CHAPTER 14. Human Effects: Air Pollution and Heat Islands
Chapter Overview:
The principal types of atmospheric pollutants are discussed in this chapter. Natural and
anthropogenic sources are described along with pollution effects on humans and various areas of
the ecosystem. In addition, the chapter explores causes and characteristics of urban heat island.
Chapter at a Glance:
Most air pollutants are quickly dispersed and removed from the atmosphere so that they are rather
harmless. Other pollutants may cause everything from a dull sky to death. Smog is a combination
of smoke and fog. An extreme example of smog was the 1952 London air pollution episode,
which left nearly 4000 people dead. Many industrial centers and large cities have air pollution
problems mainly stemming from industrial processes and photochemical smog developing from a
combination of automobiles and certain atmospheric conditions. Los Angeles-type smog occurs
when automobile pollutants combine with dry air. Recent concerns have led to improvements in
air quality, however, other negative aspects of the atmosphere-human relationship abound.
• Atmospheric Pollutants - Particulates, deriving from human and natural sources,
flourish throughout the atmosphere as do polluting gases. Concentrations of particulates
and certain gases in urban areas invoke concerns. Pollution may be divided into either
primary pollutants, those emitted directly into the atmosphere, or secondary pollutants,
resulting from chemical transformations. <Web>
A. Particulates - Particulates (aerosols) may be either solid or liquid materials in
the air. They stem from both natural and anthropogenic sources and range in size
from 0.1 to 100 µm. <Web>
1. Sources of Particulates - Natural sources of particulates include fires,
volcanic eruptions, salt from breaking waves, and vegetation sources
(pollen). Human sources largely involve combustion of fossil fuels in
creating primary and secondary particulates. Because many particulates act
as condensation nuclei, fog and the resulting smog may be problematic in
large cities and industrial areas. <Web>
2. Removal of Particulates - Although particulates are small and may,
therefore, remain suspended in the atmosphere for long periods, all are
eventually removed in time. Gravity and precipitation are the primary
mechanisms of removal.
3. Effects of Particulates - Particulates relate to factors such as reduced
visibility and health concerns. Smaller particulates or PM10 particulates
(particulates < 10 µm) enter the lungs and affect health. Even smaller
particulates, PM2.5s, have been deemed even more dangerous.
B. Carbon Oxides - Carbon dioxide plays an important role in the global energy
balance but is not considered a pollutant as no direct adverse effects occur from
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high concentrations. Carbon monoxide, however, is a colorless, odorless gas
emitted naturally through volcanic eruptions, forest fires, bacterial activities, etc.
In cities, unsafe levels may accumulate from incomplete combustion of gas in
automobiles. Other anthropogenic sources include furnaces, home fires, and
cigarette smoke. Carbon monoxide is extremely hazardous to the point of death
with long-term exposure to even small concentrations. <Web>
C. Sulfur Compounds - Atmospheric sulfur compounds, in gaseous and aerosol
form, derive mainly from natural sources. The most important natural source is
bacterial release of hydrogen sulfides but volcanic eruptions and sea spray also
contribute. Anthropogenic release of sulfur dioxide (SO2) and sulfur trioxide (SO3)
stems mainly from fossil fuel burning and smelting of ores. Sulfur dioxide is an
irritant to human respiratory systems. Sulfur trioxide, which usually stems from
reactions involving SO2, combines with water to form sulfuric acid (H2SO4). This
creates acid fog and acid rain, which may cause ecosystem degradation. <Web>
D. Nitrogen Oxides - Nitrogen oxides, compounds of nitrogen and oxygen, are
commonly discussed as NOx. Nitric oxide (NO) forms naturally from biological
processes in soil and water and anthropogenically through combustion in
automobiles, industry, and electricity generation. It is broken down quickly
through natural processes. Nitrogen dioxide (NO2), a toxic gas which is yellow to
reddish-brown in color, contains a pungent odor and is very corrosive. It
contributes to acid deposition and other secondary pollutants. Although it breaks
down quickly, it varies with traffic concentrations in urban areas. Long-term
exposure contributes to pulmonary health problems.
E. Volatile Organic Compounds (Hydrocarbons) - Hydrocarbons, composed
solely of hydrogen and carbon atoms, include methane, butane, propane, and
octane in addition to other compounds. Plant and animal emissions,
decomposition, and automobiles are primary hydrocarbon sources. Combined with
solar radiation, these chemicals form photochemical smog.
F. Photochemical Smog - Photochemical smog produces burning eyes, sore lungs,
odor, and poor visibility. These smogs consist of secondary pollutants to create a
Los Angeles-type smog common in many cities when smog develops in dry air.
Ozone, an important contributor of photochemical smog, may cause serious
physical and environmental harm. Physical ailments associated with ozone include
decreased lung capacity with long-term exposure leading to permanent lung
damage. <Web>
• Atmospheric Controls on Air Pollution - Natural sources of aerosols usually do not
create high concentrations due to release over large areas. Pollution problems exist when
high concentrations occur over small spatial scales. The atmosphere contributes to
pollution concentration as it is responsible for horizontal and vertical dispersion.
A. Effect of Winds on Horizontal Transport - Strong winds disperse pollutants
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through rapid transport from the high concentration source. Pollutant
concentrations are inversely proportional to wind speed. Increased winds also
indirectly reduce concentrations as eddies mix air vertically through forced
convection. Wind direction is also important in dispersing pollutants through many
directions or by concentrating pollutants in one or a few directions.
B. Effect of Atmospheric Stability - Stability is also important with regard to
pollution dispersion. A stable atmosphere restricts vertical motions, thereby,
allowing increased pollution concentrations near the surface. Radiation induced
inversions exacerbate these situations, especially during early morning hours.
Subsidence inversions, usually associated with photochemical smogs, may extend
for long temporal periods. Mixing depth is proportional to the inversion base.
• Urban Heat Islands - Urbanized regions typically maintain higher surface temperatures
than surrounding areas. These urban heat islands are usually proportional to the size of the
city and the density of population. Intensity of the heat island varies spatially with highest
temperatures usually located within the city core. <Web>
A. Radiation Effects - Urban particulates affect the intensity of urban heat island
by affecting the local radiation balance. Increased particulate concentrations absorb
and scatter insolation and also increase the amount of longwave radiation release.
The latter offsets the former leading to higher temperatures. Indirectly, these
particulates acting as condensation nuclei relate to cloud cover and precipitation
over cities leading to higher precipitation totals downwind of the city. City
surfaces also contribute to urban heat island as natural vegetative surfaces are
replaced by impervious ones. Heat exchange, albedo, evaporation, and radiation
balances are all offset by these surfaces.
B. Changes in Heat Storage - Buildings heat quickly during the day and these
increased temperatures are transferred to building interiors. At night, this stored
energy is released to the surface. Buildings have a much greater heat capacity than
natural surfaces, which increases the amount of stored heat energy available for
atmospheric warming.
C. Sensible and Latent Heat Transfer - Moisture near the surface uses available
energy through evaporation and transports that energy aloft as latent heat.
Decreased evaporation associated with the dry surfaces of cites decreases the
amount of vertical energy transport, thereby increasing the amount of surface
sensible heat.
1. Urban Heat Islands and the Detection of Climate Change - Most
temperature data come from urban settings. Unprecedented growth over
the past century in urban areas has likely affected temperature readings.
Therefore, caution should be used when examining temperature data at face
value to detect climate change. Contaminated temperature data are
frequently discarded or corrected for urban heat island effects.
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Chapter Boxes:
14-1 Focus on the Environment: Air Pollution - Natural or Anthropogenic? - Natural
phenomena such as El Niño events may exacerbate pollution stemming from
anthropogenic sources. Such was the case in Indonesia in 1997 in which human induced
fires raged.
14-2 Focus on the Environment: the Counteroffensive on Air Pollution - Although
local and state laws to regulate pollution existed in the US since the late 19th century, it was
not until 1963 that a national effort was initiated. The Clean Air Act sought widespread air
pollution control and stimulated research and technical development. Maximum standards
were set for many atmospheric aerosols through the act and its many amendments. One
such amendment called for the elimination of lead in gasoline. The act also required states
to establish enforcement agencies in addition to requiring automobile emission reduction
devices. Because many states and urban areas failed to meet initial standards, the
amendments of 1990 were established. These widespread standards call for mandatory
compliance.
14-3 Focus on the Environment: Smog in Southern California - The EPA classifies Los
Angeles as an extreme area of noncompliance of ozone standards. Air quality in this
region is poor due to blocking mountains, sunny days, presence of a subsidence inversion,
and a high concentration of automobiles. New regulations for automobile emissions,
special gas pump nozzles, and new low hydrocarbon gasolines have combined to reduce
the problem. Summer smogs are a common problem due to high residual primary and
secondary pollutants, weak winds in the morning hours, a low morning sun angle, and the
presence of fog. By late morning the development of a sea breeze helps alleviate the
problem.
Related Web Sites:
Smog: www.smogcheck.ca.gov/smoghome.htm
Acid Rain: www.epa.gov/acidrain/ardhome.html
Urban Heat Island: www.climatenews.com
Particulates (Aerosols): http://sasa.pmel.noaa.gov
Carbon Monoxide: www.phymac.med.wayne.edu/FacultyProfile/penney/COHQ/co1.htm
Air Pollution: www.epa.gov/oar
Media Enrichment:
ME14.1 - Weather Image - Atmospheric Pollution as Seen from the Space Shuttle
ME14.2 - Weather Image - Pollution over Amazon Basin
ME14.3 - Weather Image - Kuwait Oil Field Fires
ME14.4 - Weather Image - Industrial Pollution in Siberia
ME14.5 - Weather Image - Ozone Concentration and Air Temperature
ME14.6 - Weather Image - Atlanta Urban Heat Island I, II and III
Key Terms:
air pollution
carbon oxides
acid rain
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photochemical smog
primary pollutants
secondary pollutants
particulates
pm10
pm2.5
carbon monoxide
carbon dioxide
sulfur dioxide
acid fog
sea breeze front
nitrogen oxides
London-type smog
nitric oxides
Los Angeles-type smog
nitrogen dioxide
urban heat island
volatile organic compounds (hydrocarbons)
Review Questions:
1. Explain the distinction between primary and secondary pollutants.
Primary pollutants are those emitted directly into the atmosphere. Secondary pollutants
result from chemical transformations of pre-existing substances and/or pollutants.
2. What are particulates and how are they introduced into the atmosphere?
Particulates may be either solid or liquid materials in air. They stem from both natural and
anthropogenic sources.
3. What are the two processes most responsible for removing particulates from the atmosphere?
Gravity and precipitation are the may processes of pollutant removal.
4. What are PM10 and PM2.5? Does one pose a greater health risk than the other?
Particulates with a size less than 10 µm (PM10) are capable of entering the lungs. As such
they pose a significant health threat. PM2.5 particulates are even smaller and are more
dangerous.
5. List the most important gases that contribute to air pollution.
Carbon dioxide, sulfur compounds, nitrogen oxides, volatile organic compounds
(hydrocarbons) and ozone are generally the most important polluters.
6. What are the primary sources of carbon monoxide in the atmosphere? If these sources are
nonanthropogenic, why is it that CO is considered a pollutant?
Volcanic eruptions, forest fires, bacterial action and other natural processes and by humans
though combustion. Natural sources emit far more CO than human, however, in cities,
automobiles re the primary source.
7. In what way does carbon monoxide harm the human body?
CO is extremely toxic. Even low levels can cause a person to immediately experience
slowed reflexes, drowsiness, and a reduction or loss of consciousness. Exposure for 3
hours at 400 ppm is threatening and at 1600 ppm death occurs within an hour. Long-term
exposure to low levels may lead to heart disease.
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8. What are the primary sources of sulfur dioxide and sulfur trioxide in the atmosphere?
Both stem from fossil fuel burning. Specifically sulfur trioxide usually stems from
reactions involving sulfur dioxide.
9. Would a person be more likely to notice the presence of high CO or high SO2 contents in the
ambient air?
Carbon monoxide is colorless and odorless, so sulfur dioxide would be most noticeable.
10. Which primary pollutant is most likely to promote the formation of acid fog or acid rain?
Acid fog and rain stem from sulfur trioxide, which combines with water to form sulfuric
acid.
11. Why is it that nitric oxide is much less common in the atmosphere than nitrogen dioxide?
Nitric oxide breaks down quickly through natural processes. Nitrogen dioxide does as well
but it increases with traffic concentrations in urban areas.
12. Describe the general composition of volatile organic compounds.
Hydrocarbons are composed solely of hydrogen and carbon atoms. They include methane,
butane, propane, and octane in addition to other compounds.
13. How do London-type and Los Angeles-type smog differ from each other?
Los Angeles-type smog is common in many cities when smog develops in dry air. It
develops from photochemical reactions between hydrocarbons and sunlight. London-type
smog is actually a mix of smoke and fog, typical of wet environments.
14. Which pollutant gases cause a noticeable coloration of the atmosphere? Which have a
characteristic odor?
Nitrogen dioxide is yellow to reddish-brown in color and contains a pungent odor.
15. Describe the various atmospheric controls that affect the concentration of air pollutants.
Wind is the primary means of transporting high concentrations of pollutants from a
location. As they horizontally transport pollutants the pollutants disperse. Forced
convection also contributes to mixing. Wind direction is important in dispersion or by
concentrating pollutants in one or a few directions. Stability is also important as this
regulates vertical transport. For instance, mixing depth is proportional to inversion base
associated with early morning hour smog.
16. How does the construction of buildings in cities alter the radiation exchange near the surface
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and contribute to the urban heat island effect?
Buildings heat quickly during the day and these increased temperatures are transferred to
building interiors. At night, this stored energy is released to the surface. Buildings have a
much greater heat capacity than natural surfaces, which increases the amount of stored heat
energy available for atmospheric warming.
17. Describe the way in which heat storage in cities differs from that of surrounding rural areas.
Rural areas contain natural surfaces which are permeable and typically vegetated. These
surfaces allow for the evaporation of water from the surface that transports heat energy
vertically. Cities, with impervious surfaces have a much greater sensible heat flux than
rural areas. This results in higher temperatures for the cities.
18. What effect does urbanization have on the exchange of sensible and latent heat?
Decreased evaporation rates associated with the dry surfaces of cities decreases the amount
of vertical energy transport, thereby increasing the amount of surface sensible heat.
19. Does the urban heat island manifest itself equally during the day and night? Are there seasonal
differences in the magnitude of the heat island effect?
Stored energy is available for transfer to the lower atmosphere during the evening and
nighttime, which increases nocturnal temperatures. Heating is greatest during the winter,
which partially explains why heat islands are most pronounced during that season.
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