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Chapter 11 Atmospheric science and air pollution
Natural atmospheric conditions and human-made pollutants contribute to the destructive actions of air
pollution. Some properties influencing our dynamic atmosphere are pres- sure, density, relative
humidity, and temperature. The four layers of the atmosphere have boundaries invisible to the human
eye, but they can be delineated by changes in temperature, density, and composition. The oceans
and their interaction with the atmosphere also affect weather and climate. Because human pollution
of the atmosphere affects ecological systems and health, the Environmental Protection Agency (EPA)
pays special attention to six "criteria pollutants" that pose the greatest threats to health and welfare.
Most outdoor air pollution impacts are due to chronic low-level emissions that result from recurrent
conditions such as urban smog. The depletion of stratospheric ozone threatens to be a great global
problem. Recently, scientists found that the effects of acid precipitation are worse than first predicted,
and the mandates of the 1990 Clean Air Act do not adequately address this problem. The health
effects from indoor air pollution are worse than outdoor air pollution, and cause, 14 times as many
deaths. The highest health risks in the developing world are particulate matter and chemicals from
wood and charcoal smoke, while the top risks in developed nations are cigarette smoke and radon.
Outdoor air pollution has been reduced in many countries because of limits on toxic emissions
through legislation. Indoor air pollution is minimized by reducing toxic materials, increasing ventilation,
and using efficient stoves.
The 1952 "Killer Smog" of London .
On December 5, 1952, people in London stoked their coal stoves to keep warm during that very cold
day, which caused smog, a mixture of smoke and fog, to settle over the city, causing the city's air
quality to be ten times worse than usual.
The visibility was so poor that pedestrians could not see across the street, roads became clogged
with abandoned cars, schools closed, flowers wilted, and cattle died.
Modem researchers estimate that the actual death toll of the four-day smog event, including delayed
cases that appeared over the next two months, reached 12,000.
Similar, but less severe, smog had occurred in London since the early 1800s, and also in the United
States, Mexico, and Malaysia.
Before the 1950s, smog was considered a necessary burden, but today, we view air pollution as an
environmental challenge.
Limits on toxic emissions through legislation have caused declines in air pollution.
The success of environmental advocacy and policy in reducing air pollution has made the air in many
American cities and London cleaner than in the 1950s.
However, people and cities around the world still suffer from air pollution, especially in developing
nations. For example, in 1995, airborne pollution in Delhi, India, was measured at 1.3 times the level
of London's average for the year 1952.
Atmospheric Science
The atmosphere, the thin layer of gases that surrounds Earth, supports life by providing needed
chemicals (oxygen and carbon dioxide), absorbing dangerous radiation, burning up incoming
meteors, transporting and re- cycling water and other chemicals, and moderating climate.
Today, about 78% of the atmosphere consists of nitrogen gas and 21 % oxygen gas.
Human activities have added small but significant quantities of artificial gases, as well as changing
the quantities of some atmospheric gases, particularly carbon dioxide (CO2), methane (CH4), and
ozone (03),
Important atmospheric properties include temperature, pressure, and humidity Movement of air within
the lower atmosphere results from differences in the physical properties of different air masses, which
include pressure, density, relative humidity, and temperature.
The air's density is greater near Earth's surface, and decreases with altitude because gravity pulls
gas molecules toward the surface.
Atmospheric pressure, which measures the weight per unit area produced by a column of air, also
decreases with altitude.
At sea level, atmospheric pressure is equal to 14.7 Ib/in2 or 1,013 millibar (mb).
At the top of Mount Everest (29,035 ft), the "thin air" is just over 300 mb, and a mountain climber is
standing higher than two-thirds of the atmosphere's air molecules.
Air also has a relative humidity, the ratio of water vapor a given volume of air contains to the
maximum amount it could hold, so a relative humidity of 50% means the air contains half the water
vapor it possibly could hold.
When relative humidity is high, sweat evaporates slowly and the body cannot cool itself efficiently,
making it feel hotter than it really is.
Temperature differences affect air circulation; variation over Earth's surface is due to the sun's rays
striking some areas more directly, while altitude through the layers of the atmosphere also causes
temperature differences.
The atmosphere consists of several layers
The atmosphere, about 1/100 of Earth's diameter, consists of four layers whose boundaries are not
visible to the human eye, but that can be delineated by changes in temperature, density, and
composition.
The troposphere, nearest Earth's surface, most directly affects living things because it provides gases
for breathing, and cycles nutrients from terrestrial and aquatic ecosystems.
Movement of air within the troposphere is also responsible for the weather.
Although only 7 miles high, the tropospheric air temperature declines with increasing altitude, until it
suddenly stops declining at the tropopause, the top of the troposphere.
The tropopause prevents mixing between the troposphere and the next layer, the stratosphere.
From 7 to 31 miles above sea level, the stratosphere's temperature rises with altitude.
The stratosphere is drier, less dense, and calmer than the troposphere, so there is little vertical
mixing, and substances entering it stagnate and re- main for a long time.
Most of the minute amount of ozone is concentrated in the bottom stratosphere, in a layer called
Earth's "ozone layer."
Ozone absorbs or scatters the sun's damaging ultraviolet (UV) radiation, so this layer is vital to the
maintenance of life on Earth.
In the mesosphere (31-53 miles above sea level), air pressure is extremely low and temperatures fall
with altitude.
The thermosphere extends to an altitude of 300 miles, where solar rays produce extremely high
temperatures and ions from the sun react with atmospheric molecules to produce the beautiful,
haunting displays known as the aurora.
In the thermosphere, the molecules are so few and far between that they rarely collide, so that
heavier molecules (nitrogen and oxygen) sink and light ones (hydrogen and helium) end up near the
top.
Solar energy heats the atmosphere, helps create seasons, and causes air to circulate
Radiation from the sun plays a major role in our atmosphere by driving most of its air movement,
creating seasons, and driving both weather and climate.
About 70% of the sun's energy is absorbed by the atmosphere and planetary surface, while the rest is
reflected back into space.
Given Earth's curvature, solar radiation intensity is highest near the equator and weakest near the
poles.
Because Earth tilts on its axis, the northern and southern hemispheres each face the sun for half the
year and create seasons, which are especially pronounced near the 'poles.
Land and surface water absorb solar energy, reradiating some heat and causing some water to
evaporate, making the air near Earth's surface warmer and wetter than at higher altitudes.
Warm air rises into regions of lower atmospheric pressure, expands, and releases heat, which cools
the air, which then descends, becoming denser and replacing the warm air.
This type of circular, or convection, current, with warm air rising to be replaced by colder air
descending, plays a key role in guiding both weather and climate.
The atmosphere drives weather and climate
Weather consists of the local physical properties of the troposphere such as temperature, pressure,
humidity, cloudiness, and wind over relatively short time periods and in a relatively small geographic
area.
Climate describes the pattern of atmospheric conditions found across a relatively large geographic
region over a long period of time, typically seasons, years, or millennia.
Weather is produced by interacting air masses
Weather changes occur when air masses with different physical properties meet.
A front is the boundary between two air masses that differ in temperature and density.
A warm front is the boundary where warm air displaces colder air, and
the warm front rises over the cold air mass, cools, and condenses to form clouds that produce rain.
A cold front is the boundary along which denser cold air wedges beneath warm air, pushing the warm
air upward, where the warm air cools and expands to form clouds and potentially produce
thunderstorms.
After a cold front passes through, the sky clears and the temperature and humidity drop.
A high-pressure system, which is an air mass with high atmospheric pres- sure, contains air that
descends and typically brings fair weather.
In a low-pressure system, air moves toward the low atmospheric pressure at the center of the system
and spirals upward, causing the air to expand and cool, followed by clouds and precipitation.
Usually, the air in the troposphere gets cooler as altitude increases, and warm air rises, causing
vertical mixing.
Sometimes, however, colder air is trapped near the ground, with warmer air above it, which is called a
temperature inversion or thermal inversion. Thermal inversions trap pollutants near the ground, which
can cause episodes of smog buildup in urban areas that can kill thousands of people.
Global climate patterns result from the differential heating of Earth's surface
Air movements on a larger geographic scale can create climatic patterns that are maintained over
long periods of time.
Near the equator, Hadley cells are planet-wide patterns of convection cur- rents caused by sunlight.
The intense sunlight warms surface air, causing it to rise in two columns toward the poles.
As the air is warmed, it rises and expands, which causes moisture to be released, resulting in tropical
rainforests near the equator.
As the columns of air cool, they descend at about 30° latitude and absorb moisture from the land,
resulting in desert areas.
Two other convection currents, Ferrel cells and polar cells, force air upward around 60° latitude,
resulting in precipitation in these areas.
Global wind patterns are influenced by Earth's rotation
Horizontal air currents, called wind, are caused by cooling air replacing rising warm air in a
convection current.
On a global scale, the Hadley, Ferrel, and polar cells produce wind patterns that extend across the
planet, but do not cause north-south surface winds, because of the Coriolis effect
The Coriolis effect is caused because regions near the equator spin more quickly than polar regions,
so the north-south winds are deflected from a straight path, and seem to travel partly in east-west
directions.
The interaction of the convection currents with Earth's rotation produces global circulation patterns
that sailors used for centuries to cross oceans. A region with few latitudinal winds near the equator is
known as the doldrums.
Between the equator and 30° latitude lie the west-blowing trade winds, and from 30° to 60° latitude
are the westerlies (which blow toward the east).
Outdoor Air Pollution
Throughout human history, humans have generated significant quantities of air pollution that can
affect climate and/or harm organisms.
However, in recent years a number of air pollution problems have been greatly diminished as a result
of government regulation and improved technologies.
The majority of outdoor air pollution comes from natural sources
Outdoor air pollution consists of volatile chemicals or particulate matter mixing in the troposphere.
The majority of outdoor air pollution is caused by natural sources such as the metabolism of plants,
the decay of dead plants, salt from sea spray, dust storms, volcanic eruptions, and forest fires.
Although dust storms are a natural occurrence, poor farming and grazing practices allow wind erosion
that contributes to the hundreds of millions of tons of dust blown across the oceans each year,
carrying fungal and bacterial spores that have been linked to die-offs in Caribbean coral reef systems.
Volcanic eruptions release large quantities of particulate matter and sulfur dioxide into the
troposphere and stratosphere, where it can remain for months or years.
The 1980 eruption of Mount St. Helens in Washington produced dust that circled Earth for 15 days,
while the dust from the massive 1883 eruption on the Indonesian island of Krakatau produced
gorgeous sunsets and caused global temperature to drop.
The burning of vegetation pollutes the atmosphere with smoke and soot from both natural and
human-caused fires.
In 1997, a severe drought caused fires in Indonesia, Mexico, Central America, and Africa, releasing
more carbon monoxide than the worldwide burning of fossil fuels.
Human activities create various types of outdoor air pollution
Human activity can increase the severity of natural air pollution, as well as introduce new sources of
air pollution. .
Point source pollution describes a specific spot-such as a factory's or smokestacks, where pollution is
discharged.
Non-point sources are more diffuse and consist of many small sources, such as fireplaces.
Primary pollutants such as soot and carbon monoxide are emitted into the troposphere in a form that
is directly harmful.
Secondary pollutants are hazardous substances produced when chemicals added to the atmosphere
react with chemicals normally found in the atmosphere.
Six pollutants are closely tracked by the U.S. EPA
The U.S. EPA gives special attention to six criteria pollutants judged to
pose especially great threats to human health and welfare: carbon monoxide (CO), lead (Pb),
nitrogen dioxide (N02), ozone (03), sulfur dioxide (SOJ, and particulate matter.
The EPA has established national air quality standards and monitors levels of these pollutants
throughout the country, particularly in urban areas. According to the EPA, in 2001 almost half of the
population lived in counties where at least one of these six pollutants reached unhealthy levels,
although the percentage of days citizens were exposed to unhealthy air dropped.
Carbon monoxide: This colorless, odorless gas, produced primarily by the incomplete combustion of
fuels, is the most abundant air pollutant in terms of emissions.
Vehicles account for most emissions, but lawn and garden equipment, forest fires, burning of
industrial waste, and residential wood burning also emit carbon monoxide.
Carbon monoxide is dangerous to organisms because it binds to hemoglobin in red blood cells,
preventing the hemoglobin from binding with oxygen.
Since 1982, U.S. CO emissions have decreased by 62%, due largely to cleaner-burning automobile
engines.
Sulfur dioxide: This is colorless gas that is created mostly during the combustion of coal for industry
and electricity generation.
Once in the atmosphere, S02 may react to form sulfuric acid (H2SO4)' one form of acid precipitation.
Nitrogen dioxide: A highly reactive gas, N02 contributes to smog and acid rain and comes from
vehicles, electrical utilities, and industrial combustion.
Nitrogen dioxide, along with nitrogen monoxide (NO), belongs to a family of related compounds called
nitrogen oxides (NOx) that form when atmospheric nitrogen and oxygen react at the high
temperatures created by combustion engines.
Although N02 emissions have dropped, emissions from nitrogen oxides as a whole have increased,
due to more construction, recreation equipment, and diesel engines.
Tropospheric ozone: Although stratospheric ozone is necessary to shield organisms from UV
radiation, tropospheric ozone from human activity builds up and acts as a pollutant.
Ozone is a colorless gas that forms over cities when air pollution interacts with sunlight, heat, nitrogen
oxides, and volatile chemicals.
Besides being a component of smog, ozone can cause problems because one of the oxygen atoms
can be released from its bonds, and is then able to participate in reactions that injure living tissues.
Although ozone concentrations fell from 1982 to 2001, this pollutant most frequently exceeds EPA
standards in urban areas.
Lead: Lead is added to gasoline to improve engine performance, but exhaust from the gasoline
injects lead into the atmosphere, where it settles on the land and water.
Lead enters the food chain, accumulates within body tissues, and causes central nervous system
malfunction, mental retardation in children, and many other ailments.
Leaded gasoline has been phased out in the United States, and lead emissions have remained
steady and low, showing the positive effects that legislation can have on air quality.
Many developing nations add lead to gasoline and suffer significantly from lead pollution.
Particulate matter: Particulate matter consists of any solid or liquid particles small enough to be
carried aloft, and can damage respiratory tissues when inhaled.
Particulate matter includes primary pollutants (such as dust and soot) as well as secondary pollutants
(such as sulfates and nitrates).
The largest source of particulate matter is wind-blown dust; although emissions of large particles,
have decreased, small-particle emissions have increased.
Thermal inversions can trap particulate matter emitted from coal burning, resulting in smog that can
seriously damage health.
Photochemical smog is produced by a complex series of atmospheric reactions
Photochemical smog, or brown-air smog, is formed through light-driven reactions between primary
pollutants and normal atmospheric compounds.
Due to high levels of NOx, photochemical smog forms a brownish haze over cities.
Urban areas with hot, sunny days provide perfect conditions, because sun- light promotes the
production of ozone and other ingredients of photo- chemical smog.
Levels of photochemical smog in urban areas peak around mid-afternoon, irritating peoples' eyes,
noses, and throats, as well as harming living tissues in animals and plants and damaging buildings.
Photochemical smog afflicts many major cities such as Athens, Greece, especially those with
topography and weather conditions that promote it.
Stratospheric ozone depletion is caused by human-made chemicals.
Although ozone is a pollutant at low altitudes, in the lower stratosphere it is a highly beneficial gas,
where it is concentrated in the so-called "ozone layer."
Despite low concentrations, ozone molecules absorb incoming ultraviolet radiation from the sun, thus
protecting life on Earth's surface.
In the 1960s, scientists noticed that ozone levels were lower than predicted, due to a group of humanmade compounds called chlorofluorocarbons (CFCs).
CFCs deplete stratospheric ozone by splitting 03 molecules and creating O2 molecules from them.
When ozone is lost, more UV radiation can reach Earth's surface, which can result in thousands more
skin cancer cases each year.
In 1985, scientists found that stratospheric ozone levels over Antarctica
had declined by 40-60% in the past decade, leaving a thinned ozone concentration that was called
the ozone hole.
Scientists were also concerned over the effects of increased UV radiation on ecosystems, including
harm to crops and to the productivity of ocean phytoplankton, the base of the marine food chain.
Ozone depletion was halted by the Montreal Protocol
In 1987, 180 nations signed the Montreal Protocol, in which (along with the five later agreements)
they agreed to cut CFC production in half, advance the timetables for compliance, and address
related ozone-depleting chemicals.
Today the production and use of ozone-depleting compounds has fallen 95%, and the ozone layer is
beginning to recover; unfortunately, 5 billion kg of CFCs have not yet diffused into the stratosphere.
Industry shifted to alternative, environmentally safer chemicals, which are also cheaper and more
efficient.
The Montreal Protocol and its amendments are considered the most spectacular success story so far
in addressing any global environmental problem, because policymakers and industry worked together
in solving the problem, and came up with technological fixes and replacement chemicals.
Also, the process followed an adaptive management approach, which allows strategies to change in
response to new scientific data, technological advances, or economic figures.
Acid precipitation represents another trans-boundary pollution problem
Acid precipitation (acid rain, fog, and snow) forms when pollutants (mostly S02 and NOx) react with
water, oxygen, and oxidants to form sulfuric acid and nitric acid.
Sulfur dioxide and nitrogen oxides are produced primarily through fossil fuel combustion by
automobiles, electric utilities, and industrial facilities.
Acid droplets may travel for days or weeks, covering hundreds or thousands of kilometers before
falling in precipitation.
Acid precipitation leaches many minerals like calcium and magnesium from the soil, changing soil
chemistry and harming plants and soil organisms.
Thousands of lakes now have water acidic enough to kill all fish.
Acid fog with a pH of 2.3 (equal to vinegar) kills trees and damages agricultural crops.
Acid precipitation defaces buildings and cars, and causes billions of dollars of damage to ancient
European cathedrals, monuments in Washing- ton, D.C., and temples in Asia.
Regions of greatest acidification are often downwind of areas with major point sources of pollution,
which has led to political bickering among states and nations.
Acid precipitation has not been reduced as markedly as scientists had hoped
To reduce acid precipitation, primary pollutants must be reduced through technology such as
"scrubbers" that filter pollutants from factory smoke- stacks.
Although the average sulfate precipitation has decreased, the average nitrate precipitation has
increased nationally.
Recently, scientists have found that the effects of acid precipitation are worse than first predicted, and
the mandates of the 1990 Clean Air Act
are not adequate to restore ecosystems in the northeastern United States.
Indoor Air Pollution
Indoor spaces, including industrial workplaces, offices, schools, and homes, have higher
concentrations of pollutants than outdoor spaces, killing 6,000 to 8,000 people each day.
The health effects from indoor air pollution are worse than those from outdoor air pollution, and there
are 14 times as many deaths globally due to indoor air pollution than to outdoor air pollution.
People are harmed by indoor pollution because they spend 90% of their time indoors, where there
are synthetic materials that may not have been tested for health effects.
Also, to reduce heat loss and improve energy efficiency, ventilation in existing buildings was sealed
off, and new buildings were constructed with little ventilation and windows that did not open.
These steps may have saved energy, but they worsened indoor air pollution by trapping
contaminated air inside.
Indoor air pollution in the developing world arises from fuelwood burning
Over 90% of the deaths from indoor air pollution occur in the developing world, mostly in rural areas
that burn wood or charcoal in unventilated homes for cooking and heating.
Poverty forces many people to cook with indoor fires, although they do
not know that they and their children can suffer from pneumonia, bronchitis, allergies, sinus
infections, cataracts, asthma, emphysema, heart disease, cancer, and premature death by doing so.
Recognizing indoor air pollution as a problem is still fairly novel.
Most countries do not recognize that indoor air pollution is a serious problem, and we know far less
about indoor air pollution than we do about outdoor air pollution.
The United States does not monitor indoor pollutants comprehensively,
and our knowledge of indoor pollution has come mostly from individual independent studies, not
comprehensive, coordinated federal or international efforts.
Scientists are finding new sources of indoor air pollution, such as reactions between chlorinecontaining detergents and organic food remains in dish- washers that can release pollutants that build
up inside the home.
The highest health risks in the developing world are particulate matter
and chemicals from wood and charcoal smoke, while the top risks in developed nations appear to be
cigarette smoke and the naturally occurring radioactive gas, radon.
Tobacco smoke and radon are the most deadly indoor pollutants in the developed world
Secondhand smoke, or environmental tobacco smoke, is inhaled by a nearby nonsmoker.
Secondhand smoke causes many of the same problems, such as irritation of the eyes, nose, and
throat, asthma, and lung cancer, as directly inhaled cigarette smoke.
Because of reduced smoking in the United States and some other nations, exposure of young
children to secondhand smoke has been cut in half.
Radon gas is the second-leading cause of lung cancer for Americans, causing 15,000-22,000 radonrelated lung cancer deaths per year in the United States. Radon, a radioactive gas resulting from the
natural decay of uranium in soil, rock, or water, seeps up from the ground and infiltrates buildings.
The only way to determine whether radon is entering a building is to purchase and use a test kit.
A great diversity of volatile organic compounds pollutes indoor air
Because CO is so deadly and hard to detect, many homes are equipped with detectors that sound an
alarm if dangerous levels of CO build up.
An enormous number of types of volatile organic compounds (VOCs) existing plastics, oils, perfumes,
paints, and pesticides, which evaporate or leak from furnishings, building materials, color film,
carpets, laser printers, fax machines, and sheets of paper.
Fortunately VOCs are emitted in very small amounts, but these amounts are still much higher than
outdoors.
The impacts of such chronic, low levels of VOC exposure on human health are unclear, except for
formaldehyde, which does have clear and known health impacts.
VOCs also include pesticides, and 90% of people's pesticide exposure formerly came from indoor
sources, including banned pesticides, which had apparently been used in previous years against
termites, then seeped into houses through floors and walls.
Living organisms can pollute indoor spaces .
Tiny living organisms, such as dust mites, fungi, mold, and bacteria can also be indoor pollutants;
These pollutants can cause headaches, allergies, asthma, respiratory diseases, and cardiovascular
effects.
Heating and cooling systems of buildings make ideal breeding grounds for microbes, because they
provide moisture and air currents to carry them aloft.
Building-related illness is a sickness produced by indoor pollution where the specific cause is
identifiable.
Sick-building syndrome occurs when the cause of such an illness is a mystery, and when symptoms
are nonspecific.
VOCs are often thought to be responsible for sick-building syndrome.
We can reduce indoor air pollution
Using nontoxic materials, limiting exposure to trapped air and products such as pesticides and
cleaning fluids, and removing toxicants from the house can reduce indoor pollution.
The most important factor is keeping indoor spaces well ventilated to minimize exposure to
concentrations of chronic contaminants.
Developing countries can use remedies such as drying wood before burning it, shifting to lesspolluting fuels (such as natural gas), and using cleaner, more efficient stoves.