was lost.”

Researchers monitored the forest over the next 15 years, comparing the treatment area with an adjacent watershed that had the same characteristics, but did not get the added calcium.

Hubbard Brook Experimental Forest with a view of the south-facing experimental watersheds. (Photo courtesy of the Hubbard Brook Ecosystem Study)

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

“This study has important implications that go well beyond the forests of the northeastern United States,” said Dave Schindler, a professor of ecology at the University of Alberta in Canada who was not part of this research. “Similar depletion of soil nutrients by acid precipitation has occurred in much of eastern Canada and Europe. This long-term study indicates that the calcium problem can be reversed, and that is heartening.”

Both Schindler and Battles noted that the high cost of adding calcium to the soil would likely limit its use to targeted watersheds rather than as a treatment for vast areas of affected forests.

“Prevention is always preferable, and with our study’s clear evidence that acid rain is hurting forests, other countries will hopefully be motivated to intervene sooner by implementing air pollution standards to reduce emissions,” said Battles.

Funding from the National Science Foundation helped support this research.

What Is Acid Rain: Tips For Safeguarding Plants From Acid Rain Damage

Acid rain has been an environmental buzzword since the 1980s, even though it started falling from the sky and eating through lawn furniture and ornaments as early as the 1950s. Although common acid rain isn’t acidic enough to burn skin, the effects of acid rain on plant growth can be dramatic. If you live in an acid rain-prone area, read on to learn about safeguarding plants from acid rain.

What is Acid Rain?

Acid rain forms when sulfur dioxide and nitrogen oxide react with chemicals like water, oxygen and carbon dioxide in the atmosphere to form sulfuric acid and nitric acid. Water containing these acidic compounds falls back to the earth as rain, harming plants and other immobile objects below. Although the acid from acid rain is weak, normally no more acidic than vinegar, it can seriously alter the environment, damaging plants and aquatic ecosystems.

Does Acid Rain Kill Plants?

This is a straightforward question with a not very straightEnforward answer. Acid rain and plant damage go hand in hand in areas prone to this type of pollution, but the changes to a plant’s environment and tissues are gradual. Eventually, a plant exposed to acid rain will die, but unless your plants are incredibly sensitive, the acid rain unusually potent and frequent or you’re a very bad gardener, the damage is not fatal.

The way that acid rain damages plants is very subtle. Over time, the acidic water alters the pH of the soil where your plants are growing, binding and dissolving vital minerals and carrying them away. As the soil pH falls, your plants will suffer increasingly obvious symptoms, including yellowing between the veins on their leaves.

Rain that falls on leaves can eat away the outer waxy layer of tissue that protects the plant from drying out, leading to the destruction of the chloroplasts that drive photosynthesis. When a lot of leaves are damaged at once, your plant may become very stressed and attract a host of pests and diseases organisms.

Safeguarding Plants from Acid Rain

The best way to protect plants from acid rain is to prevent rain from falling on them, but with larger trees and shrubs this may be impossible. In fact, many experts recommend planting more tender specimens under large trees to protect them from damage. Where trees aren’t available, moving these delicate plants to gazebos or covered porches will do. When all else fails, some thick plastic draped over stakes surrounding the plant can hold off acid damage, provided that you place and remove the covers promptly.

The soil is another matter entirely. If you live in an area where acid rain is common, soil testing every six to 12 months is a good idea. Frequent soil tests will alert you to problems in the soil so you can add extra minerals, nutrients or lime when necessary. Staying one step ahead of acid rain is vital to keeping your plants healthy and happy.

Acid Rain: Do you need to start wearing a rain hat?

Depending on where you live, maybe you've heard of acid rain. Now, acid rain is not pure acid falling from the sky, but rather it is rainfall or atmospheric moisture that has been mixed with elements and gases that have caused the moisture to become more acidic than normal. Pure water has a pH of 7, and, generally, rainfall is somewhat on the acidic side (a bit less than 6). But, acid rain can have a pH of about 5.0-5.5, and can even be in the 4 range in the northeastern United States, where there are a lot of industries and cars.

Causes of acid rain

Acidic precipitation can be caused by natural (volcanoes) and man-made activities, such as from cars and in the generation of electricity. The precursors, or chemical forerunners, of acid rain formation result from both natural sources, such as volcanoes and decaying vegetation, and man-made sources, primarily emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_x) resulting from fossil fuel combustion. The burning of fossil fuels (coal and oil) by power-production companies and industries releases sulfur into the air that combines with oxygen to form sulfur dioxide (SO₂). Exhausts from cars cause the formation of nitrogen oxides in the air. From these gases, airborne sulfuric acid (H₂SO₄) and nitric acid (HNO₃) can be formed and be dissolved in the water vapor in the air. Although acid-rain gases may originate in urban areas, they are often carried for hundreds of miles in the atmosphere by winds into rural areas. That is why forests and lakes in the countryside can be harmed by acid rain that originates in cities.

Effects of acid rain

The environment can generally adapt to a certain amount of acid rain. Often soil is slightly basic (due to naturally occurring limestone, which has a pH of greater than 7). Because bases counteract acids, these soils tend to balance out some of the acid rain's acidity. But in areas, such as some of the Rocky Mountains and parts of the northwestern and southeastern United States, where limestone does not naturally occur in the soil, acid rain can harm the environment.

Some fish and animals, such as frogs, have a hard time adapting to and reproducing in an acidic environment. Many plants, such as evergreen trees, are damaged by acid rain and acid fog. I've seen some of the acid-rain damage to the evergreen forests in the Black Forest of Germany. Much of the Black Forest was indeed black because so much of the green pine needles had been destroyed, leaving only the black trunks and limbs! You also might notice how acid rain has eaten away the stone in some cities' buildings and stone artwork.

Geographic distribution of acid rain

Acidity in rain is measured by collecting samples of rain and measuring its pH. To find the distribution of rain acidity, weather conditions are monitored and rain samples are collected at sites all over the country. The areas of greatest acidity (lowest pH values) are located in the Northeastern United States. This pattern of high acidity is caused by the large number of cities, the dense population, and the concentration of power and industrial plants in the Northeast. In addition, the prevailing wind direction brings storms and pollution to the Northeast from the Midwest, and dust from the soil and rocks in the Northeastern United States is less likely to neutralize acidity in the rain.

Acid rain and stone

When you hear or read in the media about the effects of acid rain, you are usually told about the lakes, fish, and trees in New England and Canada. However, we are becoming aware of an additional concern: many of our historic buildings and monuments are located in the areas of highest acidity. In Europe, where buildings are much older and pollution levels have been ten times greater than in the United States, there is a growing awareness that pollution and acid rain are accelerating the deterioration of buildings and monuments.

Stone weathers (deteriorates) as part of the normal geologic cycle through natural chemical, physical, and biological processes when it is exposed to the environment. This weathering process, over hundreds of millions of years, turned the Appalachian Mountains from towering peaks as high as the Rockies to the rounded knobs we see today. Our concern is that air pollution, particularly in urban areas, may be accelerating the normal, natural rate of stone deterioration, so that we may prematurely lose buildings and sculptures of historic or cultural value.

What about buildings?

Many buildings and monuments are made of stone, and many buildings use stone for decorative trim. Granite is now the most widely used stone for buildings, monuments, and bridges. Limestone is the second most used building stone. It was widely used before Portland cement became available in the early 19th century because of its uniform color and texture and because it could be easily carved. Sandstone from local sources was commonly used in the Northeastern United States, especially before 1900. Nationwide, marble is used much less often than the other stone types, but it has been used for many buildings and monuments of historical significance. Because of their composition, some stones are more likely to be damaged by acidic deposition than others. Granite is primarily composed of silicate minerals, like feldspar and quartz, which are resistant to acid attack. Sandstone is also primarily composed of silica and is thus resistant. A few sandstones are less resistant because they contain a carbonate cement that dissolves readily in weak acid. Limestone and marble are primarily composed of the mineral calcite (calcium carbonate), which dissolves readily in weak acid; in fact, this characteristic is often used to identify the mineral calcite.

How does acid precipitation affect marble and limestone buildings?

Acid precipitation affects stone primarily in two ways: dissolution and alteration. When sulfurous, sulfuric, and nitric acids in polluted air react with the calcite in marble and limestone, the calcite dissolves. In exposed areas of buildings and statues, we see roughened surfaces, removal of material, and loss of carved details. Stone surface material may be lost all over or only in spots that are more reactive.

You might expect that sheltered areas of stone buildings and monuments would not be affected by acid precipitation. However, sheltered areas on limestone and marble buildings and monuments show blackened crusts that have spalled (peeled) off in some places, revealing crumbling stone beneath. This black crust is primarily composed of gypsum, a mineral that forms from the reaction between calcite, water, and sulfuric acid. Gypsum is soluble in water; although it can form anywhere on carbonate stone surfaces that are exposed to sulfur dioxide gas (SO₂), it is usually washed away. It remains only on protected surfaces that are not directly washed by the rain. Gypsum is white, but the crystals form networks that trap particles of dirt and pollutants, so the crust looks black. Eventually the black crusts blister and spall off, revealing crumbling stone.