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Chapter 12
Air Pollution
Summary
Air pollutants and air pollution meteorology are covered in detail in this chapter. Many
concepts introduced earlier in the text, such as atmospheric stability and small scale wind
circulation patterns, are relevant to this topic and are integrated into the discussion.
The chapter begins with a brief historical review of air pollution. Many students will be
surprised, perhaps, to see that concern over air pollution dates back to the 13th century.
Students may also be unaware of the very serious air pollution events that occurred earlier in
this century in Europe and the United States. The London smog of 1952, for example, remains
today the world's worst air pollution disaster.
Sources and environmental effects of the primary air pollutants, carbon monoxide, sulfur
dioxide, nitrogen oxides, particulate matter, and volatile organic compounds are discussed next.
Tropospheric ozone, a pollutant and component of photochemical smog, is discussed together with
stratospheric ozone in this chapter. The pollutant standards index provides a quantitative, readily
understood measure of air quality. We see that while air quality has improved following the
passage of clean air legislation, many urban areas still frequently exceed these standards. Weather
conditions, local topography, and urban environment can all influence the buildup of air pollutants
in particular regions. The chapter concludes with a discussion of acid deposition.
Teaching Suggestions, Demonstrations and Visual Aids
1.
Heat a small piece of coal using a propane torch in class. The burning coal will often
produce a lot of black smoke and a pungent, sulfurous odor. Students will get a better appreciation
of how unhealthy conditions must have been during the London smog episodes.
2.
Effective classroom demonstrations of some of the common reactions in air pollution
chemistry have been given by J. L. Hollenberg et al. (ref: "Demonstrating the Chemistry of Air
Pollution," J. Chem. Educ., 64, 893-894, 1987).
3.
It is a relatively easy matter to dissolve carbon dioxide gas in water and show, using an
acid/base indicator solution, that this turns the water acidic. Bromothymol blue is an indicator that
turns yellow in an acid, blue in a base, and green in a neutral solution.
Fill three beakers with distilled water and add a sufficient quantity of bromothymol blue to each to
turn the color of the water a deep green. Add white vinegar to one beaker and ammonia to another.
The water in these beakers will turn yellow and a blue, respectively.
Fill a fourth beaker with distilled water and add a small amount of baking soda to make the water
slightly alkaline (in some parts of the country, tap water is already alkaline and can be used). Add
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 12: Air Pollution
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some bromothymol blue indicator to the water. Next add a few small pieces of dry ice to a flask
with a side spout. Stopper the flask and connect a small piece of flexible tubing to the side arm of
the flask. Immerse the other end of the tubing in the beaker containing the water. As the dry ice
sublimates, the CO2 gas will bubble through the water and slowly make it acidic. The changing
color of the indicator solution will show that this conversion is taking place.
Student Projects
1.
A daily air quality summary is given in many local papers. Have students report on
conditions in their location. What emissions are of most concern? What are the primary sources of
these emissions? How do the reported values correlate with observed atmospheric conditions,
visibility, and synoptic weather conditions?
2.
Have students investigate and report on measures being taken locally to reduce air
pollution. This might include carpooling, subsidized bus passes or other use of mass transit, use
of oxygenated fuels during certain times of the year, vehicle emissions inspections, and home fire
place no-burn nights.
3.
Use the Smog section of the ThomsonNow web site to answer the following
question. Is it possible to have low values of ozone (< 20 ppb) with high concentrations
(> 4 ppb) of NOx? Explain.
Answers to Questions for Review
1.
Natural sources: windblown dust and soot, volcanoes, forest fires. Human-induced
sources: industrial complexes, power plants, homes, office buildings, motor vehicles, ships,
aircraft. Secondary sources: chemical reactions in the atmosphere.
2.
Certain pollutants are called primary air pollutants because they enter the atmosphere
directly—from smokestacks and tail pipes, for example. Other pollutants, known as secondary air
pollutants, form only when a chemical reaction occurs between a primary pollutant and another
component of air, such as water vapor or another pollutant.
3.
Soot, dust, smoke, pollen, asbestos fibers, arsenic, sulfuric acid droplets, PCBs, oil,
pesticides.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 12: Air Pollution
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4.
Finer particles with diameters smaller than 10 µm are referred to as PM-10. Particles with
diameters smaller than 5 µm are referred to as PM-2.5. PM-2.5 pose the greater health risk because
they can penetrate further into the lung when inhaled, and because they frequently consist of toxic
or carcinogenic compounds.
5.
Washout by rain and snow; incorporation into cloud or fog droplets.
6.
a. Incomplete combustion of carbon-containing fuels. It starves blood hemoglobin of
oxygen. b. Burning of sulfur-containing fossil fuels. When inhaled into the lungs, high
concentrations of sulfur dioxide aggravate respiratory problems, such as asthma, bronchitis, and
emphysema. c. Industrial processes and vehicles. Certain VOCs, such as benzene (an industrial
solvent) and benzo-a-pyrene (a product of burning wood, tobacco and barbecuing), are known to be
carcinogens—cancer-causing agents. Although many VOCs are not intrinsically harmful, some
will react with nitrogen oxides in the presence of sunlight to produce secondary pollutants, which
are harmful to human health. d. High-temperature combustion of fuel from motor vehicles, power
plants and waste disposal systems. High concentrations are believed to contribute to heart and lung
problems, as well as lowering the body’s resistance to respiratory infections.
7.
London smog: mixture of sulfurous smoke and fog. Los Angeles or photochemical smog:
forms when chemical reactions take place in the presence of sunlight.
8.
The main component of photochemical smog is the gas ozone (O3). Ozone is a noxious
substance with an unpleasant odor that irritates eyes and the mucous membranes of the respiratory
system, aggravating chronic diseases, such as asthma and bronchitis.
9.
Because sunlight is required to produce ozone, concentrations of tropospheric ozone are
normally higher during the afternoons and during the summer months, when sunlight is more
intense.
10.
Stratospheric ozone absorbs harmful ultraviolet radiation from the sun, preventing it from
causing cancer in living cells at the earth's surface. Tropospheric ozone, when inhaled, causes
respiratory problems.
11.
If ozone were removed from the atmosphere, the amount of ultraviolet radiation reaching
the earth's surface would reach dangerous levels, causing widespread cases of skin cancer.
12.
The Clean Air Act of 1970 empowered the federal government to set emission standards
that each state was required to enforce. The Clean Air Act was revised in 1977 and updated by
Congress in 1990 to include even stricter emission requirements for autos and industry. The new
version of the Act also includes incentives to encourage companies to lower emissions of those
pollutants contributing to the current problem of acid rain. Moreover, amendments to the Act have
identified 189 toxic air pollutants for regulation. In 2001, the United States Supreme Court, in a
unanimous ruling, made it clear that cost need not be taken into account when setting clean air
standards.
13.
a. A pollutant is considered unhealthful when its AQI value exceeds 100. b. Very
unhealthy. c. Significant aggravation of symptoms and decreased exercise tolerance in persons
with heart or lung disease, with widespread symptoms in the healthy population. Active children
and adults with existing heart or lung disease should avoid outdoor activities and exertion.
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Instructor’s Manual
Chapter 12: Air Pollution
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Everyone else, especially children, should limit outdoor exertion.
14.
The wind speed determines how quickly the pollutants mix with the surrounding air and, of
course, how fast they move away from their source.
15.
Atmospheric stability controls the extent to which air mixes and becomes diluted. Stable
conditions are not conducive to mixing and result in high concentrations; unstable conditions dilute
the air and reduce concentrations.
16.
An inversion is an example of extreme atmospheric stability, resulting in little or no mixing
and leading to high pollution concentrations.
17.
An inversion represents an extremely stable atmosphere where warm air lies above cool
air. Mixing of air is drastically reduced, allowing pollution concentrations to increase to high
levels.
18.
Shorter smokestacks are likely to have weaker winds, are less likely to rise above an
inversion layer, and are capable of mixing air through a smaller volume of air as compared to taller
smokestacks.
19.
Since the atmosphere tends to be most unstable in the afternoon and most stable in the
early morning, we typically find the greatest mixing depth in the afternoon and the most shallow
one (if one exists at all) in the early morning.
20.
During the afternoon, because the atmosphere is likely to be unstable then, providing
favorable conditions for mixing and dilution.
21.
These areas are likely to provide limited mixing (slow moving air), and subsidence
inversions accompanying high pressure systems provide extremely stable conditions.
22.
Los Angeles and Denver: the city of Los Angeles is surrounded on three sides by hills and
mountains. Cool marine air from off the ocean moves inland and pushes against the hills, which
tend to block the air’s eastward progress. Unable to rise, the cool air settles in the basin, trapping
pollutants from industry and millions of autos. Baked by sunlight, the pollutants become the
infamous photochemical smog. By the same token, the “mile high” city of Denver, Colorado, sits
in a broad shallow basin that frequently traps both cold air and pollutants. Mountainous terrain: at
night, cold air tends to drain downhill, where it settles into low-lying basins and valleys. The cold
air can have several effects: It can strengthen a preexisting surface inversion, and it can carry
pollutants downhill from the surrounding hillsides.
23.
Many sources of air pollution, a deep high-pressure area that becomes stationary over a
region, light surface winds that are unable to disperse the pollutants, a strong subsidence inversion
produced by the sinking of air aloft, a shallow mixing layer with poor ventilation, a valley where
the pollutants can accumulate, clear skies so that radiational cooling at night will produce a surface
inversion, which can cause an even greater buildup of pollutants near the ground, for
photochemical smog, adequate sunlight to produce secondary pollutants, such as ozone.
24.
In cities, where less vegetation and exposed soil exists, the majority of the sun’s energy is
absorbed by urban structures and asphalt. Hence, during warm daylight hours, less evaporative
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 12: Air Pollution
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cooling in cities allows surface temperatures to rise higher than in rural areas. At night, the solar
energy (stored as vast quantities of heat in city buildings and roads) is slowly released into the city
air. Additional city heat is given off at night (and during the day) by vehicles and factories, as well
as by industrial and domestic heating and cooling units. The release of heat energy is retarded by
the tall vertical city walls that do not allow infrared radiation to escape as readily as do the
relatively level surfaces of the surrounding countryside. The slow release of heat tends to keep
nighttime city temperatures higher than those of the faster cooling rural areas. Overall, the heat
island is strongest at night.
25.
A breeze blowing from the countryside into the city. Thermal gradients caused by the
urban heat island are stronger at night than during the day, so the breeze is stronger at night. It is
more easily developed in the summer, when more solar radiation is available.
26.
Air pollution emitted from industrial areas, especially products of combustion, such as
oxides of sulfur and nitrogen, can be carried many kilometers downwind. Either these particles and
gases slowly settle to the ground in dry form (dry deposition) or they are removed from the air
during the formation of cloud particles and then carried to the ground in rain and snow (wet
deposition).
27.
High concentrations of acid deposition can damage plants and water resources. Emissions
of sulfur dioxide (SO2) and oxides of nitrogen may settle on the local landscape, where they
transform into acids as they interact with water, especially during the formation of dew or frost.
The remaining airborne particles may transform into tiny dilute drops of sulfuric acid (H 2SO4) and
nitric acid (HNO3) during a complex series of chemical reactions involving sunlight, water vapor,
and other gases.
Answers to Questions for Thought and Exploration
1.
No, you'd expected a looping plume.
2.
Nighttime inversions would prevent mixing, allowing pollution to become more
concentrated. Also, in the absence of convection forced by incoming solar radiation, the polluted
air found near the ground will not warm and rise to higher altitudes.
3.
Stratospheric ozone absorbs dangerous ultraviolet radiation from the sun, but because of its
high altitude, you don't have to breathe it! Ozone is damaging to the respiratory system.
4.
The taller smokestack will likely reduce air quality problems in its immediate vicinity.
However, the taller stack will create new air quality problems for areas downwind.
5.
People generally wear clothing when walking in the rain, so most of their skin is protected.
Also, most people bathe frequently, thus washing the acidic rain off their bodies.
6.
Acid fog. The droplets are smaller so the acid will be more concentrated.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 12: Air Pollution
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