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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London’s historical problems
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King Edward I forbid sea coal in 1273
London known for “pea-soup” (dangerous)
One “pea-soup” incident caused 700 deaths
Term “smog” created around 1911
Disastrous London smog event of 1952
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Winds died, smog settled over London
People could see where they were walking for 5 days
Killed 5,000 lives
Caused Parliament to pass Clean Air Act of 1956
A Brief History of Air Pollution
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Donora, Pennsylvania pollution episode,
1948
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Pollution trapped in Monongahela River Valley
by inversion (20 people died)
U.S. Clean air act, 1970, 1990
Due to high pollution, Congress passed Clear
Air Act (revised in 1977, 1990)
 Sets emission standards
 Incentives for companies to lower emission
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Principal Air Pollutants
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Primary pollutants
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Enter atmosphere directly
Secondary pollutants
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Enter atmosphere from chemical reactions
• Globally, a large percentage of air pollution sources are
natural. Within localized areas, however, humancaused sources are often the largest contributors.
Principal Air Pollutants
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Particulate matter – solid particles and liquid
droplets small enough to remain suspended
(aerosols).
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Not poisonous - soot, smoke, dust, pollen
More dangerous – asbestos, arsenic, oil, sulfuric acid
PM10 – Particulates with diameters < 10
micrometers
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Small enough to penetrate into lungs
Can remain in air for long time
Wind can carry great distances
Asian dust can transport dust long distances
Principal Air Pollutants
Principal Air Pollutants
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PM2.5 – Particulates with diameters < 2.5
micrometers
Penetrate farther into lungs
 Found in diesel soot and are carcinogenic
(school buses, traffic corridors)
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Removed by rain, snow, ice crystals
Principal Air Pollutants
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Carbon Monoxide – major pollutant in
cities. Colorless, odorless, poisonous gas
60 million metric tons enter U.S. air annually.
Half from highway vehicles.
 Extremely dangerous and can kill quickly in
poorly ventilated areas.
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Principal Air Pollutants
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Sulfur dioxide – comes from sulfurcontaining fossil fuels. Colorless. Comes
from power plants, heating devices, paper
mills, volcanoes
Can form into secondary pollutants (sulfur
trioxide, sulfuric acid
 Hurt breathing and certain plants
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Principal Air Pollutants
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Volatile organic compounds – organic
compounds mainly made of hydrogen and
carbon
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Methane, benzene, formaldehyde,
chlorofluorocarbans
Nitrogen oxides – formed when nitrogen
combines with oxygen during fuel combustion
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Nitrogen dioxide, nitric oxide (NOx)
Vehicles, power plants, garbage dumps
• Along with ozone, VOCs and NOx are major
components of photochemical smog.
Ozone in the Troposphere
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Photochemical smog – formed when chemical
reaction takes place with sunlight
Ozone – Dangerous at surface. Irritates eyes,
lungs. Can cause asthma and bronchitis
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Causes several billions of dollars in loss from crop
yield
• Photochemical smog is a problem on most major
cities of the world.
Ozone in the Troposphere
Ozone in the Stratosphere
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Relationship to
ultraviolet radiation
Ozone layer shields
surface from UV
radiation
 UV can cause skin
cancer
 Eye cataracts
 Hurt immune system
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Ozone in the Stratosphere
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Ozone broken down into
O2 and O when UV hits it
Chloroflucarbons (CFCs)
where used in hairsprays,
deodorants, propellants
until 1987 (Montreal
Protocol)
Stable at surface, enter
stratosphere from t-storms,
near jet streams
Ozone in the Stratosphere
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Sunlight breaks off the
Chlorine and it breaks
up ozone
CFCs are very longlasting
One CFC can destroy
100,000 Ozone
molecules
Ozone in the Stratosphere
Ozone in the Stratosphere
Air Pollution: Trends and
Patterns
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Primary air quality standards – standards
set up to protect human health
secondary air quality standards- standards
set up to protect visibility, crops, and
buildings
Nonattainment areas – areas that do no
meet air quality standards
• Secondary air pollutants are particularly difficult to
control, because they are not emitted directly into the
atmosphere.
Air Pollution: Trends and
Patterns
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80 millions Americans breathing air not
meeting one standard
Air Quality Index (AQI) – includes carbon
monoxide, sulfur dioxide, nitrogen dioxide,
particulate matter, ozone
Highest pollutant is one used in index
 Anything higher than 100 is unhealthy
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Fig. 12-11, p. 338
The Role of the Wind
The Role of the Wind
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Dilution – mixing air pollution in a greater
volume of air. Can create problems
downwind
• “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 inhibit vertical
movement. Keep
pollutants in a layer
Mixing depth – the
depth of the mixing layer
(the layer from the
surface to the top of the
inversion)
• The mixing layer can often be easily seen from an airplane.
The Role of Stability and
Inversions
The Role of Topography
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Cold air drainage – air drains downhill over
night, carrying pollutants
Air blockage by mountain ranges.
Mountains block the progress of flow.
Blocks pollutants from moving out of the
region
The Role of Topography
Severe Air Pollution Potential
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Many sources (clustered close together)
High pressure (becomes stationary, light
winds)
Inversions (inhibits vertical movement)
Stagnation (pollutants unable to disperse)
Valley (pollutants accumulate)
• 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 – cities are warmer than rural
areas
Country breeze – Return breeze in circulation
Acid Deposition
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Dry deposition – when
pollutants slowly settle to
ground
Wet deposition – when
pollutants are carried to
ground by rain, snow
Acid rain effects
• Strangely enough, an acidified lake looks crystal clear
and beautiful. But the beauty is false; it’s only clear
because nothing is alive in it.