Chapter 12: Air
Pollution
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A brief history of air pollution
Types and sources of air pollutants
Factors that affect air pollution
Air pollution and the urban environment
Acid deposition
A Brief History of Air Pollution
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disastrous London smog event of 1952
smog: smoke and fog;
5 days, nearly 4000 deaths;
Clean Air Act in 1956
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Los Angeles: photochemical smog
forms in sunny weather and irritates the eyes
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U.S. Clean Air Act, 1970, 1990
set federal emission standards for states to implement
and enforce
Types and Sources of Air
Pollutants
Air pollutants are airborne substances (either solids,
liquids, or gases) that occur in concentrations high
enough to threaten the health of people and animals,
to harm vegetation and structures, or to toxify a given
environment.
They come from natural sources and human activities:
Natural: dust, volcano, forest fire, ocean waves, …
Human: fixed sources (power plants, homes, …)
mobile sources (cars, ships, …)
Principal Air Pollutants
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primary and secondary pollutants
particulate matter: a group of solid particles and liquid
droplets that are small enough to remain suspended in the air
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PM10, PM2.5: <10 or 2.5 micrometer in diameter
Carbon monoxide: colorless, odorless, poisonous
• Globally, a large percentage of air pollution sources are
natural. Within localized areas, however, humancaused sources are often the largest contributors.
Over U.S.
Principal Air Pollutants
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Volatile organic compounds (VOCs): represent a
class of organic compounds that are mainly hydrocarbons –
individual organic compounds composed of hydrogen and carbon
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nitrogen oxides: nitrogen dioxide (NO2) and nitric
oxide (NO), together called NOx
• Along with ozone, VOCs and NOx are major
components of photochemical smog.
• Photochemical smog is a problem on most
major cities of the world.
Ozone in the Troposphere
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Photochemical smog: in the presence of sunlight
Ozone: unpleasant odor, irritates eyes and hurt human health,
reduce crop yield
Ozone in the Stratosphere
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relationship to ultraviolet radiation
chlorine compounds
chlorofluorocarbons (CFCs); a single
chlorine removes as many as 100,000
ozone molecules
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Montreal Protocol
• When scientists first measured
extremely low ozone values in the
Antarctic stratosphere, they
thought the instruments were
malfunctioning.
Ozone hole in 2006;
mainly due to changes in
polar stratospheric
temperatures
Figure 1, p. 337
Air Pollution: Trends and
Patterns
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Air Quality Index (AQI):
includes the pollutants CO, SO2, NO2,
particulate matter, and O3
• Secondary air pollutants (e.g., O3)
are particularly difficult to control,
because they are not emitted
directly into the atmosphere.
Fig. 12-11, p. 338
Factors affecting air pollution
The role of the wind
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dilution
turbulence
mixing
• “Dilution is the solution to pollution” - in the 1950s this motto
led to the construction of tall smokestacks for large factories.
Pollution was released higher in the atmosphere where winds
were stronger. Air quality improved locally but suffered
downwind.
The Role of Stability and
Inversions
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temperature lapse rates
inversions
mixing depth
• The mixing layer can often be
easily seen from an airplane.
The Role of Topography
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cold air drainage
air blockage by mountain ranges
Severe Air Pollution Potential
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Sources (clustered close together)
high pressure (for inversion and weak wind)
Inversions
Stagnation (unable to disperse pollutants)
A valley (for accumulation of pollutants)
• Some locations, like Los Angeles and Mexico City, have an
unfortunate combination of surrounding topography, frequent
inversions, abundant emissions and plentiful sunlight perfect conditions for photochemical smog.
Air Pollution and the Urban
Environment
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urban heat island
country breeze
Acid Deposition
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pH
wet deposition
dry deposition
acid fog: SO2 and NOx
acid rain effects
Precipitation pH values