Transcript Urbanization and the..

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?
Absorption and Scattering of
solar radiation
• Solar radiation is attenuated as it passes through the
earth’s 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.
Earth Radiation Budget
3. Molecules
absorb infrared
light – warms
atmosphere by
trapping radiation
4.Particles
scatter light – net
cooling effect;
absorb lightheating
Earth’s
surface
5. Clouds scatter and absorb
light – cool the atmosphere
during day and keep surface
warm at night.
1. Light that reaches the ground
(mostly visible) is absorbed and
reflected; absorbed light increases
ground temperature
2. Ground radiates in
the infrared, cooling the
surface
Convection Mixes the
Atmosphere and Helps Dilute
Pollutants
3. Water vapor in air
condenses and forms
clouds.
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).
1.Solar radiation is reflected
and absorbed. The absorbed
radiation heats the surface.
cold air
hot air
"Urban Heat Islands."
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.
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.
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.
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 70°F, the
incidence of smog increases by 3%.
Cool Pavements Lower
Temperatures
Dark materials absorb more heat from the sun. Black surfaces in the sun can
become up to 70°F (40°C) hotter than the most reflective white surfaces. If
those dark surfaces are roofs, some of the heat collected by the roof is
transferred inside.
PRE-INDUSTRIAL TROPOSPHERE
The chemical composition of the troposphere in preindustrial 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.
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?
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).
Aerosol Sources
• Natural and Anthropogenic Sources:
– Sulfates from volcanic eruptions,
– Black carbon (soot) and sulfates from
combustion
– Mineral dust
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
Dust Devil
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
Earth’s radiation balance and contribute to
climate change.
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).
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.
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.
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.
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.
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
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.
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
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.
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 earth’s surface: 0.02 ppm
• Ozone concentration may exceed 0.5 ppm in thick
photochemical smog.
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
pH of familiar items
pH
7.8 to 8.4
6.3 to 6.6
5.6 to 6.0
5.2 to 5.4
4.0 to 5.0
3.2 to 4.0
2.8 to 3.8
2.4 to 3.4
3.0 to 3.3
1.8 to 2.0
1. to 3.
2. to 4.
Item
Sea water
Cow’s milk
Potatoes
Cabbage
Beer
Cherries
Wine
Vinegar
Grapefruit
Limes
Human gastric juices
Soft drinks
ACID RAIN
Precipitation pH
pH  5.6
5.0  pH  5.6
pH < 5.0
Comments
Virtually no impact
from pollution
Within the range of
natural variation;
little influence prom
pollution
Reasonable to
assume that
pollution has
lowered the pH.
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.
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.
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.
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.
Convection Mixes the
Atmosphere and Helps Dilute
Pollutants
3. Water vapor in air
condenses and forms
clouds.
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).
1.Solar radiation is reflected
and absorbed. The absorbed
radiation heats the surface.
cold air
hot air
Atmospheric Stability
(km)
Unstable Situation
Mixing Occurs
-
height
15
Temperature of air parcel
5
-
Atmospheric Temperature
temperature
Atmospheric Stability
(km)
Very stable situation
No mixing occurs
-
height
15
-
5
-
Temperature of air parcel
temperature
Atmospheric Temperature:
Temperature inversion
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.
Temperature Inversions
Temperature inversions
Stable Air
Traps pollutants
Radiation Inversion
• At night the surface cools by emission of
infrared radiation, so that the coldest air is
adjacent to the Earth’s 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?
URBAN SMOG
Characteristics
First recognized
Source
Primary pollutants
Secondary pollutants
Temperature
Relative humidity
Light levels
Time of peak pollution
Type of temperature
inversion
London Smog
(Sulfurous)
Centuries ago
Burning high-sulfur coal
SO2 (sulfur dioxide),
sooty particles
H2SO4, sulfates, aerosols,
… 
Cool ( 2o C [35o F])
High; usually foggy
Dark, foggy
Early morning
Radiation
Los Angeles Smog
(Photochemical)
Mid- 1940’s
Burning oil in power plants and gasoline in
automobiles
Organic Molecules, NOx
O3, PAN (peroxyacetyl nitrate), aldehydes,
nitrates,
sulfates, particulates,…

Warm ( 24oC[75oF])
Low; usually hot and dry
Bright sunlight
Afternoon
Subsidence
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.
Subsidence Inversion
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.