The Urban Heat Island and Air Pollution

1
The Urban Heat Island and Air Pollution
2
The Effect of Cities on Air Temperature

• When a city is built, changes are made to the
  natural environment that affect the temperature.
• The changes affect energy transfers and the
  storage of energy.

3
The Effect of Building Materials on the Albedo

• (1) Building materials generally have a lower
  albedo than soil and vegetation.
• The result is that buildings, streets,
• parking lots, etc. absorb more solar
• radiation than soil and vegetation. The
• increased absorption of solar radiation
• makes the city warmer than the
• surrounding rural area on sunny days.

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higher albedo reflects more solar radiation -
cooler
lower albedo absorbs most solar radiation city
has higher temperatures
5
The Effect of Building Materials on Storage of
Internal Energy

• (2) Building materials generally have higher heat
  capacities than soil and vegetation.
• This means that buildings, streets and
• parking lots tend to retain internal energy
• longer and stay warmer than
• surrounding rural areas especially at
• night.

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vegetation has lower heat capacity and cannot
store as much internal energy
building materials have higher heat capacities
and store more internal energy
temperature decreases more quickly in rural areas
cooler at night
temperature inside a city stays higher at night
7
Generation of Internal Energy By Human Activity

• (3) Many human activities generate internal
  energy as a waste byproduct (e.g. power plants,
  industrial processes, energy loss from
  buildings).
• The much higher population densities in cities
  mean that these processes generate much more
  energy than in surrounding rural areas.

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Internal energy generated as a result of human
activity
very little internal energy generated as a result
of human activity
lower temperatures
higher temperatures
9
Reduced Evaporation

• (4) Storm water drainage systems in cities are
  designed to remove rainfall as quickly as
  possible. As a result the surface in a city is
  usually much drier than in the surrounding rural
  areas.
• There is normally much less evaporation in the
  city than in surrounding rural areas.

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Reduced Evaporation (Cont.)

• The reduced evaporation in the city means that
  the relative humidity is lower than it is in
  surrounding rural areas.
• Since water absorbs energy when it evaporates,
  the reduced evaporation in the city also
  contributes to higher temperatures than in the
  surrounding rural areas.

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much more water evaporates in the surrounding
rural areas
much less evaporation occurs over a city
the relative humidity is higher and the
temperature is lower
the relative humidity tends to be lower over a
city and the temperature higher
12
Increased Friction

• (5) Buildings are generally taller than most
  natural vegetation and they obstruct the airflow
  and create more friction.
• The result is to slow the air and produce lower
  average wind speeds.
• Note Channeling between buildings may produce
  higher wind speeds in some locations.

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taller buildings make the surface rougher and
generate more friction
shorter vegetation produces less friction
average wind speeds are higher
average wind speeds are lower over a city
14
Urban Heat Island

• The combined effects of these processes is to
  produce warmer temperatures over the city.
• Temperatures tend to be highest near the center
  of the city and to decrease as you move away from
  the center of the city.

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Urban Heat Island (Cont.)

• The temperature may be as much as 8-12F higher
  at the center of a large city than over the
  surrounding rural area.
• The warmer region centered on the city is
  sometimes called the urban heat island.
• The biggest temperatures differences tend to
  occur during calm, clear winter nights.

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the warmest temperatures occur near the center of
the city
temperatures are slightly higher near the edge of
the city
temperatures are cooler over the surrounding
rural area
Urban Heat Island
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Summary of Urban-Rural Differences

• Temperatures are higher in cities.
• Relative humidities are lower in cities.
• Average wind speeds are lower in cities.

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Air Pollution

• Other byproducts of human activities may include
  undesirable gases, liquids and solids (lumped
  together as air pollutants).

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Primary Sources of Air Pollution in the U.S.

• Transportation 46
• Stationary fuel combustion 29
• Industrial processes 16
• Solid waste disposal 2
• All other sources 7

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Principal Air Pollutants in the U.S.

• Carbon monoxide (CO) 48
• Sulfur oxides (SOx) 16
• Nitrogen oxides (NOx) 16
• Volatile organic compounds 15
• Particulates 5

21
Carbon Monoxide

• Carbon monoxide (CO) is an odorless, colorless
  gas that forms during incomplete combustion of
  carbon-based fuels.
• Roughly half of the carbon monoxide is produced
  by vehicles.

22
Carbon Monoxide (Cont.)

• Outdoors carbon monoxide is quickly removed by
  microorganisms in the soil.
• It can build up to dangerous levels in poorly
  ventilated structures, tunnels, etc.

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Carbon Monoxide (Cont.)

• The main danger to humans is that carbon monoxide
  can combine with hemoglobin in the blood and
  prevent the blood from carrying sufficient oxygen
  to the brain.

24
Sulfur Oxides

• Sulfur oxides (SOx) are produced when fossil
  fuels (coal, petroleum, natural gas) that contain
  sulfur are burned.
• Sulfur dioxide (SO2) is the most common sulfur
  oxide.

25
Sulfur Oxides (Cont.)

• Primary sources of sulfur oxides include power
  plants, furnaces, smelters, petroleum refineries
  and paper mills.
• Volcanic eruptions also inject sulfur oxides into
  the atmosphere.

26
Sulfur Oxides (Cont.)

• Sulfur dioxide may combine with water in the air
  to produce sulfuric acid and contribute to the
  acidity of rainfall and fogs.
• Sulfur dioxide tends to aggravate respiratory
  problems in humans including asthma, bronchitis
  and emphysema.

27
Nitrogen Oxides

• Nitrogen oxides (NOx) are molecules that include
  nitrogen and oxygen.
• Nitrogen oxides form when nitrogen in the air
  combines with oxygen during combustion.

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Nitrogen Oxides (Cont.)

• The primary sources of nitrogen oxides are
  vehicles, power plants and waste disposal
  systems.
• Nitrogen oxides may combine with water in the
  atmosphere to produce acidic rainfall or fog.

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Nitrogen Oxides (Cont.)

• High concentration of nitrogen oxides may also
  aggravate respiratory problems in humans.

30
Smog

• In the presence of ultraviolet radiation the
  oxygen in nitrogen oxides may combine to produce
  ozone (O3) which is a major component of
  photochemical smog that forms in places like Los
  Angeles.

31
Volatile Organic Compounds

• Volatile organic compounds are molecules that
  contain hydrogen and carbon and are also know as
  hydrocarbons.
• Many of these compounds are naturally found in
  the atmosphere (e.g. methane CH4.

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Volatile Organic Compounds

• Primary sources of volatile organic compounds
  include solid waste disposal and industrial
  processes.
• Laboratory tests show that sufficiently high
  concentrations are statistically linked to higher
  rates of certain cancers in test animals.

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Particulates

• Particulates are solid particles and liquid
  droplets (i.e. aerosols) that are small enough to
  remain suspended in the air.
• Primary sources are combustion and industrial
  processes.

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Particulates (Cont.)

• Particulates may eventually settle out of the
  atmosphere due to gravity (sometimes called dry
  deposition) or they may be washed out of the air
  by precipitation.
• Particulates may aggravate respiratory problems
  or be linked to other health issues depending on
  their chemical composition.