MCTP
Maryland Collaborative
for Teacher Preparation
The Air We Breathe:
Is Dilution the Solution to Pollution?
Thomas C. O'Haver
Department of Chemistry and Biochemistry
University of Maryland
College Park, MD 20742
(301) 4051831
to2@umail.umd.edu
NSF Cooperative Agreement No. DUE 9255745
Topic
The atmosphere; air quality and pollution.
Primary Expected Outcomes
The student should be able to identify the general regions of the
atmosphere with respect to altitude and the relationship of air
pressure to altitude; recognize the composition of air and reasons
for local and regional variations in air composition; understand
factors behind air quality and the chief components of air
pollution; interpret air quality data in terms of concentrations
units and pollution levels, including the unreasonableness of
totally "pollution-free" levels; use scientific notation and
significant figures in performing basic calculations; differentiate
among elements, compounds, and mixtures; understand the differences
between atoms and molecules, between symbols for elements and
formulas for chemical compounds; write and interpret simple
chemical formulas and balanced equations; name selected chemical
elements and compounds; describe the involvement of automobile
emissions with photochemical smog and other forms of air pollution;
evaluate conditions significant in risk/benefit analysis; describe
the nature of air quality policies in this country and abroad in
terms of effectiveness of air pollution control.
Scientific and/or Math Concepts
AAAS benchmarks: Physical Setting: 4B The Earth; 4D The Structure
of Matter; Habits of Mind: 12B Estimation and Computation; 12C
Manipulation and Observation.
Disciplines integrated
Earth science, chemistry, mathematics.
Prerequisite knowledge
Basic arithmetic with decimal numbers; exponents; percents;
fractions; area and volume.
Student's Preconceptions
From the AAAS Benchmarks: Students of all ages ... lack an
appreciation of the very small size of particles.
Multicultural aspects
Air quality is an important aspect of global interdependence (AAAS
Benchmarks: Human Society 7G) that couples all nations large and
small and has been a source of considerable social conflict between
the industrial nations and developing nations.
Activities and Experiences
1. Student share personal experiences of the change of air pressure
with altitude (e.g. popping ears, air travel, boiling eggs in
Denver, etc). Discussion: Why does atmospheric pressure decrease
with altitude?
2. Student share personal experiences of variation in local air
quality (visibility; lung irritation, etc.). What are the normal
components of air and what is components are pollutants?
3. Review of some basic concepts: elements, compounds, simple
chemical formulas, combustion reactions, conservation of mass.
4. What creates most of the air pollution? Students who drive cars
are asked to bring in their last auto emissions test ticket.
Results are collected, interpreted, compared to the current
National Ambient Air Quality Standards. The units of measurement
used on the ticket (e.g. ppm) are discussed and related to percent
and to molecules per liter.
5. Is air pollution getting better or worse? Class looks at and
interprets Environmental Protection Agency data on major air
pollutants for selected cities in the U.S. and changes in average
concentrations over the last two decades. Discuss trends.
6. How big is the atmosphere? Class looks at some typical textbook
drawings of the earth and its atmosphere. Using data in the book,
students attempt to draw a scale model of the earth and the
atmosphere on a plain piece of paper, representing the earth as a
large circle and the atmosphere as an enclosing concentric circle.
(Students discover that this is impossible to draw, since most of
the atmosphere lies in a layer that is 1/1000th the radius of the
earth. The typical textbook drawings are discovered to be wrong -
not to scale).
7. Challenge question: Is dilution the solution to pollution? If
I took one liter of a toxic but otherwise stable substance and
released it into the outside air and waited until it is completely
mixed with the entire atmosphere of the earth, what would be the
concentration of the substance expressed in ppm by volume? How
does that compare to the ambient air quality standards? Map out a
strategy for estimation. How could we estimate the total volume of
the atmosphere in liters? What assumptions are reasonable (earth
is a sphere; atmosphere is a spherical shell). Look up equation
for volume of a sphere. Decide on a way to estimate the average
thickness of the atmosphere based on a plot of air pressure vs
altitude. Perform calculation and units conversion using a
calculator or spreadsheet. Help students with exponential notation
as needed. How many molecules of that substance would I breathe
in each breath of air for the rest of my life? How can the
concentration expressed in ppm seem so low but the number of
molecules in a breath seem so large? Do they not express the same
concentration?
Student handouts
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Chemistry 121/122 Name________________________________
Chapter 1: The Air We Breathe
This is not a quiz, but rather a class exercise. The papers will
be collected and graded. You may talk to your classmates, but you
must write your own person answers to each question.
1. List the air pollutants that are discussed in Chapter 1 (give
the names or chemical formulas).
2. Of the air pollutants listed on page 8 of the textbook, which do
you think is the most hazardous?
3. a. On the basis of the data on page 15, would you say that air
pollution has been getting better or worse since 1975?
b. Which air pollutant has had the largest change since
1975?
c. What do you think is the largest source of this
pollutant?
d. Why do you think this pollutant has decreased so much
since 1975?
e. Why were lead compounds (e.g. tetraethyl lead) added to
most gasoline that was sold before 1975?
f. Why do you think that the gas companies were able to "get
away" with adding lead - a substance that has been known to be
toxic for many years? In other words, why was there not a public
outcry from the very beginning?
4. a. Some copies of recent Maryland Vehicle Emission Test Reports
are being circulated around the class. Look at these and list the
pollutants that are measured here (HC = hydrocarbons).
b. Convert the test reading for carbon monoxide (CO) on the
test ticket from percent (PCT = percent) to PPM (parts per
million).
c. How does the test reading for CO compare to the permissible
limit for CO listed on page 8 of the textbook?
d. How can the car pass the test when the CO emission is so much
greater than the permissible limit?
5. As you know, normal (clean) air is composed of mostly nitrogen
and oxygen. One of the most toxic air pollutants (Table 1.2) is
nitrogen oxide, which from the name is also composed of nitrogen
and oxygen. What is the difference between nitrogen oxide and the
nitrogen and oxygen in normal air that would account for the
toxicity of nitrogen oxide?
6. Look at the textbook drawings of the earth and its atmosphere on
pages 54 and 64 of the textbook. Using the data in the textbook,
draw a scale model of the earth and the atmosphere on the back of
this paper, representing the earth as a large circle and the
atmosphere as an enclosing concentric circle. Rulers and compasses
are available for your use. Based only your experience, what can
you say about the accuracy of the scale of the drawings on pages 54
and 64?
7. Consider the following hypothetical experiment. Suppose you
were to take one liter (12 liter = 1000 cubic centimeters) of a
toxic but otherwise stable gas and release it into the outside air
and waited until it is completely mixed with the entire atmosphere
of the earth, what would be the concentration of the substance
expressed in ppm by volume?
a.Without actually performing this calculation, what other piece of
information would you have to know to obtain the solution of this
problem?
b. If you were to look up the formula for the volume of a sphere,
and supposing that you had a number for the "thickness" of the
atmosphere, how could you estimate the volume of the atmosphere?
c. How could you estimate the "thickness" of the atmosphere? Does
the atmosphere have a sharp upper boundary? Based on Figure 1.2
(page 6), what value for the thickness of the atmosphere would you
personally use for your calculation? Why?
d. What would you guess is the probability that one molecule of
that toxic substance substance, after it is completely mixed with
the entire atmosphere of the earth, would be found in one breath of
air?
e. The accepted value for the number of molecules in one liter of
air at atmospheric pressure is given by the book (page 21) as 2 X
1022 molecules. If you accept this number, and assuming that you
were able to calculate the volume of the entire atmosphere in
liters, how could you actually calculate the number of molecules of
your hypothetical toxic substance in one liter of air? You needn't
actually do the calculation, just explain how you would do it.
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Bibliography
American Chemical Society, "Chemistry in Context: Applying
Chemistry to Society", Wm. C. Brown Publishers, 1994.