Transcript Chapter 18: Air Pollution

Chapter 18: Air Pollution
In this chapter the following topics will be covered:
• The major categories and sources of air pollution
• Conventional & unconventional pollutants
• The origins and dangers of some indoor pollutants
• The effects of stratospheric ozone depletion and radon in indoor air
• How air pollution damages human health, vegetation and buildings
• Different approaches to air pollution control
The Air Around Us
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Smoke, haze, dust, odors, corrosive gases, noise, and toxic
compounds are present nearly everywhere, even in the most
remote, pristine wilderness.
Air pollution is generally the most widespread and obvious kind
of environmental damage.
Over the past twenty years, air quality has improved appreciably
in most cities in Western Europe, North America and Japan.
Air quality in the developing world has been getting much
worse.
Natural Sources of Air Pollution
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There are many natural sources of air quality degradation.
- Natural fires release smoke.
- Volcanoes spew out ash, acid mists, hydrogen sulfide, and
other toxic gases.
- Sea spray and decaying vegetation are major sources of
reactive sulfur compounds in the air.
- Trees and bushes emit millions of tons of volatile organic
compounds.
- Pollen, spores, viruses, bacteria, and other small bits of organic
material are present in the air.
- Bacterial metabolism of decaying vegetation in swamps and of
cellulose in the guts of termites and ruminant animals is
responsible for large methane releases.
Human-Caused Air Pollution
Primary and Secondary Pollutants
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Primary pollutants: those released directly from the source into the
air in a harmful form.
Secondary pollutants: modified to a hazardous form after they enter
the air or are formed by chemical reactions as components of the air
mix and interact.
- Solar radiation often provides the energy for these reactions.
Fugitive emissions: those that do not go through a smokestack (e.g.
dust from soil erosion, strip mining, rock crushing, and building
construction).
Conventional or Criteria Pollutants
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The U.S. Clean Air Act of 1970 designated seven major pollutants
for which maximum ambient air levels are mandated.
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These seven conventional or criteria pollutants contribute the
largest volume of air-quality degradation.
• Sulfur compounds
- Natural sources: evaporation of sea spray, erosion of sulfate
containing dust from arid soils, fumes from volcanoes and
fumaroles, and biogenic emissions of hydrogen sulfide and
organic sulfur-containing compounds.
- The predominant form of anthropogenic sulfur is sulfur dioxide
from combustion of sulfur-containing fuel.
* Sulfur dioxide is a colorless corrosive gas that is
directly damaging to both plants and animals.
* Can be oxidized to sulfur trioxide, which reacts with
water vapor or dissolves in water droplets to form
sulfuric acid (major component of acid rain).
• Nitrogen compounds
- Nitrogen oxides: highly reactive gases formed when nitrogen in
fuel or combustion air is heated to temperatures above
650o C in the presence of oxygen, or when bacteria in soil or
water oxidize nitrogen-containing compounds.
- Nitrogen oxides combine with water to make nitric acid, which
is a major component of atmospheric acidification.
- Excess nitrogen also causes fertilization and eutrophication of
inland waters and coastal seas.
• Carbon Oxides
- Carbon dioxide (CO2) is the predominant form of carbon in the
air.
- Usually considered nontoxic and innocuous, increasing levels
of carbon dioxide appears to be causing a global climate
warming.
- Burning of fossil fuels is estimated to add between 5 and 5.5
billion tons of carbon to the atmosphere each year.
- Uncertainty exists about where the extra carbons goes.
- Carbon monoxide: colorless, odorless, nonirritating but highly
toxic gas.
- About 90 percent of the carbon monoxide in the air is
consumed in photochemical reactions that produce ozone.
• Metals and Halogens
- Many toxic metals are mined and used in manufacturing
processes or occur as trace elements in fuels, especially coal.
- Lead
* Worldwide lead emissions amount to about 2 million metric tons per year,
or two-thirds of all metallic pollution.
* Most lead is from leaded gasoline.
* An estimated 20 percent of all inner-city children suffer some degree of
mental retardation from high environmental lead levels.
- Mercury
* Two largest sources of atmospheric mercury appear to be coal-burning
power plants and waste incinerators.
- Other toxic metals of concern are nickel, beryllium, cadmium, thallium,
uranium, cesium, and plutonium.
- Halogens (fluorine, chlorine, bromine, and iodine) are highly reactive and
generally toxic in their elemental form.
- About 600 million tons of highly persistent chlorofluorocarbons (CFCs) are
used annually worldwide in spray propellants, refrigeration compressors,
and for foam blowing.
* CFCs diffuse into the stratosphere where they release chlorine and
fluorine atoms that destroy the ozone shield that protects the earth from
U.V. radiation.
• Particulate material
- Particulate material: all atmospheric aerosols, whether solid or
liquid.
- Includes dust, ash, soot, lint, smoke, pollen, spores, algal cells,
and many other suspended materials.
- Particulates often are the most apparent form of air pollution
since they reduce visibility and leave dirty deposits on
windows, painted surfaces, and textiles.
- Respirable particles smaller than 2.5 micrometers are among
the most dangerous of this group because they can be drawn
into the lungs.
• Volatile organic compounds
- Volatile organic compounds (VOCs): organic chemicals that
exist as gases in the air.
- Plants are the largest source of VOCs.
- A large number of other synthetic organic chemicals, such as
benzene, toluene, formaldehyde, vinyl chloride, phenols,
chloroform, and trichloroethylene, are released into the air by
human activities.
* These chemicals play an important role in the formation
of photochemical oxidants.
- Of the 188 air toxics listed in the Clean Air Act, about two
thirds are VOCs and most of the rest are metal compounds.
- EPA has identified 33 chemical compounds considered to be
the greatest threat to public health in urban areas.
Table 18.2 -- Urban air toxics of greatest concern
Acetaldehyde
Acrolein
Acrylonitrile
Arsenic compounds
Benzene
Beryllium compounds
1,3-butadiene
Cadmium compounds
Carbon tetrachloride
Chloroform
Chromium compounds
Coke oven emissions
Dioxins
1,2-dibromoethane
1,3-dichloropropane
Propylene dichloride
Ethylene dichloride
Ethylene oxide
Formaldehyde
Hexachlorobenzene
Hydrazine
Lead compounds
Source: U.S. EPA 1999
Manganese compounds
Mercury compounds
Methylene chloride
Nickel compounds
Polychlorinated biphenyls
Polycyclic organic matter
Quinoline
1,1,2,2-tetrachlorethane
Tetrachloroethylene
Trichloroethylene
Vinyl chloride
• Photochemical oxidants
- Photochemical oxidants: products of secondary atmospheric
reactions driven by solar energy.
- One of the most important reactions involves formation of
singlet (atomic) oxygen by splitting nitrogen dioxide (NO2).
- Then the atmoic oxygen reacts with another molecule of O2 to
make ozone (O3).
- Ozone formed in the stratosphere provides a valuable shield
for the biosphere by absorbing incoming ultraviolet radiation.
- In ambient air, however, O3 is a strong oxidizing reagent and
damages vegetation, building materials, and sensitive tissues.
Unconventional Pollutants
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EPA has authority under the Clean Air Act to set emission standards
(regulating the amount released) for certain unconventional or noncriteria pollutants that are considered especially hazardous or toxic.
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Examples of these unconventional pollutants include asbestos,
benzene, beryllium, mercury, polychlorinated biphenyls, and vinyl
chloride.
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Aesthetic degradation: includes any undesirable changes in the
physical characteristics or chemistry of the atmosphere (e.g. noise,
odors and light pollution).
Indoor Air Pollution
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The EPA has found that indoor concentrations of toxic air pollutants
are often higher than outdoors.
People generally spend more time inside than out and therefore are
exposed to higher doses of these pollutants.
Smoking is the most important air pollutant in the United States in
terms of human health.
In some cases, indoor air in homes has concentrations of chemicals
that would be illegal outside or in the workplace.
"Green design" principles can make indoor spaces both healthier and
more pleasant.
In less-developed countries of Africa, Asia, and Latin America
where such organic fuels as firewood, charcoal, dried dung, and
agricultural wastes make up the majority of household energy,
smoky, poorly ventilated heating and cooking fires represent the
greatest source of indoor air pollution.
Climate, Topography, and Atmospheric Processes
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Topography, climate, and physical processes in the atmosphere play
an important role in transport, concentration, dispersal, and removal
of many air pollutants.
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Wind speed, mixing between air layers, precipitation, and
atmospheric chemistry all determine whether pollutants will
remain in the locality where they are produced or will go elsewhere.
Inversions
• Temperature inversions: occur when a stable layer of warmer air
overlays cooler air, reversing the normal temperature decline with
increasing height and preventing convection currents from
dispersing pollutants.
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Several mechanisms create inversions.
- Cold front slides under an adjacent warmer air mass or
when cool air subsides down a mountain slope to displace
warmer air in the valley.
- Rapid nightime cooling in a valley or basin where air
movement is restricted.
* The cool air slides in under contaminated air, squeezing
it up against the cap of warmer air above and
concentrated the pollutants accumulated during the day.
Dust Domes and Heat Islands
• Tall buildings in large cities create convective updrafts that sweep
pollutants into the air.
• Temperatures in the center of large cities are frequently 3o to 5oC
higher than surrounding countryside.
• Stable air masses created by this "heat island" over the city
concentrate pollutants in a "dust dome".
Long-Range Transport
• Fine aerosols and industrial pollutants can be carried great distances
by the wind.
• Some of the most toxic and corrosive materials delivered by long
range transport are secondary pollutants, produced by the mixing and
interaction of atmospheric contaminants as they travel through the
air.
• Somoa, Greenland, and even Antactica and the North Pole, all have
heavy metals, pesticides, and radioactive elements in their air.
• The Inuit people of Broughton Island, well above the Arctic Circle,
have higher levels of polychlorinated biphenyls in their blood than
any other known population, except victims of industrial accidents.
Stratospheric Ozone
• In 1985, a disturbing discovery was announced: ozone levels in the
stratosphere over the South Pole were dropping precipitously during
September and October every year as the sun reappears at the end of
the long polar winter.
• Why are we worried about stratospheric ozone?
- In the upper atmosphere, where it screens out dangerous U.V.
rays from the sun, ozone is an irreplaceable resource.
• Exceptionally cold temperatures in Antactica play a role in ozone
losses.
• Humans release a variety of chlorine-containing molecules into the
atmosphere (e.g. chlorofluorocarbons and halon gases).
- Because these molecules are so stable, they persist for decades or
even centuries once released.
- When they diffuse out into the stratosphere, the intense U.V.
irradiation releases chlorine atoms that destroy ozone.
- At a 1989 conference, eighty-one nations agreed to phase out
CFC production by the end of the century.
- Alternatives to CFCs exist including hydrochlorofluorocarbons
(HCFCs) which release much less chlorine per molecule.
CFC production in industrialized countries has fallen nearly 80%
since 1989.
Effects of Air Pollution
Human Health
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Heart attacks, respiratory diseases, and lung cancer all are significantly higher in people who
breathe dirty air, compared to matching groups in cleaner environments.
Conditions are often much worse in other countries than Canada or the United States.
The United Nations estimates that at least 1.3 billion people around the world live in areas
where air is dangerously polluted.
The most common route of exposure to air pollutants is by inhalation, but direct absorption
through the skin or contamination of food and water are also important pathways.
Because they are strong oxidizing agents, sulfates, SO2, NOx, and O3 act as irritants that
damage delicate tissues in the eyes and respiratory passages.
Carbon monoxide binds to hemoglobin and decreases the ability of red blood cells to carry
oxygen.
Some important chronic health effects of air pollutants include bronchitis and emphysema.
- Bronchitis: persistent inflammation of bronchi and bronchioles (large and small
airways in the lung) that cause a painful cough and involuntary muscle spasms
that constrict airways.
- Emphysema: an irreversible obstructive lung disease in which airways become
permanently constricted and alveoli are damaged or even destroyed.
Half of all lungs examined at autopsy in the United States have some degree of alveolar
deterioration.
Smoking is undoubtedly the largest cause of obstructive lung disease and preventable death
in the world.
Plant Pathology
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In the early days of industrialization, fumes from furnaces, smelters,
refineries, and chemical plants often destroyed vegetation and created
desolate, barren landscapes around mining and manufacturing
centers.
- Copper-nickel smelter at Sudbury, Ontario, is a notorious
example of air pollution effects on vegetation and ecosystems.
There are two probable ways that air pollutants damage plants.
- They can be directly toxic, damaging sensitive cell membranes
much as irritants do in human lungs.
- They can act as metabolic regulators or plant hormones and
disrupt normal patterns of growth and development.
Synergistic effects: effects caused following exposure to two factors
which together is more than the sum of exposure to each factor
individually.
Pollutant levels too low to produce visible symptoms of damage may
still have important effects.
Acid Deposition
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Acid precipitation: the deposition of wet acidic solutions or dry
acidic particles from the air.
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By the 1940's, it was known that pollutants, including atmospheric
acids, could be transported long distances by wind currents.
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pH and atmospheric acidity
- acidity is described in terms of pH (the negative logarithm of the
hydrogen ion concentration in a solution).
- pH scale ranges from 0 to 14 with 7, the midpoint, being neutral.
- Values less than 7 indicate progressively greater acidity, while
above 7 are progressively more alkaline.
- Normal, unpolluted rain generally has a pH of about 5.6 due to
carbonic acid created by CO2 in the air.
• Aquatic effects
- Generally, reproduction is the most sensitive stage in fish life cycles.
- Eggs and fry of many species are killed when the pH drops to about 5.0.
- This level of acidification (pH 5.0) can also disrupt the food chain by killing
aquatic plants, insects, and invertebrates on which fish depend for food.
- There are several ways acids kill fish.
* Alters body chemistry
* Destroys kills and prevents oxygen uptake
* Causes bone decalcification
* Disrupts muscle contraction.
- Acid water leaches toxic metals, such as mercury and aluminum, out of soil
and rocks.
- Studies in the Adirondack Mountains of New York revealed that about half
of the high altitude lakes are acidified and have no fish.
- Much of the western United States has relatively alkaline bedrock and
carbonate-rich soil, which counterbalance acids from the atmosphere.
- Sulfates account for about two-thirds of the acid deposition in eastern North
America and most of Europe, while nitrates contribute most of the remaining
one-third.
• Forest damage
- In the early 1980s, disturbing reports appeared of rapid forest declines in
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both Europe and North America.
A 1980 survey on Camel's Hump Mountain in Vermont showed that
seedling production, tree density, and viability of spruce-fir forests at high
elevations had declined about 50 percent in 15 years.
* By 1990, almost all the red spruce, once the dominant species on the
upper part of the mountain, were dead or dying.
European forests also are dying at an alarming rate.
* In 1982, German foresters estimated only 8 percent of their forests
showed pollution damage.
* By 1983, some 34 percent of the forest was affected.
* By 1985, more than 4 million hectares (about half the total) were
reported to be in a state of decline.
Similar damage is reported in Czechoslovakia, Poland, Austria, and
Switzerland.
Researchers at the Hubbard Brook Experimental Forest in New Hampshire
have shown that forest soils have become depleted of natural buffering
reserves of basic cations such as calcium and magnesium through years of
exposure to acid rain.
Plant pathogens and insect pests may damage trees or attack trees debilitated
by air pollution.
• Buildings and monuments
- In cities throughout the world, some of the oldest and most glorious
buildings and works of art are being destroyed by air pollution.
- Air pollution also damages ordinary buildings and structures by corroding
steel in reinforced concrete in the buildings as well as roads and bridges.
• Visibility reduction
- Foul air obscuring the skies above industrialized cities has long been
recognized as a problem.
- Pollution affects rural areas as well (e.g. Grand Canyon National Park and
Shenandoah National Park).
Air Pollution Control
Moving Pollution to Remote Areas
• Among the earliest techniques for improving local air quality was
moving pollution sources to remote locations and/or dispersing
emissions with smokestacks.
Particulate Removal
• Filters remove particle physically by trapping them in a porous mesh
of cotton cloth, spun glass fibers, or asbestos-cellulose, which allows
air to pass through but holds back solids.
• Electrostatic precipitators are the most common particulate controls
in power plants.
- Fly ash particles pick up an electrostatic surface charge as they
pass between large electrodes in the effluent stream.
- Performance depends on particle size and chemistry, strength
of the electric field, and flue gas velocity.
Sulfur Removal
• Sulfur removal can be done a variety of ways either by using low-sulfur fuel or by removing
sulfur from effluents.
• Fuel switching and fuel cleaning
- Switching from soft coal with a high sulfur content to low-sulfur coal can greatly reduce
sulfur emission.
- Changing to another fuel, such as natural gas or nuclear energy, can eliminate all
sulfur emissions as well as those of particulates and heavy metals.
- Alternative energy sources, such as wind and solar power, are preferable to either fossil fuel
or nuclear power, and are becoming economically competitive.
- Coal can be crushed, washed, and gassified to remove sulfur and metals before combustion.
• Limestone injection and fluidized bed combustion
- Sulfur emissions can be reduced as much as 90 percent by mixing crushed limestone with
coal before it is fed into a boiler.
- A relatively new technique for burning, called fluidized bed combustion, offers several
advantages in pollution control.
• Flue gas desulfurization
- Crushed limestone, lime slurry, or alkali can be injected into a stack gas stream to
remove sulfur after combustion.
• Sulfur recovery processes
- Sulfur can be removed from effluent gases by processes that yield a usable product, such
as elemental sulfur, sulfuric acid, or ammonium sulfate.
Nitrogen Oxide Control
• Staged burners, in which the flow of air and fuel are carefully
controlled, can reduce nitrogen oxide formation by as much as 50%.
• The approach adopted by U.S. automakers for NOx reductions has
been to use selective catalysts to change pollutants to harmless
substances.
• Raprenox (rapid removal of nitrogen oxides) is a new technique for
removing nitrogen oxides that was developed by the U.S. Department
of Energy Sandia Laboratory in Livermore, California.
Hydrocarbon Controls
• Closed systems that prevent escape of fugitive gases can reduce
many hydrocarbon emissions (e.g. positive crankcase ventilation
(PCV) systems in automobiles).
• Controls on fugitive losses from valves, pipes, and storage tanks in
industry can have a significant impact on air quality.
• Afterburners are often the best method for destroying volatile
organic chemicals in industrial exhaust stacks.
Clean Air Legislation
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The Clean Air Act of 1963 was the first national legislation in the
United States aimed at air pollution control.
- Federal grants were provided to states to combat pollution, but the act was
careful to preserve states' rights to set and enforce air quality regulations.
- It became obvious that some pollution problems cannot be solved on a local basis.
In 1970, an extensive set of amendments essentially rewrote the Clean Air Act.
- These amendments identified the "criteria pollutants" and established national
ambient air quality standards.
- Standards are divided into two categories.
* Primary standards: intended to protect human health.
* Secondary standards: set to protect materials, crops, climate, visibility, and
personal comfort.
In 1990, the Clean Air Act was extensively rewritten and updated including
provisions to address the following issues: acid rain, urban smog, toxic air
pollutants, ozone protection, marketing pollution rights, and volatile organic
compounds.
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In 1997, further changes were made to the Clean Air Act; ambient ozone
standards will be lowered from 0.12 ppm to 0.08 ppm.
The EPA estimates that costs of these measures could be as high as $8.5 billion per
year, but that they should save 15,000 lives, cut hospital admissions for respiratory
illnesses by 9.000, and reduce chronic bronchitis cases by 60,000 each year.
The EPA won't fully implement these latest standards for ozone and fine soot until
2008 to give states a chance to set up monitoring systems and to find ways to
eliminate pollution in the most cost-effective manner.
California has gone further than the federal government in making specific plans for
air pollution control.
Current Conditions and Future Prospects
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Clean Air Act goals have not been achieved; however, air quality
has improved dramatically in the last decade in terms of the
major large-volume pollutants.
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The EPA estimates that emissions of particulate materials
decreased 78 percent, lead fell 98%, SO2 declined 32%
percent, and CO shrank 23%.
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Because automobiles are the main source of NOx, cities where
pollution is largely from traffic still have serious air quality
problems.
• The major metropolitan areas of many developing countries are
growing at explosive rates to incredible sizes and environmental
quality is still abysmal in many of them (e.g. Mexico City and many
large cities in China).
• As political walls came down across Eastern Europe and the Soviet
Union at the end fo the 1980s, horrifying environmental conditions in
these centrally planned economies were revealed.
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Not all is pessimistic, however. There have been some spectacular
successes in air pollution control (e.g. Sweden and West Germany).