.
 |
Emissions of sulfur dioxide and nitrogen oxides react with water vapor
in the atmosphere to create sulfuric and nitric acids. |
Acid Rain
How Acid Rain Works
BY SARAH DOWDEY
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.
The pH of Acid Rain
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.
 |
| The pH scale is a measure of acidity and alkalinity. Acid rain has a pH of 5.0 or less. |
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.
 |
| Acid deposition weakens trees and pollutes surface waters. |
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.
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.
The Effects of Acid Rain
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.
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| Acid rain can eat through stone and metal. It has acceleratedthe natural weathering process of this scarred stone angel's face. |
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.
Reducing Acid Rain
Acid rain has existed since
the first factories of the Industrial Revolution began spitting out toxic
emissions.
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Power plants must limit emissions of SO and NOx
to meet targets set by the Acid Rain Program. |
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, solear 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.
Sarah Dowdey,
Contributing Editor
Sarah Dowdey holds a bachelor's degree in English from the
University of Georgia.
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