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CHAPTER 16 Air Pollution 347
16.1 The Air Around Us How does the air taste, feel, smell, and look in your neighbor-
hood? Chances are that wherever you live, the air is contaminated
to some degree. Smoke, haze, dust, odors, corrosive gases, noise,
and toxic compounds are among our most widespread pollutants.
According to the Environmental Protection Agency (EPA), human
activities release some 147 million metric tons of air pollutants
(not counting carbon dioxide or windblown soil) into the atmo-
sphere each year in the United States alone. Worldwide emissions
are around 2 billion metric tons per year ( table 16.1 ).
Table 16.1 Estimated Fluxes of Pollutants and Trace Gases to the Atmosphere
Approximate Annual Flux (Millions of Metric Tons/Yr)
Species Sources Natural Anthropogenic
CO 2 (carbon dioxide) Respiration, fossil fuel burning, land clearing, industrial processes 370,000 * 29,600
CH 4 (methane) Rice paddies and wetlands, gas drilling, landfills, animals, termites 155 350
CO (carbon monoxide) Incomplete combustion, CH 4 oxidation, plant metabolism 1,580 930
Non-methane hydrocarbons Fossil fuel burning, industrial uses, volatile compounds from plants 860 92
NO x (nitrogen oxides) Fossil fuel burning, lightning, biomass burning, soil microbes 90 140
SO x (sulfur oxides) Fossil fuel burning, industry, biomass burning, volcanoes, oceans 35 79
SPM (suspended particulate materials ) Biomass burning, dust, sea salt, biogenic aerosols 583 362 *Natural flux to atmosphere is balanced over time by capture, deposition, or decomposition of gases or SPM.
Especially in the burgeoning megacities of rapidly indus-
trializing countries (chapter 22), air pollution often exceeds
World Health Organization standards. In many Chinese cities,
for example, airborne dust, smoke, and soot often are ten times
higher than levels considered safe for human health ( fig. 16.1 ).
Currently, 16 of the 20 smoggiest cities in the world are in China.
Worldwide we continue to have low-level, chronic exposure
to pollutants. When millions of people are exposed over many
years to these risks, the cumulative number of injuries and deaths
may actually be greater than from notable events like those in
London in 1952.
FIGURE 16.1 On a smoggy day in Shanghai ( left ) visibility is less than 1 km. Twenty-four hours later, after a rainfall ( right ), the air has
cleared dramatically.
348 CHAPTER 16 Air Pollution http://www.mhhe.com/cunningham12e
FIGURE 16.2 Natural pollution sources, such as volcanoes,
can be important health hazards.
can absorb. Many substances are innocuous at naturally occur-
ring levels, but high concentrations in cities or industrial areas can
exceed our physical ability to tolerate them. In many cities and
agricultural regions, for example, more than 90 percent of the air-
borne particulate matter is anthropogenic (human-caused). Effects
include asthma, allergies, and heart and lung ailments.
16.2 Major Types of Pollutants Throughout history, countless ordinances have prohibited emis-
sion of objectionable smoke, odors, and noise. Air pollution tradi-
tionally has been treated as a local problem. The Clean Air Act of
1963 was the first national legislation in the United States aimed
at air pollution control. The act provided federal grants to states to
combat pollution but was careful to preserve states’ rights to set
and enforce air quality regulations. But it soon became obvious
that some pollution problems could not be solved on a local basis.
Criteria pollutants were addressed first Amendments to the law in 1970 essentially rewrote the U.S. Clean
Air Act. Congress designated new standards, to be applied evenly
across the country, for six major pollutants: sulfur dioxide, nitro-
gen oxides, carbon monoxide, ozone, lead, and particulate matter.
These standards were set according to health criteria and environ-
mental quality. National ambient air quality standards (NAAQS)
identify maximum allowable limits for these ( ambient air is the
air around us). These six conventional or criteria pollutants were
addressed first because they contributed the largest volume of air
quality degradation and also are considered the most serious threat
of all air pollutants to human health and welfare. Primary standards
( table 16.2 ) are intended to protect human health. Secondary stan-
dards are also set to protect crops, materials, climate, visibility, and
personal comfort.
Table 16.2 National Ambient Air Quality Standards (NAAQS)
Pollutant Primary (Health-Based) Averaging Time
Standard Concentration
TSPa Annual geometric mean b 50 μg/m 3
24 hours 150 μg/m 3
SO 2 Annual arithmetic mean c 80 μg/m 3 (0.03 ppm)
24 hours 120 μg/m 3 (0.14 ppm)
CO 8 hours 10 mg/m 3 (9 ppm)
1 hour 40 mg/m 3 (35 ppm)
NO 2 Annual arithmetic mean 80 μg/m 3 (0.05 ppm)
O 3 Daily max 8 hour avg. 157 μg/m 3 (0.08 ppm)
Lead Maximum quarterly avg. 1.5 μg/m 3 a Total suspended particulate material, PM2.5 and PM10.
b The geometric mean is obtained by taking the nth root of the product of n numbers. This tends to reduce the impact of a few very large numbers in a set.
c An arithmetic mean is the average determined by dividing the sum of a group of data points by the number of points.
Despite these challenges, most developed countries no longer
have acute air pollution episodes like London’s killer smog. Many
people are surprised to learn that a generation ago most American
cities were much dirtier than they are today. We’ve cleaned up many
of the worst pollution sources, especially those that are large, cen-
tralized, and easy to monitor, and we have better standards and tech-
nology for many smaller sources. The many improvements in air
quality demonstrate that dramatic progress can be made in solving
environmental problems. Continuing industry challenges to clean
air rules also indicate that we can’t be complacent. Public attention
is always needed to protect the safeguards we now rely on.
There are many natural air pollutants It is difficult to give a simple, comprehensive definition of pollu-
tion. The word comes from the Latin pollutus , which means “made
foul, unclean, or dirty.” Some authors use the term only for dam-
aging materials that are released into the environment by human
activities. There are, however, many natural sources of air quality
degradation. Volcanoes spew out ash, acid mists, hydrogen sulfide,
and other toxic gases ( fig. 16.2 ). 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
(terpenes and isoprenes). These compunds create, for example, the
blue haze that gave the Blue Ridge Mountains their name. Storms
in arid regions raise dust clouds that transport millions of tons of
soil. Bacterial metabolism of decaying vegetation in swamps and
of cellulose in the guts of termites and ruminant animals is respon-
sible for as much as two-thirds of the methane in the air. For these compounds, the difference between natural and
human-caused sources is mainly in concentrations, as in cities,
and in our production of amounts greater than natural systems
CHAPTER 16 Air Pollution 349
FIGURE 16.3 Primary pollutants are released directly from
a source into the air. Coal-burning power plants like this one
produce about two-thirds of the sulfur oxides, one-third of the
nitrogen oxides, and one-half of the mercury emitted in the United
States each year.
Carbon monoxide
Nitrogen oxides
Volatile Organic Compounds (VOCs) Sulfur dioxide
Particulate materials
Transportation
Power plantsOther
Industry
Transportation
WastedisposalSolvents
Other
Power plantsIndustry
Other
Transportation
Other
Agriculture
Lead
Smeltingand processing
Transportation
Waste
Other
Transportation
Metals
Solvents, wastedisposal, etc.
Other
Non-road
Non-road
Construction
Non-roadengines
FIGURE 16.4 Anthropogenic sources of primary “criteria” pollutants in the United States. Volatile organic compounds are an
important precursor of ozone, one of the 6 criteria pollutants.
We also distinguish pollutants according to how they are
produced. Primary pollutants are those released directly from
the source into the air in a harmful form ( fig. 16.3 ). Secondary pollutants are converted 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. Photochemical oxidants and atmospheric acids formed
by these mechanisms are among our most important pollutants in
terms of health and ecosystem damage. Fugitive emissions are those that do not go through a smoke-
stack. By far the most massive example of this category is dust
from soil erosion, strip mining, rock crushing, and building con-
struction (and destruction). Fugitive industrial emissions are
hard to monitor, but they are extremely important sources of air
pollution. Leaks around valves and pipe joints, and evaporation
of volatile compounds from oil-processing facilities, contribute
as much as 90 percent of the hydrocarbons and volatile organic
chemicals emitted from oil refineries and chemical plants.
Transportation and power plants are the dominant sources of
criteria pollutants ( fig. 16.4 ). We’ll examine each of these, then
we’ll look at additional pollutants that are also monitored under
the Clean Air Act.
Sulfur Dioxide (SO 2 ) Natural sources of sulfur in the atmosphere include evaporation
of sea spray, erosion of sulfate-containing dust from arid soils,
fumes from volcanoes and hot springs, and biogenic emissions of
hydrogen sulfide (H 2 S) and organic sulfur-containing compounds.
Total yearly emissions of sulfur from all sources amount to some
114 million metric tons. Worldwide, anthropogenic sources repre-
sent about two-thirds of the all airborne sulfur, but in most urban
areas they contribute as much as 90 percent of the sulfur in the air.
The predominant form of anthropogenic sulfur is sulfur dioxide(SO 2 ) from combustion of sulfur-containing fuel (coal and oil),
purification of sour (sulfur-containing) natural gas or oil, and
industrial processes, such as smelting of sulfide ores. China and
the United States are the largest sources of anthropogenic sulfur,
primarily from coal burning and smelting.
Sulfur dioxide was a major contaminant, along with particulate
matter, responsible for illness and death in London’s smog of 1952
(opening case study). This colorless corrosive gas is directly damaging
350 CHAPTER 16 Air Pollution
to both plants and animals ( fig. 16.5 ). Once in the atmosphere, it can
be further oxidized to sulfur trioxide (SO 3 ), which reacts with water
vapor or dissolves in water droplets to form sulfuric acid (H 2 SO 4 ),
a major component of acid rain. Very small solid particles or liquid
droplets can transport the acidic sulfate ion (SO 4 −2 ) long distances
through the air or deep into the lungs where it is very damaging. Sul-
fur dioxide and sulfate ions are probably second only to smoking as
causes of air-pollution-related health damage. Sulfate particles and
droplets reduce visibility in the United States as much as 80 percent.
Some of the smelliest and most obnoxious air pollutants are sulfur
compounds, such as hydrogen sulfide from pig manure lagoons or
mercaptans (organosulfur thiols) from paper mills ( fig. 16.6 ).
Nitrogen Oxides (NO x ) Nitrogen oxides are highly reactive gases formed when nitro-
gen in fuel or in air is heated (during combustion) to tem-
peratures above 650°C (1,200°F) in the presence of oxygen.
Bacteria can also form NO as they oxidize nitrogen-containing
compounds in soil or water. The initial product, nitric oxide(NO), oxidizes further in the atmosphere to nitrogen dioxide(NO 2 ), a reddish-brown gas that gives photochemical smog its
distinctive color. In addition, nitrous oxide (N 2 O) is an inter-
mediate form that results from soil denitrification. Nitrous
oxide absorbs ultraviolet light and is an important greenhouse
gas (chapter 15). Because nitrogen readily changes from one of
these forms to another by gaining or losing O atoms, the general
term NO x is used to describe these gases. Nitrogen oxides com-
bine with water to make nitric acid (HNO 3 ), a major component
of acid rain.
Anthropogenic sources account for 60 percent of the global
emissions of about 230 million metric tons of reactive nitrogen
compounds each year (see table 16.1 ). About 95 percent of all
human-caused NO x in the United States is produced by fuel com-
bustion in transportation and electric power generation. Because
FIGURE 16.5 Sulfur dioxide concentrations and deaths during the London smog of December 1952. The EPA standard limit is
0.08 mg/m 3 (dashed line, (a). The soybean leaf at right (b) was exposed to 2.1 mg/m 3 sulfur dioxide for 24 hours. White patches show
where chlorophyll has been destroyed.
(a)
1000
750
Deaths
Sulfurdioxide
Fog
EPA standard
500
Dea
ths
250
0
4
3
2
1
0
Con
cent
ratio
n of
sul
fur
diox
ide
(SO
2), m
g/m
3
1 3 5 7Date
9 11 13 15
(b)
FIGURE 16.6 The most annoying pollutants from this paper
mill are pungent organosulfur thiols and sulfides. Chlorine bleaching
can also produce extremely dangerous organochlorines, such as
dioxins.
CHAPTER 16 Air Pollution 351
we continue to drive more miles every year, and to consume abun-
dant electricity, we have had less success in controlling NOx than
other pollutants.
Excess nitrogen from agricultural fertilizer use and produc-
tion is also an important, but little understood, contributor to air-
borne NO x . Fertilizers washing from farmlands also cause excess
fertilization and eutrophication of inland waters and coastal seas.
Environmental dispersal of nitrogen from fertilizers also may
be adversely affecting terrestrial plants by fertilizing weedy and
invasive plants.
Carbon Monoxide (CO) Carbon monoxide (CO) is a colorless, odorless, nonirritating, but
highly toxic gas. CO is produced mainly by incomplete combus-
tion of fuel (coal, oil, charcoal, or gas), as in furnaces, incinerators,
engines, or fires, as well as in decomposition of organic matter. CO
blocks oxygen uptake in blood by binding irreversibly to hemoglobin
(the protein that carries oxygen in our blood), making hemoglobin
unable to hold oxygen and deliver it to cells. Human activities pro-
duce about half of the 1 billion metric tons of CO released to the
atmosphere each year. In the United States, two-thirds of the CO
emissions are created by internal combustion engines in transporta-
tion. Land-clearing fires and cooking fires also are major sources.
About 90 percent of the CO in the air is converted to CO 2 in pho-
tochemical reactions that produce ozone. Catalytic converters on
vehicles are one of the important methods to reduce CO production
by ensuring complete oxidation of carbon to carbon dioxide (CO 2 ).
Carbon dioxide is the predominant form of carbon in the
air. Growing recognition of the health and environmental risks
associated with climate change (chapter 15) have led to recent
regulations on CO 2 , which are discussed below.
Ozone (O 3 ) and Photochemical Oxidants Ozone (O 3 ) high in the stratosphere provides a valuable shield for
the biosphere by absorbing incoming ultraviolet radiation. But at
ground level, O 3 is a strong oxidizing reagent that damages vegeta-
tion, building materials (such as paint, rubber, and plastics), and
sensitive tissues (such as eyes and lungs). Ozone has an acrid, bit-
ing odor that is a distinctive characteristic of photochemical smog.
Ground-level O 3 is a product of photochemical reactions (reactions
initiated by sunlight) between other pollutants, such as NO x or
volatile organic compounds. A general term for products of these
reactions is photochemical oxidants . One of the most important
of these reactions involves splitting nitrogen dioxide (NO 2 ) into
nitrous oxide (NO) and oxygen (O). This single O atom is then
available to combine with a molecule of O 2 to make ozone (O 3 ).
Hydrocarbons in the air contribute to the accumulation of
ozone by combining with NO to form new compounds, leaving
single O atoms free to form O3 (fig. 16.7). Many of the NO com-
pounds are damaging photochemical oxidants. A general term for
organic chemicals that evaporate easily or exist as gases in the air
is volatile organic compounds (VOCs) . Plants are the largest
source of VOCs, releasing an estimated 350 million tons of iso-
prene (C 5 H 8 ) and 450 million tons of terpenes (C 10 H 15 ) each year.
About 400 million tons of methane (CH 4 ) are produced by natural
wetlands and rice paddies and by bacteria in the guts of termites
and ruminant animals. These volatile hydrocarbons are generally
oxidized to CO and CO 2 in the atmosphere. In addition to these natural VOCs, a large number of other
synthetic organic chemicals, such as benzene, toluene, formalde-
hyde, vinyl chloride, phenols, chloroform, and trichloroethylene,
are released into the air by human activities. About 28 million
tons of these compounds are emitted each year in the United
States, mainly unburned or partially burned hydrocarbons from
transportation, power plants, chemical plants, and petroleum
refineries. These chemicals play an important role in the forma-
tion of photochemical oxidants.
Lead Our most abundantly produced metal air pollutant, lead is toxic to our
nervous systems and other critical functions. Lead binds to enzymes
and to components of our cells, such as brain cells, which then can-
not function normally. Airborne lead is produced by a wide range of
industrial and mining processes. The main sources are smelting of
metal ores, mining, and burning of coal and municipal waste, in which
lead is a trace element, and burning of gasoline to which lead has been
added. Until recently, leaded gasoline was the main source of lead in
the United States, but leaded gas was phased out in the 1980s. Since
1986, when the ban was enforced, children’s average blood lead lev-
els have dropped 90 percent and average IQs have risen three points.
Banning leaded gasoline in the United States was one of the most
successful pollution-control measures in American history. Now,
50 nations have renounced leaded gasoline. The global economic
benefit of this step is estimated to be more than $200 billion per year.
Worldwide atmospheric lead emissions amount to about
2 million metric tons per year, or two-thirds of all metallic air
pollution. Globally, most of this lead is still from leaded gasoline,
as well as metal ore smelting and coal burning.
Particulate Matter Particulate matter includes solid particles or liquid droplets sus-
pended in a gaseous medium. Very fine solid or liquid particulates
suspended in the atmosphere are aerosols . This includes dust,
ash, soot, lint, smoke, pollen, spores, algal cells, and many other
suspended materials. Particulates often are the most obvious
Atmospheric oxidant production:
1. NO + VOC NO2 (nitrogen dioxide)
2. NO2 + UV NO + O (nitric oxide + atomic oxygen)
3. O + O2 O3 (ozone)
4. NO2 + VOC PAN, etc. (peroxyacetyl nitrate)
Net results:
NO + VOC + O2 + UV O3, PAN, and other oxidants
FIGURE 16.7 Secondary production of urban smog oxidants
by photochemical reactions in the atmosphere.
352 CHAPTER 16 Air Pollution http://www.mhhe.com/cunningham12e
form of air pollution because they reduce visibility and leave dirty
deposits on windows, painted surfaces, and textiles.
Particulates small enough to breathe are monitored under the
Clean Air Act. Particles smaller than 2.5 micrometers in diameter,
such as those found in smoke and haze, and produced by fires,
power plants, or vehicle exhaust, are among the most dangerous
particulates because they can be drawn into the lungs, where they
damage respiratory tissues. Asbestos fibers and cigarette smoke are
among these dangerous fine particles. This fine particulate matter is
referred to as PM2.5, in reference to its size. Reducing sulfur in coal
and diesel fuel, which produces aerosol droplets of sulfuric acid, is
one important strategy for controlling PM2.5 particulates.
Coarse inhalable particles are larger than 2.5 micrometers but
less than 10 micrometers in diameter. These are known as PM10,
and they are typically found near roads or other visible dust sources.
The “dust bowl” of the 1930s involved this kind of particulates. At
that time farmland soils were often left bare, especially during severe
drought, and billions of tons of topsoil blew away from farmlands.
Soil conservation on farmlands is one strategy for reducing PM10;
another strategy is better management of dust at construction sites.
Dust storms can travel remarkable distances. Dust from Africa’s
Sahara desert regularly crosses the Atlantic and raises particulate lev-
els above federal health standards in Miami and San Juan, Puerto
Rico ( fig. 16.8 ). Amazon rainforests receive mineral nutrients carried
in dust from Africa; more than half the 50 million tons of dust trans-
ported to South America each year has been traced to the bed of the
former Lake Chad in Africa. In China, vast dust storms blow
out of the Gobi desert every spring, choking Beijing and clos-
ing airports and schools in Japan and Korea. The dust plume follows
the jet stream across the Pacific to Hawaii and then to the west coast
of North America, where it sometimes makes up as much as half
the particulate air pollution in Seattle, Washington. Some Asian dust
storms have polluted the U.S. skies as far east as Georgia and Maine.
Epidemiological studies have shown that cities with chronically
high levels of particulates have higher death rates, mostly from heart
and lung disease. Emergency-room visits and death rates rise in days
following a dust storm. Some of this health risk comes from the par-
ticles themselves, which clog tiny airways and make breathing diffi-
cult. The dust also carries pollen, bacteria, viruses, fungi, herbicides,
acids, radioactive isotopes, and heavy metals between continents.
Airborne dust is considered the primary source of allergies
worldwide. Saharan dust storms are suspected of raising asthma
rates in Trinidad and Barbados, where cases have increased
17-fold in 30 years. Aspergillus sydowii, a soil fungus from
Africa, has been shown to be causing death of corals and sea fans
in remote reefs in the Caribbean. Europe also receives airborne
pathogens via dust storms. Outbreaks of foot-and-mouth disease
in Britain have been traced to dust storms from North Africa.
Mercury and other metals are also regulated In addition to criteria pollutants or conventional pollutants, many
other pollutants are regulated to protect public health and our envi-
ronment. Standards for these pollutants continue to evolve, as do
definitions of which pollutants require regulation. These changes
reflect increases in certain pollutants, such as airborne mercury;
the introduction of new pollutants, such as newly developed
organic compounds; and increasing recognition of risks, as in the
case of carbon dioxide.
Many toxic metals are released into the air by burning coal and
oil, mining, smelting of metal ores, or manufacturing. Lead, mer-
cury, cadmium, nickel, arsenic (a highly toxic metalloid), and others
are released in the form of metal fumes or suspended particulates by
fuel combustion, ore smelting, and disposal of wastes. Among these,
lead and mercury are the most abundantly produced toxic metals.
Mercury has become regulated relatively recently. Like lead,
mercury is toxic in minute doses, causing nerve damage and
other impairments, especially in young children and developing
fetuses. Volcanoes and rock weathering can produce mercury, but
70 percent of airborne mercury derives from coal-burning power
plants, metal processing (smelting), waste incineration, and other
industrial combustion.
About 75 percent of human exposure to mercury comes from
eating fish. This is because aquatic bacteria are mainly respon-
sible for converting airborne mercury into methyl mercury, a form
that accumulates in living animal tissues. Once methyl mercury
enters the food web, it bioaccumulates in predators. As a conse-
quence, large, long-lived, predatory fish contain especially high
levels of mercury in their tissues. Contaminated tuna fish alone is
responsible for about 40 percent of all U.S. exposure to mercury
( f ig. 16.9 ). Swordfish, shrimp, and other seafood are also impor-
tant mercury sources in our diet.
Freshwater fish also carry risks. Mercury contamination is
the most common cause of impairment of U.S. rivers and lakes,
and 45 states have issued warnings against frequent consumption
of fresh-caught fish. A 2007 study tested more than 2,700 fish
FIGURE 16.8 A massive dust storm extends more than
1,600 km (1,000 mi) from the coast of western Sahara and
Morocco. Storms such as this can easily reach the Americas,
and they have been linked both to the decline of coral reefs in
the Caribbean and to the frequency and intensity of hurricanes
formed in the eastern Atlantic Ocean.
CHAPTER 16 Air Pollution 353
from 636 rivers and streams in 12 western states, and mercury
was found in every one of them.
Global air circulation also deposits airborne mercury on
land. Half or more of the mercury that falls on North America
may come from abroad, much of it from Asian coal-burning
power plants. Similarly, North American mercury travels to
Europe. A 2009 report by the U.S. Geological Survey found
that mercury levels in Pacific Ocean tuna have risen 30 percent
in the past 20 years, with another 50 percent rise projected by
2050. Increased coal burning in China, which is building two
new coal-burning power plants every week, is understood to be
the main cause of growing mercury emissions in the Pacific.
Much of our understanding of mercury poisoning comes from a
disastrous case in Minamata, Japan, in the 1950s, where a chemical
factory regularly discharged mercury-laden waste into Minamata
Bay. Babies whose mothers ate mercury-contaminated fish suffered
profound neu rological disabilities, including deafness, blindness,
mental retardation, and cerebral palsy. In adults, mercury poison-
ing caused numbness, loss of muscle control, and dementia. The con-
nection between “Minamata disease” and mercury was established in
the 1950s, but waste dumping didn’t end for another ten years.
The U.S. National Institutes of Health (NIH) estimates that
1 in 12 American women has more mercury in her blood than
the 5.8 μg/l considered safe by the EPA. Between 300,000 and
600,000 of the 4 million children born each year in the United
States are exposed in the womb to mercury levels that could
cause diminished intelligence or developmental impairments.
According to the NIH, elevated mercury levels cost the U.S.
economy $8.7 billion each year in higher medical and educational
costs and in lost workforce productivity.
Mercury emissions in the United States have declined since
the Clean Air Act began regulating mercury emissions, and many
states have instituted rules for capturing mercury before it leaves
the smokestack. In 2009 the EPA took another step in controlling
mercury emissions when it issued new rules controlling emis-
sions from cement plants, one of the largest sources of the toxin.
Health advocates continue to lobby for international standards
on emissions, espe cially
from coal-burn-
ing power plants
(What Do You
Think? p. 354).
Carbon dioxide and halogens are key greenhouse gases Some 370 billion tons of CO 2 are emitted each year from res-
piration (oxidation of organic compounds by plant and animal
cells; table 16.1 ). These releases are usually balanced by an
equal uptake by photosynthesis in green plants. At normal con-
centrations, CO 2 is nontoxic and innocuous, but atmospheric
levels are steadily increasing (about 0.5 percent per year) due
to human activities and are now causing global climate change,
with serious implications for both human and natural communi-
ties (chapter 15).
Regulating CO 2 has been a subject of intense debate since
the 1990s. On the one hand, policymakers have widely acknowl-
edged that climate change is likely to have disastrous effects. On
the other hand, CO 2 is difficult to consider limiting because we
produce abundant quantities, reductions involve changes to both
technology and behavior, and CO 2 production historically has
been closely tied to our economic productivity. Although future
economic growth is likely to depend on efficiencies and new tech-
nologies, these con cerns remain an important part of the debate.
Since the midterm elections of 2010, many members of Con-
gress have been intent on eliminating this and other pollution reg-
ulation, arguing that it is too costly for industry and the economy
(see further discussion in section 16.5). Energy companies and
their representatives, in particular, have lobbied to prevent legal
limits on greenhouse gases. The 2011 congressional budget pro-
posed to slash EPA funding by one-third, in part to reduce pollu-
tion monitoring and regulation.
The question of whether the EPA should regulate greenhouse
gases was so contentious that it went to the Supreme Court in
2007. The Court ruled that it was the EPA’s responsibility to limit
these gases, on the grounds that greenhouse gases endanger pub-
lic health and welfare within the meaning of the Clean Air Act.
The Court, and subsequent EPA documents, noted that these risks
include increased drought, more frequent and intense heat waves
and wildfires, sea-level rise, and harm to water resources, agricul-
ture, wildlife, and ecosystems. In addition to these risks, the U.S.
military has cited climate change as a security threat. A coalition
of generals and admirals signed a report from the Center for Naval
Analyses stating that climate change “presents significant national
security challenges” including violence resulting from scar-
city of water, and migration due to sea-level rise and
crop failure.
Since the Supreme Court ruling, the EPA is
charged with regulating six greenhouse gases: carbon
dioxide, methane, nitrous oxide, hydrofluorocarbons,
perfluorocarbons, and sulfur hexafluoride. These
are gases whose emissions have grown dramati-
cally in recent decades.
Three of these six greenhouse gases contain
halogens, a group of lightweight, highly reactive
elements (fluorine, chlorine, bromine, and iodine).
Because they are generally toxic in their elemen-
tal form, they are commonly used as fumigants and
on emissions, espe cially
from coal-burn-
ing power plants
(What Do You
Think? p. 354).
ture, wildlife, a
military has cit
of generals and
Analyses statin
security ch
city
crop
cha
dio
p
a
c
FIGURE 16.9 Airborne mercury bioaccumulates in seafood, especially in top
predators such as tuna. Mercury contamination is also the most common cause of
fish consumption advisories in U.S. lakes and rivers.
354 CHAPTER 16 Air Pollution http://www.mhhe.com/cunningham12e
disinfectants, but they also have hundreds of uses in indus-
trial and commercial products. Chlorofluorocarbons (CFCs)
have been banned for most uses in industrialized countries, but
about 600 million tons of these compounds are used annually
worldwide in spray propellants and refrigeration compressors
and for foam blowing. They diffuse into the stratosphere, where
they release chlorine and fluorine atoms that destroy ozone
molecules that protect the earth from ultraviolet radiation
(see section 16.3).
Halogen compounds are also powerful greenhouse gases: they
trap more energy per molecule than does CO 2 , and they persist
in the atmosphere for decades to centuries. Perfluorocarbons will
persist in the atmosphere for thousands of years. The global warm-
ing potential (per molecule, over time) of some types of CFCs is
10,900 times that of CO 2 ( table 16.3 ).
What Do You Think?
Cap and Trade for Mercury Pollution?
Often referred to as quicksilver, mercury is used in a host of products,
including paints, batteries, fluorescent lightbulbs, electrical switches, pes-
ticides, skin creams, antifungal agents, and old thermometers. Mercury
also is a powerful neurotoxin that destroys the brain and central nervous
system at high doses. Minute amounts can cause nerve damage and devel-
opmental defects in children. Exposure results mainly from burning gar-
bage, coal, or other mercury-laden materials—the mercury falls to the
ground and washes into lakes and wetlands, where it enters the food web.
In a survey of freshwater fish from 260 lakes across the United States, the
EPA found that every fish sampled contained some level of mercury.
In 1994 the EPA declared mercury a hazardous pollutant regu-
lated under the Clean Air Act. Municipal and medical incinerators were
required to reduce their mercury emissions by 90 percent. Industrial and
mining operations also agreed to cut emissions. However, the law did not
address the 1,032 coal-burning power plants, which produce nearly half
of total annual U.S. emissions, some 48 tons per year.
Finally in 2000 the EPA declared mercury from power plants, like
that from other sources, a public health risk. The agency could have
applied existing air-toxin regulations and required power plants to reduce
their emissions by 90 percent in 5 years with existing control technology.
But the EPA in 2000 opted instead for a “cap and trade” market mecha-
nism, which should reduce mercury releases 70 percent in about 30 years.
Cap-and-trade approaches set limits (caps) and allow utilities to
buy and sell unused pollution credits. This strategy is widely supported
because it uses a profit motive rather than rules, and it allows industries
to make their own decisions about emission controls. It also allows con-
tinued emissions if credits are cheaper than emission controls, and traders
have the opportunity to make money on the exchanges.
On the other hand, public health advocates argue that although cap-and-
trade systems work well for some pollutants, they are inappropriate for a sub-
stance that is toxic at very low levels, and they object that utilities are allowed
to continue emitting mercury for years longer than necessary. Many eastern
states are especially concerned because they suffer from high mercury pol-
lution generated in the Midwest and blown east by prevailing winds ( fig. 1 ). Meanwhile, in the Allegheny Mountains of West Virginia, a huge
coal-fired power plant is adding fuel to the mercury debate. The enormous
1,600-megawatt Mount Storm plant ranked second in the nation in mer-
cury emissions just a few years ago. When Mount Storm installed new
controls to capture sulfur and nitrogen oxides from its stack, this equip-
ment also caught 95 percent of its mercury emissions, at no extra cost.
This is excellent news, but it also raises a policy question: If existing tech-
nology can cut mercury economically, why wait 30 years to impose simi-
larly cost-effective limits on other power plants?
This case illustrates the complexity of regulating air pollution. Highly
mobile, widely dispersed, produced by a variety of sources, and having
diverse impacts, air pollutants can be challenging to regulate. Often air
quality controversies—such as mercury control—pit a diffuse public inter-
est (improving general health levels or child development) against a very
specific private interest (utilities that must pay millions of dollars per year
to control pollutants). How would you set the rules if you were in charge?
Would you impose rules or allow for trading of mercury emission permits?
Why? How would you negotiate the responsibility for controlling pollutants?
0
0
150 300
150 300
1.0–2.02.0–5.05.0–10.010.0–20.0>20.0
Deposition in µg/m2/yr
Miles
Kilometers
FIGURE 1 Atmospheric mercury deposition in the United States. Due to prevailing westerly winds, and high levels of industrialization, eastern states have high mercury deposition.
Source: EPA, 1998.
Table 16.3 Global Warming Potential (GWP) of Several Greenhouse Gases
GAS Global warming
potential 1 Atmospheric
lifetime (years) 2
Carbon dioxide (CO 2 ) 1 ̃ 100
Methane (CH 4 ) 25 124
Nitrous oxide (N 2 O) 298 1144
CFC-12 (CCl 2 F 2 ) 10,900 100
HCFC-142b(CH 3 CClF 2 ) 2,310 18
Sulfur hexafluoride (SF 6 ) 22,800 3200
1 A measure of radiative effects, integrated over a 100-yr time horizon, relative to an equal mass of CO 2 emissions. CO 2 is set as 1 for comparison. 2Average residence times shown; actual range for CO 2 is decades to centuries.
Source: Carbon Dioxide Information Analysis Center, 2011 .
CHAPTER 16 Air Pollution 355
Developing rules and standards for greenhouse gases will
take time and considerable debate. Many strategies have been
proposed, including subsidies for alternative energy, reducing tax
breaks and other subsidies for fossil fuels, imposing a tax on coal,
oil, and gas, and cap-and-trade systems, including carbon-trading
markets. The last of these options has been the most acceptable,
and carbon trading is now worth billions of dollars every year.
Data remain inconclusive regarding whether this has produced an
overall decline in emissions.
Hazardous air pollutants (HAPs) can cause cancer and nerve damage Although most air contaminants are regulated because of their
potential adverse effects on human health or environmental qual-
ity, a special category of toxins is monitored by the U.S. EPA
because they are particularly dangerous. Called hazardous air pollutants (HAPs), these chemicals include carcinogens, neu-
rotoxins, mutagens, teratogens, endocrine system disrupters, and
other highly toxic compounds (chapter 8). Twenty of the most
“ persistent bioaccumulative toxic chemicals” (see table 8.2)
require special reporting and management because they remain
in ecosystems for long periods of time and accumulate in animal
and human tissues. Most of these chemicals are either metal com-
pounds, chlorinated hydrocarbons, or volatile organic compounds.
Gasoline vapors, solvents, and components of plastics are all
HAPs that you may encounter on a daily basis.
Only about 50 locations in the United States regularly
measure concentrations of HAPs in ambient air. Often the best
source of information about these chemicals is the Toxic Release Inventory (TRI) collected by the EPA as part of the community
right-to-know program. Established by Congress in 1986, the
TRI requires 23,000 factories, refineries, hard rock mines, power
plants, and chemical manufacturers to report on toxin releases
(above certain minimum amounts) and waste management meth-
ods for 667 toxic chemicals. Although this total is less than 1 per-
cent of all chemicals registered for use, and represents a limited
range of sources, the TRI is widely considered the most compre-
hensive source of information about toxic pollution in the United
States ( fig. 16.10 ). Most HAP releases are decreasing, but discharges of mercury
and dioxins—both of which are bioaccumulative and toxic at
extremely low levels—have increased in recent years. Dioxins are
created mainly by burning plastics and medical waste containing
chlorine. The EPA reports that 100 million Americans live in areas
where the cancer rate from HAPs exceeds 10 in 1 million, or ten
times the normally accepted standard for action. Benzene, formal-
dehyde, acetaldehyde, and 1,3 butadiene are responsible for most
of this HAP cancer risk. Furthermore, twice that many Americans
(70 percent of the U.S. population) live in areas where the risk
of death from causes other than cancer exceeds 1 in 1 million.
To help residents track local air quality levels, the EPA recently
estimated the concentration of HAPs in localities across the con-
tinental United States (over 60,000 census tracts). You can access
this information on the Environmental Defense Fund web page at
www.scorecard.org/env-releases/hap/ .
Aesthetic degradation also results from pollution Aesthetic degradation is any undesirable change in the physi-
cal characteristics or chemistry of the atmosphere, such as noise,
odors, and light pollution. These factors rarely threaten life or
health directly, but they can strongly impact our quality of life.
They also increase stress, which affects health. We are often espe-
cially susceptible to noises and odors. Often the most sensitive
device for odor detection is the human nose. We can smell sty-
rene, for example, at 44 parts per billion (ppb). Trained panels
of odor testers often are used to evaluate air samples. Factories
that emit noxious chemicals sometimes spray “odor maskants” or
perfumes into smokestacks to cover up objectionable odors. Light
pollution also is a concern in most urban areas, where ambient
light confuses birds and hides the stars.
Indoor air can be worse than outdoor air We have spent a considerable amount of effort and money to con-
trol the major outdoor air pollutants, but we have only recently
begun to address indoor air pollutants. The EPA has found that
indoor concentrations of toxic air pollutants are often higher than
outdoors. Furthermore, people generally spend more time inside
than out, so they are exposed to higher doses of these pollutants.
In some cases, indoor air in homes has concentrations of
chemicals that would be illegal outside or in the workplace. The
FIGURE 16.10 Harmful air toxics from large industrial
sources, such as chemical plants, petroleum refineries, and paper
mills, have been reduced by nearly 70 percent since the EPA
began regulating them. Many smaller sources remain unregulated.
356 CHAPTER 16 Air Pollution http://www.mhhe.com/cunningham12e
EPA has found that concentrations of such compounds as chlo-
roform, benzene, carbon tetrachloride, formaldehyde, and styrene
can be seventy times higher in indoor air than in outdoor air, as
plastics, carpets, paints, and other common materials off-gas these
compounds. Finding less-toxic paints and fabrics can make indoor
spaces both healthier and more pleasant.
In the less-developed countries of Africa, Asia, and Latin
America, where such organic fuels as firewood, charcoal, dried
dung, and agricultural wastes provide the majority of household
energy, smoky, poorly ventilated heating and cooking fires are
the greatest source of indoor air pollution ( fig. 16.11 ). The World
Health Organization (WHO) estimates that 2.5 billion people—
over a third of the world’s population—are adversely affected by
pollution from this source. Women and small children spend long
hours each day around open fires or unventilated stoves in enclosed
spaces. Levels of carbon monoxide, particulates, aldehydes, and
other toxic chemicals can be 100 times higher than would be legal
for outdoor ambient concentrations in the United States. Design-
ing and building cheap, efficient, nonpolluting energy sources for
the developing countries would not only save shrinking forests but
would make a major impact on health as well.
16.3 Atmospheric Processes Topography, climate, and physical processes in the atmosphere
play an important role in the transport, concentration, dispersal,
and removal of many air pollutants. Cities concentrate dust and
pollutants in urban “dust domes”; winds cause mixing between
air layers, precipitation, and atmospheric chemistry. All these fac-
tors determine whether pollutants will remain in the locality where
they are produced or go elsewhere. In this next section we will
survey some environmental factors that affect air pollution levels.
Temperature inversions trap pollutants As in London’s smog of 1952, temperature inversions can
greatly concentrate air pollutants. Inversions occur when a stable
layer of warmer air lies above cooler air. The normal conditions,
where temperatures decline with increasing height, are inverted,
and these stable conditions prevent convection currents from dis-
persing pollutants. Often these conditions occur when cold air
settles in a valley that is surrounded by hills or mountains. When
a 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 below, the cold air becomes trapped, as in a bowl.
Inversions might last from a few hours to a few days.
The most stable inversion conditions are usually created by
rapid nighttime cooling in a valley or basin where air movement
is restricted. Los Angeles is a classic example, with conditions
that create both temperature inversions and photochemical smog
( fig. 16.12 ). The city is surrounded by mountains on three sides and
the climate is dry, with abundant sunshine for photochemical oxi-
dation and ozone production. Millions of automobiles and trucks
create high pollution levels. Skies are generally clear at night,
allowing heat to radiate from the ground. The ground and the lower
FIGURE 16.11 Smoky cooking and heating fires may cause
more ill health effects than any other source of indoor air pollution
except tobacco smoking. Some 2.5 billion people, mainly women
and children, spend hours each day in poorly ventilated kitchens
and living spaces where carbon monoxide, particulates, and
cancer-causing hydrocarbons often reach dangerous levels.
Alti
tude
Temperature
Alti
tude
Temperature
Night
Cooler
Cool
Warm
Day
Cooler
Cool
Warm
FIGURE 16.12 Atmospheric temperature inversions occur
where ground-level air cools more quickly than upper levels. This
temperature differential prevents mixing and traps pollutants close
to the ground.
CHAPTER 16 Air Pollution 357
layers of air cool quickly at night, while upper air layers remain
relatively warm. During the night, cool, humid, onshore breezes
also slide in under the contaminated air, which is trapped by a wall
of mountains to the east and by the cap of warmer air above.
Morning sunlight is absorbed by the concentrated aerosols
and gaseous chemicals caught near the ground by the inversion.
This complex mixture quickly cooks up a toxic brew of hazard-
ous compounds. As the ground warms later in the day, convec-
tion currents break up the temperature gradient and pollutants are
carried back down to the surface, where more contaminants are
added. Nitric oxide (NO) from automobile exhaust is oxidized to
a brownish haze of nitrogen dioxide (NO 2 ). As nitrogen oxides are
used up in reactions with unburned hydrocarbons, the ozone level
begins to rise. By early afternoon an acrid brown haze fills the air,
making eyes water and throats burn. In the 1970s, before pollution
controls were enforced, the Los Angeles basin often would reach
0.34 ppm or more by late afternoon and the pollution index could
be 300, the stage considered a health hazard.
Wind currents carry pollutants worldwide Dust and contaminants can be carried great distances by the wind.
Areas downwind from industrial complexes often suffer serious
contamination, even if they have no pollution sources of their
own ( fig. 16.13 ). Pollution from the industrial belt between the
Great Lakes and the Ohio River Valley, for example, regularly
contaminates the Canadian Maritime Provinces, and sometimes
can be traced as far as Ireland. As noted earlier, long-range trans-
port is a major source of Asian mercury in North America. Studies of air pollutants over southern Asia reveal a 3 km
thick toxic cloud of ash, acids, aerosols, dust, and photochemi-
cal reactants that regularly covers the entire Indian subcontinent
and can last for much of the year. Nobel laureate Paul Crutzen
estimates that up to 2 million people in India alone die each year
from atmospheric pollution. Produced by forest fires, the burn-
ing of agricultural wastes, and dramatic increases in the use of
fossil fuels, the Asian smog layer cuts by up to 15 percent the
amount of solar energy reaching the earth’s surface beneath it.
Meteorologists suggest that the cloud—80 percent of which is
human-made—could disrupt monsoon weather patterns and may
be disturbing rainfall and reducing rice harvests over much of
South Asia. As UN Environment Programme executive director
Klaus Töpfer said, “There are global implications because a pol-
lution parcel like this, which stretches three km high, can travel
half way round the globe in a week.”
An increase in monitoring activity has revealed industrial
contaminants in places usually considered among the cleanest in
the world. Samoa, Greenland, Antarctica, and the North Pole all
have heavy metals, pesticides, and radioactive elements in their
air. Since the 1950s, pilots flying in the high Arctic have reported
dense layers of reddish-brown haze clouding the arctic atmo-
sphere. Aerosols of sulfates, soot, dust, and toxic heavy metals,
1000
0
0
1000 2000 Miles
2000 3000 KilometersScale: 1 to 138,870,000
Pollution of the Atmosphere
Land areas with significant acidprecipitation
Land areas with significantatmospheric pollution
Land areas of secondaryatmospheric pollution
Air pollution plume: average winddirection and force
Land areas with significant acidprecipitation and atmosphericpollution
Wind blows in the direction of the tapered end of the air pollution plume and the force of the wind is indicated by the size of the plume.
FIGURE 16.13 Long-range transport carries air pollution from source regions thousands of kilometers away into formerly pristine
areas. Secondary air pollutants can be formed by photochemical reactions far from primary emissions sources.
358 CHAPTER 16 Air Pollution http://www.mhhe.com/cunningham12e
such as vanadium, manganese, and lead, travel to the pole from the
industrialized parts of Europe and Russia.
A process called “grasshopper” transport, or atmosphere dis-
tillation, helps deliver contaminants to the poles. Volatile com-
pounds evaporate from warm areas, travel through the atmosphere,
then condense and precipitate in cooler regions ( fig. 16.14 ). Over
several years, contaminants accumulate in the coldest places,
generally at high latitudes where they bioaccumulate in food
chains. Whales, polar bears, sharks, and other top carnivores in
polar regions have been shown to have dangerously high levels
of pesticides, metals, and other HAPs in their bodies. The Inuit
people of Broughton Island, well above the Arctic Circle, have
higher levels of polychlorinated biphenyls (PCBs) in their blood
than any other known population, except victims of industrial
accidents. Far from any source of this industrial by-product, these
people accumulate PCBs from the flesh of fish, caribou, and other
animals they eat. This exacerbates the cultural crisis caused by cli-
mate change.
Stratospheric ozone is destroyed by chlorine In 1985 the British Antarctic Atmospheric Survey announced a
startling and disturbing discovery: stratospheric ozone concen-
trations 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 ( fig. 16.15 ). This ozone depletion has been
occurring at least since the 1960s but was not recognized because
earlier researchers programmed their instruments to ignore
changes in ozone levels that were presumed to be erroneous. Chlorine-based aerosols, especially chlorofluorocarbons
(CFCs) and other halon gases, are the principal agents of ozone
depletion. Nontoxic, nonflammable, chemically inert, and cheaply
produced, CFCs were extremely useful as industrial gases and in
refrigerators, air conditioners, Styrofoam inflation, and aerosol
spray cans for many years. From the 1930s until the 1980s, CFCs
Atmosphere
Equator
FIGURE 16.14 Air pollutants evaporate from warmer areas
and then condense and precipitate in cooler regions. Eventually
this “grasshopper” redistribution leads to accumulation in the
Arctic and Antarctic.
were used all over the world and widely dispersed through the
atmosphere.
What we often call an ozone “hole” is really a vast area of
reduced concentrations of ozone in the stratosphere. Although
ozone is a pollutant in the ambient air, ozone in the stratosphere
is important because it absorbs much of the harmful ultraviolet
(UV) radiation that enters the outer atmosphere. UV radiation
damages plant and animal tissues, including the eyes and the skin.
A 1 percent loss of ozone could result in about a million extra
human skin cancers per year worldwide, if no protective measures
are taken. Excessive UV exposure could reduce agricultural pro-
duction and disrupt ecosystems. Scientists worry that, for example,
high UV levels in Antarctica could reduce populations of plank-
ton, the tiny floating organisms that form the base of a food chain
that includes fish, seals, penguins, and whales in Antarctic seas. In
2006 the region of ozone depletion covered 29.5 million km 2 (an
area larger than North America).
Antarctica’s exceptionally cold winter temperatures
(–85 to –90°C) help break down ozone. During the long, dark
winter months, strong winds known as the circumpolar vor-
tex isolate Antarctic air and allow stratospheric temperatures
to drop low enough to create ice crystals at high altitudes—
something that rarely happens elsewhere in the world. Ozone
and chlorine-containing molecules are absorbed on the surfaces
of these ice particles. When the sun returns in the spring, it
provides energy to liberate chlorine ions, which readily bond
with ozone, breaking it down to molecular oxygen (table 16.4).
FIGURE 16.15 The region of stratospheric ozone depletion
grew steadily to an area of nearly 30 million km 2 in 2006 (shown
here). This ozone “hole” has shown signs of decline since the
Montreal Protocol went into effect.