The URBAN AND INDUSTRIAL ENVIRONMENT: The Atmosphere

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The URBAN AND INDUSTRIAL ENVIRONMENT The
Atmosphere

• How do cities affect local and regional weather?
• What is the heat island effect?
• Which are the main atmospheric pollutants
• How are photochemical smog and acid rain
  produced?
• How does weather affect pollutants?

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Wavelength
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Absorption and Scattering of solar radiation

• Solar radiation is attenuated as it passes
  through the earths atmosphere. The atmosphere
  contains particles and gases, which affect the
  incoming radiation through the mechanisms of
  scattering and absorption. 
• Scattering causes changes in direction and
  intensity of radiation.
• Absorption causes molecules in the atmosphere to
  absorb energy at various wavelengths.  The net
  result of absorption is that energy is attenuated
  or lost.  Ozone, carbon dioxide, and water vapor
  are the three main atmospheric constituents that
  absorb radiation. 

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Convection Mixes the Atmosphere and Helps Dilute
Pollutants
3. Water vapor in air condenses and forms
clouds.
3.
cold air
2. The air near the surface absorbs solar
radiation and radiation emitted by the surface.
Warm air from near the surface moves upward and
is replaced by cold air from above (convection).
hot air
2.
1.Solar radiation is reflected and absorbed. The
absorbed radiation heats the surface.
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"Urban Heat Islands."
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What causes this to happen?

• There are fewer trees, shrubs, and other plants
  to shade buildings, intercept solar radiation,
  and cool the air by "evapotranspiration."
• Buildings and pavement made of dark materials
  absorb the sun's rays instead of reflecting them
  away, causing the temperature of the surfaces and
  the air around them to rise.
• Surface roughness serves to reduce the wind speed
  and reduce ventilation.

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Evapotranspiration

• Evapotranspiration occurs when plants secrete or
  "transpire" water through pores in their leaves. 
• The water absorbs energy as it evaporates,
  cooling the air in the process.  
• A single mature, properly watered tree with a
  crown of 30 feet can "evapotranspire" up to 40
  gallons of water in a day, which is like removing
  all the heat produced in four hours by a small
  electric space heater.

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Heat Islands and Energy Use

• Higher temperatures in urban heat islands bring
  with them increased energy use, mostly due to a
  greater demand for air conditioning. As power
  plants burn more fossil fuels, they increase both
  the pollution level and energy costs.

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Urban heat islands and smog

• Urban heat islands are not only uncomfortably
  hot, they are also smoggier.
• Smog is created by photochemical reactions of
  pollutants in the air. These reactions are more
  likely to occur and intensify at higher
  temperatures.
• In Los Angeles, for example, for every degree
  Fahrenheit the temperature rises above 70F, the
  incidence of smog increases by 3.

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Cool Pavements Lower Temperatures
Dark materials absorb more heat from the sun.
Black surfaces in the sun can become up to 70F
(40C) hotter than the most reflective white
surfaces. If those dark surfaces are roofs, some
of the heat collected by the roof is transferred
inside.
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PRE-INDUSTRIAL TROPOSPHERE

• The chemical composition of the troposphere in
  pre-industrial times was the result of sunlight
  acting on natural chemicals emitted by the
  biosphere, from volcanoes, and biomass burning
  (forest fires).
•      biogenic hydrocarbons,
•      minor releases of nitrogen oxides from
  biogenic emissions and lighting
•      sulfur compounds and particulates from
  volcanoes and forest fires.  
• The most important biogenic hydrocarbon was
  methane, but isoprene and other biogenic
  hydrocarbons released by trees were also
  important.

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Atmospheric Pollution

• Many gases and aerosols that can be air
  pollutants are normal constituents of the
  atmosphere.
• These substances become pollutants when their
  concentration increase to levels that can be
  harmful to humans, animals or plants.
• Which are the main pollutants?
• What are their natural sources or sinks?
• What are the anthropogenic sources?

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Atmospheric Aerosols

•   Atmospheric aerosols are particles suspended in
  air, with very short lifetimes compared to
  greenhouse gases.
• Their diameters range from a few nanometers to
  ten micrometers.
• They are generated in two ways
• by direct emission to the atmosphere, for example
  from automobile exhaust and sea-spray (primary
  aerosols),
• by gas-to-particle conversion of chemical species
  in the atmosphere (secondary aerosols).

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Aerosol Sources

• Natural and Anthropogenic Sources
• Sulfates from volcanic eruptions,
• Black carbon (soot) and sulfates from combustion
• Mineral dust

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Aerosol Sources

• The aerosol loading in the atmosphere has
  increased significantly with human activity.
• Mineral dust is a major contributor to aerosol
  loading and optical thickness, especially in
  subtropical and tropical regions.
• Deserts, dry lake beds, semi-arid desert fringes,
  and drier regions where vegetation has been
  reduced or soil surfaces have been disturbed by
  human activities, are the main sources of dust in
  the lower atmosphere. It has been estimated that
  up to 50 of the current atmospheric dust load
  originates from disturbed soil surfaces

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Dust Devil
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Aerosols are of interest because

• Aerosols are the primary cause of visibility
  degradation in polluted areas.
• Aerosols can either absorb or scatter light. In
  this way they directly influence the Earths
  radiation balance and contribute to climate
  change.

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Aerosols are of interest because

• Aerosols are a major component of urban smog and
  several recent epidemiological studies have shown
  that aerosols in urban areas have a significant
  negative impact on human health.
• Aerosols control the formation of clouds.
  When the relative humidity exceeds 100 they are
  able to take up water, and grow to become
  droplets. The result is formation of clouds or
  fog. Aerosols with this ability are called Cloud
  Condensation Nuclei (CCN).

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Carbon Dioxide Contributes to Greehouse warming

• Natural sources and sinks
• Cellular respiration of organisms release carbon
  dioxide to the atmosphere, and through
  photosynthesis, plants take up carbon dioxide.
  Other natural sources are forest and brush fires
  and volcanic activity.
• Anthropogenic sources
• Combustion of fossil fuels (coals, oil, and
  natural gas) for electric power generation,
  transportation, and heating.

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Carbon Monoxide Asphyxiating agent that
constitutes a serious health hazard

• Natural sources and sinks
• It is produced by combination of oxygen with
  methane and other volatile organic compounds it
  is removed from the atmosphere by the activity of
  certain soil microorganisms and by chemical
  reactions that convert CO to CO2. 
• Anthropogenic sources
• Incomplete combustion of fossil fuels,
  especially by motor vehicles burning of forests
  and savannas to clear land. 

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Mercury rain

• As coal burns, it releases traces of mercury that
  come out of
• smokestacks.
• Much of the mercury stays airborne for up to two
  years and
• spreads around the globe.
• But some is emitted as a water soluble compound
  formed when
• mercury reacts with chlorine, an element often
  found in coal from
• eastern states.
• Precipitation quickly washes this form of mercury
  into lakes,
• rivers and oceans, where microorganisms take it
  up and convert,
• it into toxic methylmercury.
• The mercury passes up the food chain into fish
  and eventually into
• people.

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Mercury rain

• High levels can cause learning problems or
  retardation in children and neurological damage
  in developing fetuses.
• One recent study found fetus-harming levels of
  mercury in about 8 of U.S. women of childbearing
  age.

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Methane Greenhouse gas explosive at
concentrations of 5 or bigger. 

• Natural sources and sinks
• Methane is produced when organic material decays
  in the absence of oxygen (anaerobic decay), for
  example in rice paddies and marshes biological
  activity in termites and the stomachs of cattle
  and sheep non-reactive at normal background
  levels.
• Anthropogenic sources
• Landfills

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Volatile Organic Compounds Produce photochemical
smog carcinogens

• Natural sources and sinks
• All vegetation emits various hydrocarbons.
  Terpenes are reactive and responsible for the
  aromas of pine, eucalyptus and sandalwood trees. 
  
• Anthropogenic sources
• Incomplete combustion of gasoline by motor
  vehicles (hundreds of different hydrocarbons)
  escape during gasoline delivery and refueling
  (15 of the total release into the atmosphere)
  solvents used in industrial and commercial
  processes (painting and cleaning) chemical
  manufacturing and petroleum refining.

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Oxides of Nitrogen (NO and NO2)NO2 contributes
to heart, lung, liver, and kidney damage linked
to incidence of bronchitis and pneumonia reduces
visibility precursor of photochemical smog and
acid rain.

• Natural sources and sinks
• NO is produced by soil bacteria it combines
  readily with O2 to form NO2. 
• Anthropogenic sources
• Power plants and motor vehicles are the main
  source. Oxides of nitrogen form when high
  combustion temperatures, such as those inside an
  automobile engine cause nitrogen and oxygen in
  the air to combine oxidation of nitrogen
  compounds in coal and other fuels

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Compounds of Sulfur Acid rain and London smog

• Natural sources and sinks
• Sulfur dioxide (SO2) is produced by volcanic
  eruptions sulfate particles are injected into
  the atmosphere by sea spray and hydrogen sulfide
  (H2S) is produced in anaerobic decay. These
  sulfur compounds are removed from the atmosphere
  by precipitation and transfer to the soil,
  vegetation and surface water. Anthropogenic
  sources
• Fossil fuel (coal and oil) contain sulfur as an
  impurity and emit sulfur dioxide when burned.
  Certain industrial activities, such as paper and
  pulp processing emit hydrogen sulfide and other
  sulfur-containing gases.

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Photochemical Smog

• Photochemical smog forms when oxides of nitrogen
  in motor vehicle exhaust and hydrocarbons (from
  various anthropogenic and biogenic sources) react
  in the presence of sunlight to produce a mixture
  of aerosols and gases (ozone (O3), formaldehyde
  (CH2O), ketones, and PAN (peroxyacetyl nitrates).
• Average ozone level at the earths surface 0.02
  ppm
• Ozone concentration may exceed 0.5 ppm in thick
  photochemical smog.

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ACID RAIN

• Oxides of sulfur and nitrogen in the atmosphere
  interact with moisture to form droplets of
  sulfuric acid and nitric acid. These acids
  dissolve in precipitation and increase its
  acidity nearly 200 times

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pH of familiar items
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ACID RAIN
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Effects of Acid Rain

• Lowers the pH of lakes and streams, affecting the
  reproduction cycle of fish.
• Leach metals from the soil, washing them into
  lakes and streams where they may harm fish and
  aquatic plants.
• Responsible for the decline and dieback of
  coniferous forests.
• Accelerated weathering of building materials and
  metal corrosion.

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Atmospheric Factors that Affect the Concentration
of Pollutants

• Once pollutants enter the atmosphere their
  concentration decreases as they mix with clean
  air. The rate of dilution depends on atmospheric
  conditions. Wind speed and atmospheric stability
  are important factors that determine the rate of
  dilution.

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The Urban Wind

• When it is windy, turbulence is responsible for
  mixing polluted air with clean air, accelerating
  dilution.
• When the wind is not present, the much slower
  molecular diffusion processes control the rate of
  dilution.
• In the urban environment winds are affected by
  the surface, which produces friction and slows
  the wind down. Dilution of pollutants is thus
  hampered in urban localities. 

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The frictional interaction of winds with the
urban surface forms zones of light and irregular
winds that can trap pollutants.
(A) If a smokestack is too low, effluents may be
trapped within the wake of nearby buildings or
the chimney itself.  (B) If a smokestack is
constructed to the height of a good engineering
practice (2.5 times the height of the nearest
obstacle), effluents clear the wake, and downwash
and trapping are avoided.
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Convection Mixes the Atmosphere and Helps Dilute
Pollutants
3. Water vapor in air condenses and forms
clouds.
3.
cold air
2. The air near the surface absorbs solar
radiation and radiation emitted by the surface.
Warm air from near the surface moves upward and
is replaced by cold air from above (convection).
hot air
2.
1.Solar radiation is reflected and absorbed. The
absorbed radiation heats the surface.
1.
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Atmospheric Stability
Unstable Situation Mixing Occurs
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Atmospheric Stability
Very stable situation No mixing occurs
(km)
15-
height
-
5-
Temperature of air parcel
Atmospheric Temperature Temperature inversion
temperature
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Atmospheric Stability

• Stability affects vertical motion within the
  atmosphere.
• In the presence of stable air, convection and
  turbulence are inhibited, while they are enhanced
  if the air is unstable.
• Consequently, when pollutants are emitted into
  stable air, they are not transported upward, and
  remain in a stable layer of air that acts as a
  lid in the troposphere.

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Temperature Inversions
Temperature inversions



Stable Air



Traps pollutants


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Radiation Inversion

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• At night the surface cools by emission of
  infrared radiation, so that the coldest air is
  adjacent to the Earths surface and the air
  temperature increases with altitude.
•  
• In still air, this inversion generally persists
  until the surface is warmed again the next
  morning by absorption of sunlight. What happens
  if it is foggy?

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URBAN SMOG
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Subsidence (Overhead) Inversions

•  
•    Happens in places where there are mountains.
   
•    These inversions often come from the air
  being transported over the mountains and sinking
  into the valley.
•    As the air sinks, the atmosphere below is
  compressed by the overlying sinking air mass and
  it is warmed, producing a temperature inversion.
  

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Subsidence Inversion

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Important Similarities between London and Los
Angeles Types of Smog

• They are caused by combustion products
• The pollutants are concentrated near the ground
  due to temperature inversions 
• The primary and secondary pollutants are harmful
  to human, animals and plants.