Chapter 12: Air Pollution

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Transcript Chapter 12: Air Pollution

Chapter 12: Air Pollution
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
disastrous London smog event in December 1952
smog: smoke and fog;
5 days, nearly 4000 deaths;
Clean Air Act in 1956
Los Angeles: photochemical smog
forms in sunny weather and irritates the eyes
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.
Q1: What are the natural sources of air pollutants?
A: dust, volcano, forest fire, ocean waves, vegetation, …
Q2: What are the anthropogenic sources?
A: fixed sources (power plants, homes, …)
mobile sources (cars, ships, …)
Principal Air Pollutants
Primary pollutants: from direct emission (next slide)
secondary pollutants: from chemical reaction
particulate matter: a group of solid particles and liquid droplets
that are small enough to remain suspended in the air
PM10, PM2.5: <10 or 2.5 micrometer in diameter
PM2.5 is especially dangerous to health (e.g., lung)
• Globally, a large percentage of air pollution sources are
• Within localized areas, however, human-caused sources
are often the largest contributors.
Q3: Is CO2 a pollutant?
a) yes; b) no
Q4: What do air pollutants include?
a) gases; b) liquids; c) solids;
d) all of them
Over U.S.
Principal Air Pollutants
Carbon monoxide (CO): colorless, odorless, poisonous;
primarily from incomplete combustion of fuels (e.g., cars)
That is why we need to have CO detector at home!
Sulfur dioxide (SO2): colorless; primarily from the burning of
fossil fuels (such as coal and oil) and from volcano and ocean
Volatile organic compounds (VOCs): represent a class of
organic compounds that are mainly hydrocarbons – individual
organic compounds composed of hydrogen and carbon, such
as methane; primarily from industrial processes,
transportation, and vegetation
Nitrogen oxides: nitrogen dioxide (NO2) and nitric oxide
(NO), together called NOx; primarily from motor vehicles,
power plants, and waste disposal systems.
Dust: PM10 and PM2.5; from dust storm and agricultural and
industrial activities
Q5: What do automobiles emit?
A: CO, NOx, VOCs, particulate matter, …
Q6: Do automobiles emit O3?
a) yes; b) no
Q7: What do power plants (using coal) emit?
A: SOx, NOx, ash
Q8: What do vegetation emit?
A: VOCs, pollen
Q9: Can the dust from Asia travel across the Pacific to reach
a) yes; b) no
Ozone in the Troposphere
Photochemical smog: smog in the presence of sunlight
Ozone: unpleasant odor, irritates eyes and hurt human
health, reduce crop yield
(tropospheric) O3 forms as a secondary pollutant from a
complex series of chemical reactions involving NOx and
VOCs in the presence of sunlight.
Q10: does Tucson have
O3 problem?
a) yes, b) no
Q11: does Tucson have
particulate matter
a) yes, b) no
Ozone in the Stratosphere
Stratosphere O3 absorbs ultra-violet radiation to reduce human
skin cancer; its concentration is highest at ~25 km altitude
chlorine compounds such as chlorofluorocarbons (CFCs) are
widely used as refrigerants, propellants (e.g., in hairspray) and
these gases are safe in the
troposphere but destroy O3 in the
stratosphere: a single chlorine
removes as many as 100,000 ozone
Montreal Protocol in 1987
• When scientists first measured
extremely low ozone values in
the Antarctic stratosphere, they
thought the instruments were
Ozone hole in 2006
Q12: why are there still ozone holes when chlorine is decreasing?
A: there are still CFCs in the stratospphere;
mainly due to changes in polar stratospheric temperatures
Air Pollution: Trends and Patterns
Air Quality Index (AQI): includes
the pollutants CO, SO2, NO2,
particulate matter, and O3;
on any day the pollutant measuring
the highest value is the one used in
the index
• Secondary air pollutants (e.g., O3)
are particularly difficult to control,
because they are not emitted
directly into the atmosphere.
• Higher emission standards along with
cleaner fuels (such as natural gas)
have made the air over our large
cities cleaner than it was years ago
Q13: why was the lead emission
reduced to zero in 1980s?
a) federal regulation;
b) new technology;
c) voluntary industrial decision
Q14: Why did the emission peak
for CO, SO2, VOCs occur in early
Q15: What are the main air quality
concerns in Tucson?
a) PM2.5 and O3
b) CO and SO2
c) SO2 and O3
d) CO and PM2.5
Results based on AQI in 2003
Fig. 12-11, p. 338
Q16: Why is air in California not as good as rest of the U.S.?
A: higher anthropogenic emission; dust; weather pattern; topography
Factors affecting air pollution
Role of the wind: dilution by advection and turbulent 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
The Role of Stability and Inversions
Q17: if air temperature decreases with
height, what is the air stability?
a) stable, b) unstable, c) undecided
Q18: if air temperature increases with
height, what is the air stability?
a) stable, b) unstable, c) undecided
Inversions: temperature increases
with height (i.e., very stable)
 mixing layer: from surface to
inversion base where air is unstable
and hence well mixed
Q19: is the layer below inversion in the
figure a mixing layer?
a) yes, b) no
The Role of Topography
Nighttime drainage flow
air blockage by mountain ranges
Los Angeles
Severe Air Pollution Potential
Sources (clustered close together)
high pressure (for inversion and weak wind)
Stagnation (weak wind; 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
urban heat island
country breeze
Acid Deposition
pH: measure of the acidity or
basicity of a solution
Acid deposition: wet and dry
wet deposition: removal of
pollutants by precipitation
dry deposition: by gravity
acid rain or fog: SO2 and NOx
Q20: why is natural rain somewhat
acidic? Because of
a) CO2 dissolution
b) SO2 dissolution
c) NOx dissolution
Acid Rain
acid rain effects: damaging forest; eroding the foundations
of structures; polluting lakes
Control of acid rain is an international issue due to longdistance transport of pollutants
pH values in 2005