What is Acid Rain?
First identified in 1872 in Sweden and studied in the beginning in the 1950s, acid rain is precipitation in the form of rain, snow, hail, dew, or fog that transports sulfur and nitrogen compounds from the high atmosphere to the ground. Sulfur dioxide (SO2) and nitrogen oxides (NO, NO2) are bi-products from burning fuels in electric utilities and from other industrial and natural sources. These chemicals react with water, oxygen, carbon dioxide, and sunlight in the atmosphere to form sulfuric and nitric acids. The acids reach the ground and change the chemistry within the environment.
The acidity of any solution is determined on the pH scale of 0 to 14. A pH level of 0 to 7 is considered acidic; 7 is neural; and a level above 7 is alkaline. As the pH number decreases, acidity increases. Unopened bottled distilled water has a pH of 7, so it is neutral. In comparison, household ammonia is an alkaline with a pH of 11.5. Milk is slightly acidic with a 6.5 pH, and soft drinks, which contain phosphoric acid, have a 3.1 pH.
Although the pH scale may seem straightforward, determining the pH of “normal” rain is much more complex. When distilled water is exposed to air, an interaction with carbon dioxide increases acidity through the formation of carbonic acid, H2CO3, and the pH level falls. Many scientists agree that the normal pH of rain is a slightly acidic 5.6 because of perpetual chemical interactions in the air.
What’s more, rain pH levels can vary significantly over short distances and in a short amount of time, even during the same rainfall. Seasons, climate, and a host of other factors can also influence the acidity of rain.
Rain and snow are not the only processes that deposit sulfur and nitrogen acids from the atmosphere to the ground. These compounds are also present in gases and dry particles, which are more difficult to measure. Like wet deposition, the occurrence of “dry deposition” of acids varies in different areas, depending on distance from the emission source and climatic conditions.
The Acid Rain Problem:
“Acid rain” became a household term in the 1980s when unchecked emissions from industry and motor vehicles were blamed for causing environmental deterioration. Scientific evidence has linked acid rain to decreased fish and wildlife populations, degraded lakes and streams, and human health hazards. Although the term has since faded from public consciousness, acid rain is a complex and global problem that still exists today.
On the east side of forestry building stands a Norway Spruce 40 feet tall. Twenty years ago, according to Joe Gardner, Penn State tree surgeon for 38 years, the tree was bushy, green, full of healthy cones near the top, and, in the characteristic manner of a Norway Spruce, the lower branches drooped slightly.
Today, many of the branches hang straight down. Branches, bare except for tufts of needles at the bottom, dangle in the wind like pendulums. Aborted cones small, dark brown, unopened spot the tree at all heights, to within six feet of the ground. The bald trunk is visible from across the east lawn.
Trees in the world show the same symptoms, eventually dying from the phenomenon called "forest decline." I had heard people blame forest decline along with the death of aquatic life, reduction in yields of certain crops, and the deterioration of European cathedrals and Greek temples on acid rain.
I wondered if the charges were true. To find out, I talked to some of the 20 or so scientists at meteorologists, hydrologists, biologists, and plant pathologists whose research touched on the subject. Every time I asked about this thing that scientists, politicians, and the press carefully call the "Acid Rain Problem," I became more confused.
Finally, I went to see someone I'd heard had a broad viewpoint an engineer who had been an atmospheric scientist in Ohio, a state as embroiled in the acid rain controversy. At Ohio State University, he had organized the First International Symposium on Acid Precipitation and the Forest Ecosystem in 1975 and had served on the Governor's Scientific Advisory Task Force on Acid Rain.
"Today, scientists are advising policymakers on the best strategy in dealing with acid rain. So besides trying to understand how the natural world works for the sake of knowledge, they also have to answer to policymakers. That forces them to present the best evidence they have, even, sometimes, before they feel that it's ready to present, and it also forces them to take sides in a debate that has gone far beyond questions of science and technology. The federal government has spent more than million in the last three years on acid rain research, and some scientists question the direction research is going."
"But let's start at the beginning."What was your first question as a student of the 'Acid Rain Problem'?"
How can we tell if the rain is becoming more acidic?
I had asked Bob Long, a doctoral candidate who, with Don Davis, a plant pathology professor, is studying forest decline, which has the dubious distinction of receiving the most acidic rainfall in the country. Geological Survey Tree Ring Lab, have analyzed more than 1,000 core samples from pines, hemlocks, oaks, white ash, black cherry, tulip poplar, pignut hickory, and basswood.
Long, a tall, soft-spoken young man, wears the classic wire-framed glasses preferred by hunters, outdoorsmen, and other iconoclasts. "If you're talking about the northeastern United States and southeastern Canada," he had said, "the answer is, we don't know that the rain is becoming more acidic. As a matter of fact, in the last decade the acidity of rainwater in the eastern United States may have actually decreased because the Environmental Protection Agency reduced the allowable levels for sulfur emission with the Clean Air Act of 1970.
"It's not a simple picture, though. What we wonder about are long-term trends: 25 years, 50, 100 years ago, to the time before we started burning coal in this country. No one was measuring atmospheric chemistry so we have to use indirect methods like looking at what the trees recorded.
"See this? It's an increment core from one of the oldest white oak trees ." The core looked like a 2-foot drinking straw, light brown, with dark brown lines irregularly spaced about every millimeter. It had been sanded flat on one side, polished, and mounted on a larger stick of wood. Long had taken two cores from each of the 33 trees he is examining in a stand of white oaks, aged 130 to 425 years, in Rothrock State Forest. He knows the size and species of each tree within a certain radius of the select 33, and the slope, aspect, elevation, and soil chemistry of each tree site.
"There are 425 rings here, one for each year. If the center of this tree hadn't rotted away in later years, we'd probably see that in the first 20 years of its life, the tree grew at its fastest rate. And see here?" He pointed to a spot about half way up, where the lines were bunched together for half an inch. "This could have been a local drought in the 1820s. Rainfall, of course, affects growth. So do temperature, competition for sunlight, insects and diseases, and soil fertility. A tree grows at a different rate depending on its size and age. When you start looking for places in the record where human factors have reduced growth, you have to account for the natural stresses first.
"Through statistics, we've found out something very interesting about the 1950s: About half of the trees from the old-growth stand in our study show a slowed or decreasing growth trend. So far, we've been able to discount advancing age and increasing size, two of the most likely natural causes. We can also pretty much rule out competition for sunlight because it's more stochastic more random in a stand like ours of trees of various ages. Insects or diseases are also more random. They'll affect one tree first, then spread to others. Exceptions would be the gypsy moth or the fall web worm or the Eastern tent caterpillar, but those outbreaks are noticeable the ones in recent years have been well-documented by the state foresters.
"What I'm doing now is climatic modeling, using monthly temperature or precipitation records or both to model the climate and predict tree growth. What modeling lets you determine, finally, is what percentage of your ring width variation can be accounted for by climate.
"The problem with talking about acid rain, "is we don't know what's going on up there." Hosler is a meteorology professor, member of the EPA Science Advisory Board, He is admired for his phenomenal memory for names, dates, and numbers; glasses magnify his eyes to large, blue ovals.
"The earth is a dynamic chemical system," he continued. "Changes in weather, the power of the sun, water vapor, volcanoes, ocean currents, vegetation, and the biota, too, make this chemical mixture we call the atmosphere so complicated that we are only beginning to understand it.
"Anthropogenic sources of sulfates and nitrates those resulting from human influence on nature may be important, but when you think about all the other pollutants we're putting into this shin skin of gas we live in, methane, carbon dioxide, hydrocarbons, reacting in about 75 chemical reactions that we've identified so far all of these are important in what ultimately produces acidity in the atmosphere.
"I am not an atmospheric chemist, but I've talked to most of the best in the world and they don't know what's going on. For example, if you look at a map that shows the acidity of the rainfall all over the world, one of the surprises is that rain is more acidic in the Falkland Islands than it is in. They're not burning any coal down there. The people I've talked to think it's due to ocean processes. The ocean, as you know, covers two-thirds of the globe. It's a sink or a source for many of the chemicals in the atmosphere. A shift in ocean currents or an upwelling of the tides can radically change atmospheric composition in a relatively short amount of time. The cores they've drilled into the Antarctic and the Greenland ice sheets show marked fluctuations in acidity over history and prehistory, before any human influence."
"That is where the real rub comes in. How do you measure the long-term economic impact of acid precipitation on the environment? We can estimate some of the costs of acidity on things roads, buildings, statues the same way we estimate their construction price or their depreciation through use and aging. But how do you factor in the symbolic value of a thing like the Statue of Liberty, the cathedral at Chartres, or the Parthenon? Worse, how do you measure the possibility of losing certain forests, reducing their growth, or losing desirable aquatic life? How do you make a comparison when many of these factors are quality as opposed to quantity factors? Even quality can be quantified, but the properties you choose to define it will always be subject to debate. Ultimately, we will have to decide what 'quality' means and whether it is worth the price. That's the 'problem' of acid rain."
What Causes Acid Rain?
Acid rain is linked to both natural and man-made sources. Nitrogen oxides are formed through the extreme heating of air when a thunderstorm produces lightning. Also, sulfurous gases are discharged from erupted volcanoes and rotting vegetation.
However, most public attention has been focused on man-made sources of acid rain, which include the burning of any fuel that contains sulfur and nitrogen compounds, including public utilities, industrial broilers, motor vehicles, and chemical plants. Electric power generation accounted for 69 percent of total sulfur dioxide emissions in 2007 and 20 percent of nitrogen oxides, according to the U.S. Environmental Protection Agency (USEPA).
Many industrial sources of sulfur dioxide are located in the eastern U.S., particularly in the Midwest and the Ohio Valley where coal combustion and power generation frequently occur. Typically, the highest nitrogen oxide emissions are found in states with large urban areas, a heavy population density, and significant automobile traffic.
Acid rain is not limited to the region where sources are located. Prevailing winds can blow chemicals in the atmosphere for hundreds or even thousands of miles before being deposited, regardless of state and country boundaries. For instance, compounds from industry in China can potentially be deposited in the Midwest. For this reason, acid rain is considered a global problem.
What are its Effects?
Acid rain has been linked to detrimental effects in the environment and in human health.
Forests, lakes, and streams: Acid rain can cause widespread damage to trees. This is especially true of trees at high elevations in various regions of the U.S. Acidic deposition can damage leaves and also deplete nutrients in forest soils and in trees so that trees become more vulnerable to disease and environmental stress.
When lakes and streams become more acidic than normal, they cannot continue to support the same types of fish and aquatic life as in the past. Fish communities dwindle due to high mortality, a reduced growth rate, skeletal deformities, and failed reproduction. Lakes ultimately become home only to species that can tolerate high-acid conditions. Game fish, such as trout, are particularly sensitive to acidic water conditions.
A healthy lake has a pH of 6.5 or higher. Only a few fish species can survive at a pH of below 5; at a pH of 4, the lake is considered dead. A decrease in fish populations is often the first sign of an acidification problem.
Not all lakes are equally vulnerable to acid rain, however. In some areas, such as in Illinois, the average pH of a freshwater lake is an alkaline 8 to 9 because soils and rocks in the bottom and sides of the lake contain high levels of calcium and magnesium, which neutralize the acidity of rain. Lakes surrounded by granite, such as in New England and northern New York, don’t fare as well.
Plants and crops: Acid rain can potentially reduce agricultural production by changing the chemical properties of soil, slowing the rate of microbiological processes, and reducing soil nutrients. Roots of natural vegetation and crops can become damaged due to stunted growth. Human effects: Acidic water moving through pipes causes lead and copper to leach into the water. Most public water suppliers remove such dangerous chemicals at the plant, but tainted water could be a problem for residents who don’t rely on public water supplies for their drinking water.
Acidic fog can be more hazardous to health than acid rain as small droplets can be inhaled. These atmospheric acids can cause respiratory problems in humans such as throat, nose, and eye irritation, headache, and asthma. Acid fog is particularly dangerous for the elderly, those who are ill, and people who have chronic respiratory conditions.
Man-Made Materials: Although sunlight, heat, cold, and wind contribute to the deterioration of man-made structures and objects, acid deposition speeds up this process. Metal structures and vehicles become corroded, and limestone buildings, tombstones, statues, and monuments deteriorate faster when rain is acidic.
Acid Rain on the Forest Floor
A spring shower in the forest washes leaves and falls through the trees to the forest floor below. Some trickles over the ground and runs into streams, rivers, or lakes, and some of the water soaks into the soil. That soil may neutralize some or all of the acidity of the acid rainwater. This ability is called buffering capacity, and without it, soils become more acidic.
Differences in soil buffering capacity are an important reason why some areas that receive acid rain show a lot of damage, while other areas that receive about the same amount of acid rain do not appear to be harmed at all. The ability of forest soils to resist, or buffer, acidity depends on the thickness and composition of the soil, as well as the type of bedrock beneath the forest floor. Midwestern states like Nebraska and Indiana have soils that are well buffered. Places in the mountainous northeast, like New York's Adirondack and Catskill Mountains, have thin soils with low buffering capacity.
How Acid Rain Harms Trees
Acid rain does not usually kill trees directly. Instead, it is more likely to weaken trees by damaging their leaves, limiting the nutrients available to them, or exposing them to toxic substances slowly released from the soil. Quite often, injury or death of trees is a result of these effects of acid rain in combination with one or more additional threats.
Scientists know that acidic water dissolves the nutrients and helpful minerals in the soil and then washes them away before trees and other plants can use them to grow. At the same time, acid rain causes the release of substances that are toxic to trees and plants, such as aluminum, into the soil. Scientists believe that this combination of loss of soil nutrients and increase of toxic aluminum may be one way that acid rain harms trees. Such substances also wash away in the runoff and are carried into streams, rivers, and lakes. More of these substances are released from the soil when the rainfall is more acidic.
However, trees can be damaged by acid rain even if the soil is well buffered. Forests in high mountain regions often are exposed to greater amounts of acid than other forests because they tend to be surrounded by acidic clouds and fog that are more acidic than rainfall. Scientists believe that when leaves are frequently bathed in this acid fog, essential nutrients in their leaves and needles are stripped away. This loss of nutrients in their foliage makes trees more susceptible to damage by other environmental factors, particularly cold winter weather.
How Acid Rain Affects Other Plants
Acid rain can harm other plants in the same way it harms trees. Although damaged by other air pollutants such as ground level ozone, food crops are not usually seriously affected because farmers frequently add fertilizers to the soil to replace nutrients that have washed away. They may also add crushed limestone to the soil. Limestone is an alkaline material and increases the ability of the soil to act as a buffer against acidity.
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