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.