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Acid
Rain
EPA
Graphic
"Acid rain" is a broad
term referring to a mixture of wet and dry deposition (deposited material) from
the atmosphere containing higher than normal amounts of nitric and sulfuric
acids. 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 (NOx) resulting from fossil fuel combustion. In the United States,
roughly 2/3 of all SO2 and 1/4 of all NOx come from electric power generation
that relies on burning fossil fuels, like coal. Acid rain occurs when these
gases react in the atmosphere with water, oxygen, and other chemicals to form
various acidic compounds. The result is a mild solution of sulfuric acid and
nitric acid. When sulfur dioxide and nitrogen oxides are released from power
plants and other sources, prevailing winds blow these compounds across state and
national borders, sometimes over hundreds of miles.
Wet Deposition
Wet deposition refers to acidic rain, fog, and snow. If the acid chemicals in
the air are blown into areas where the weather is wet, the acids can fall to the
ground in the form of rain, snow, fog, or mist. As this acidic water flows over
and through the ground, it affects a variety of plants and animals. The strength
of the effects depends on several factors, including how acidic the water is;
the chemistry and buffering capacity of the soils involved; and the types of
fish, trees, and other living things that rely on the water.
Dry Deposition
In areas where the weather is dry, the acid chemicals may become incorporated
into dust or smoke and fall to the ground through dry deposition, sticking to
the ground, buildings, homes, cars, and trees. Dry deposited gases and particles
can be washed from these surfaces by rainstorms, leading to increased runoff.
This runoff water makes the resulting mixture more acidic. About half of the
acidity in the atmosphere falls back to earth through dry deposition.
Acid rain causes acidification of
lakes and streams and contributes to the damage of trees at high elevations (for
example, red spruce trees above 2,000 feet) and many sensitive forest soils. In
addition, acid rain accelerates the decay of building materials and paints,
including irreplaceable buildings, statues, and sculptures that are part of our
nation's cultural heritage. Prior to falling to the earth, sulfur dioxide (SO2)
and nitrogen oxide (NOx) gases and their particulate matter derivatives—sulfates
and nitrates—contribute to visibility degradation and harm public health.
Some of the
problems attributed to acid rain include:
- Trees lose some of the
protection in their leaves, leaving them more at risk from frost and
diseases.
- Tree roots may also become
stunted, so they can't take up as many nutrients.
- Soils lose some of their
nutrients.
- Increasing acid levels may
cause problems for aquatic animals and plants. Some fish may have trouble
breathing for example.
- Acid rain may dissolve the
stonework and mortar of buildings causing structural problems of buildings.
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.
Acid
rain, woods, Jizera Mountains, Czech Republic
Credit:
Lovecz -Prague
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.
Human Health
Acid rain looks, feels, and
tastes just like clean rain. The harm to people from acid rain is not direct.
Walking in acid rain, or even swimming in an acid lake, is no more dangerous
than walking or swimming in clean water. However, the pollutants that cause acid
rain—sulfur dioxide (SO2) and nitrogen oxides (NOx)—do damage human health.
These gases interact in the atmosphere to form fine sulfate and nitrate
particles that can be transported long distances by winds and inhaled deep into
people's lungs. Fine particles can also penetrate indoors. Many scientific
studies have identified a relationship between elevated levels of fine particles
and increased illness and premature death from heart and lung disorders, such as
asthma and bronchitis.
Based on health concerns, SO2 and
NOx have historically been regulated under the Clean Air Act, including the Acid
Rain Program. In the eastern U.S., sulfate aerosols make up about 25 percent of
fine particles. By lowering SO2 and NOx emissions from power generation, the
Acid Rain Program will reduce the levels of fine sulfate and nitrate particles
and so reduce the incidence and the severity of these health problems. When
fully implemented by the year 2010, the public health benefits of the Acid Rain
Program are estimated to be valued at $50 billion annually, due to decreased
mortality, hospital admissions, and emergency room visits.
Decreases in NOx emissions are
also expected to have a beneficial impact on human health by reducing the
nitrogen oxides available to react with volatile organic compounds and form
ozone. Ozone impacts on human health include a number of morbidity and mortality
risks associated with lung inflammation, including asthma and emphysema.
Credit:
EPA, Environment Canada, UNEP, NASA
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