Chemistry 104
Fall, 2003
Computer Modeling of Ground-Level Ozone Production
Introduction
Ozone (O3) is a molecule that is often mentioned in the news in relation to environmental issues
of several types. Many people are aware of the “ozone hole” that develops seasonally in the
stratosphere over Antarctica. In this case, the loss of ozone is a concern because ozone blocks
damaging UV radiation. With lowered stratospheric ozone levels, skin cancers become more prevalent, plant growth can be altered, and wildlife suffers, all due to the increased UV radiation
reaching the surface of the earth. Thus, in the stratosphere (10 – 50 km above ground surface),
ozone is naturally present and is desirable.
In contrast, ozone at ground level (the troposphere, 0 – 10 km above the surface of the earth) is a
substantial health risk. Near ground level, ozone can be produced in urban areas when weather
conditions (temperature and sunlight levels) are right and other air pollutants are present. High
levels of tropospheric ozone cause or worsen respiratory illnesses such as asthma. Ozone also
damages plants, can impact forests and agricultural crops, and causes many materials such as
plastics and rubber to become brittle and cracked. Since sunlight favors the chemical reactions
to produce ozone (and a number of other related pollutants), the product is often referred to as
photochemical smog.
Because of these impacts, ozone is monitored in regions where conditions favor the formation of
photochemical smog. In many areas, summer weather forecasts include a segment on air quality
and sometimes even ozone forecasts. The US EPA has developed limits for tropospheric ozone
for the protection of human health. Previously, the one-hour exposure limit was 125 ppb, but recent revisions lowered this limit to 80 ppb. Exposure to ozone concentrations exceeding this
limit for one hour or more is likely to cause respiratory damage, even in healthy people.
In the atmosphere, ozone is produced from elemental oxygen by the overall reaction:
3 O2(g)  2 O3(g)
However, this process does not occur in this one simple step and is therefore very far from this
simple to understand. The presence of other materials (especially including NO2 and hydrocarbons) can facilitate this reaction, with the energy required coming from sunlight. (Small
amounts of ozone can also be produced by lightning discharges, but this source produces relatively little ozone, and it dissipates quickly.) Some of the many reactions involved in this complicated process are:
sunlight
NO2  NO + O
O + O2  O3
O + R-H  OH + R
(R = a hydrocarbon group such as CH3 or C6H5)
R-H + OH  R + H2O
R + O2  RO2
NO + RO2  NO2 + RO
NO + O3  NO2 + O2
These reactions (and dozens more which could be written) demonstrate that nitrogen oxides
(from natural sources such as lightning, and combustion sources, especially automobiles) and
organic molecules (from natural sources such as pine trees, as well as anthropogenic sources
such as natural-gas leaks, hydrocarbon fuels, paint solvents, etc.) play an important part in the
formation and destruction of ground-level ozone. Atmospheric chemistry is discussed in Chapter
11 of your textbook, and this specific topic is covered in Section 11.13. Since there are so many
possible reactions of the relevant materials, direct experimental study of this complex system is
difficult, but these processes can be studied by using appropriate computer models.
These relationships will be explored using a computer program called the Local Atmospheric
Chemistry and Ozone Model (LACOM). Computer models are useful in several respects. First,
they give us the opportunity to make predictions based on our understanding of physical systems.
If we understand the processes that result in ground-level ozone, we can enter parameters into the
model and predict the resulting concentration of ozone. Models are also useful in that they give
us information about how well we understand a system. For example, suppose we input the parameters related to ozone production into the model for a day last year where we already have a
measured ozone concentration. If that known concentration is accurately predicted by the model,
this suggests that we have a reasonably good understanding of the processes that produce and
consume ozone. However, if the model can’t accurately calculate the ozone level that actually
occurred that day, maybe our understanding (from which we developed the model) is flawed.
In this experiment, using the LACOM computer model, we will investigate the parameters that
affect ozone production. We will vary the seasons, weather conditions, and atmospheric chemical concentrations to see which parameters have the greatest impact on ozone production.
Procedure
Part 1. Starting and Getting Familiar with the Model
1. While in Microsoft Excel, open the file LACOM.xls from the “Chem 104 Lab Files” folder
on the computer’s “C” drive. Click the button to “Enable Macros” when this prompt appears on
the screen. You should now see an Excel spreadsheet.
2. The sheet called “Input Values” contains four tables specifying the location and weather parameters. On this sheet, you will input the values of temperature, season, cloud cover, etc. appropriate for the system you are modeling. For this demo, you do not need to change any of the default values, but you will need to alter these in later parts. At the bottom of this sheet is a button
saying “To the Chemistry Worksheet.” Press this when you are finished editing the input values.
3. The Chemistry Worksheet contains information on 54 chemicals that can be included in the
model. We will only be working with a small subset of these chemicals. Each chemical has the
following parameters:
initial concentration
deposition rate (the rate at which the material is removed from the atmosphere)
emission rate (the rate at which the material is added to the atmosphere)
For NO2, you will be altering only the emission rate. For o-xylene, you will be altering both the
initial concentration and the emission rate. The normal level for air components such as O2 have
already been input as default values, as have the typical “background” concentrations for common air pollutants such as CH4 and SO2. These values do not need to be adjusted. There should
be a check mark in the first line (for ozone) in the right-hand column labeled “Chart?” No other
lines need to be marked with a check. When you are finished editing the Chemistry Worksheet,
scroll to the bottom of the page and press the “Start Calculation” button.
4. When prompted, click “Yes,” you want to start the calculation. A directory will appear specifying the location of the model files. If correct, press “Continue.”
5. A black box with text will appear and you must answer two questions to continue. In both
cases, type “1” and then “enter” in response to the questions (are you using LACOM.xls; do you
want to compute the concentrations hourly). The model will then begin calculations.
6. A window will appear telling you that the calculations are complete. Press “OK.” You can
now view graphs of the concentration of each chemical over time by pressing the appropriate tab
at the bottom of the Excel screen. You want the tab labeled “O3” for ozone.
7. Note the shape of the curve. Record the concentration and time of the minimum and maximum ozone concentrations during the day (i.e., not at midnight). Concentrations are listed in
molecules/cm3 (y-axis) and time is plotted in seconds (x-axis). If you hold the mouse arrow over
the point whose coordinates you want, a small window will appear with the x,y data for that point
in the form (time, concentration).
IMPORTANT!!! To re-run the model with new parameters, you must close the
LACOM.xls file in Excel and then re-open the file. If you try to run new calculations while
previous results are still displayed, THE PROGRAM WILL LOCK UP!!
DO NOT save changes when you close the LACOM.xls file.
Part 2. Parameters Affecting Formation of Ground-Level Ozone
The table on page 5 lists conditions for several different scenarios that might occur in the Baltimore region. In each of these scenarios, one or two parameters are varied. Enter the appropriate
information as in Part 1 above and perform the calculations. In each case, record the concentration and time for the maximum and minimum ozone concentrations. Note that each time you
close the LACOM model and re-open it, it returns to the default values for all parameters.
NOTES:
When changing temperature, you must change the values in Tables 2 and 3 on the Input
Values sheet.
When changing the month of the simulation, you must change values in Tables 1, 2, 3
and 4 on the Input Values sheet.
Part 3. Additional Scenarios
In this part, you will be asked to choose a set of conditions to model specific scenarios. Be sure
to record the conditions that you choose as well as the time and concentration of the maximum
and minimum ozone levels.
1. (Experiment 9) Table 4 of the Input Values sheet allows you to modify the type and amount
of cloud cover in the ozone model. If you put the mouse arrow over the cells for “Cloud Type”
and “Total Amount of Clouds,” menus appear giving you options for these variables. All of the
previous scenarios used zero cloud cover. Investigate the effect(s) of changing the cloud cover
on two of the scenarios from Part 2. Note that it would be best if you did a series of changes for
each scenario, rather than just one additional set of conditions.
2. (Experiment 10) Pick a season (spring or fall) that was not modeled earlier and choose appropriate and realistic parameters to investigate ozone production in that season. Record the parameters that you change from the default values. When you change dates, make sure that the
dates agree in all four tables, otherwise the model will not run and will lock up. Also, make sure
that the span of time does not change in each table (data spans 3 days for Tables 2, 3 and 4).
Again, record the concentration and time for the maximum and minimum ozone levels.
3. (Experiment 11) Because sunlight is needed for the production of ozone, the length of the
day might have an impact on ozone production. However, when you model seasons using realistic data, more than just the amount of sunlight changes. Model conditions in which all parameters are the same as one of the scenarios in Experiment 1, but the length of daylight is less.
#
Experiment
Name
Latitude
1
Summer in
Baltimore
Hot
Baltimore
Day
Humid
Baltimore
Day
Winter
Baltimore
Day
Summer
Ottawa
Day
Baltimore
Elevated
NO2
Baltimore
Elevated
o-xylene
Baltimore
Elevated
NO2 and
o-xylene
39.38
2
3
4
5
6
7
8
Max
Min
Humidity
Temperature Temperature
(C)
(C)
31
19
54
Month
7
Rate
NO2
Emissions
2.77E10
Initial
o-xylene
Conc.
4.00e10
Rate
o-xylene
Emissions
2.00e10
39.38
38
26
54
7
2.77E10
4.00e10
2.00e10
39.38
31
19
75
7
2.77E10
4.00e10
2.00e10
39.38
7
1
54
1
2.77E10
4.00e10
2.00e10
45.00
31
19
54
7
2.77E10
4.00e10
2.00e10
39.38
31
19
54
7
3.00E11
4.00e10
2.00e10
39.38
31
19
54
7
2.77E10
4.00e12
2.00e11
39.38
31
19
54
7
3.00E11
4.00e12
2.00e11
In Your Report
1. Convert all concentrations from molecules/cm3 to ppb by dividing the concentration by 2.46 x
1010. Convert all times to hours by dividing the time in seconds by 3600. Report these to three
significant figures.
2. Report your data by making a table that contains the experiment number and name, time
(hours) and concentration (ppb) of the minimum ozone level and the time and concentration of
the maximum ozone level. This should be typed.
3. Answer the Questions given below.
Questions
1. From your results, which conditions (high or low humidity, high or low temperature, long or
short daylight period, etc.) seem to be the most important in the promotion of formation of
ozone? Refer to specific results in each case to support your answer.
2. Look at the parameters that you considered in this model. What other parameters can you
think of that the model does not take into account (or oversimplifies) that might be important?
3. Comparing your results from Experiments 1, 4 and 11, which parameter has a greater effect
on ozone production, the amount of daylight or the temperature? Explain how your results justify your answer.
4. What did your results for Experiment 9 reveal about the effect(s) on ozone production of the
cloud cover conditions you chose? Why do you think this occurred?
5. When conditions favor the formation of unhealthy levels of ozone, an Ozone Action Day is
declared in which people are asked to modify their routines to minimize ozone production. On
there occasions, signs on the Baltimore Beltway urge people to fuel their cars after dark. Why
might this decrease the level of ozone production? (Hint: gasoline contains molecules like oxylene.)
6. Given what you learned from the model and the fact that NO2 is produced by automobile exhaust and o-xylene is a component of gasoline, what types of areas would be most likely to be
subject to unhealthy levels of ozone? Question 5 mentioned one way in which people can alter
their routines to help decrease the production of ozone. Given the factors that contribute to
ozone formation, what other things can people in susceptible areas do to help decrease ozone
production?
7. In general, at what time of day was the maximum concentration of ozone observed? The minimum concentration? Why do you think the maximum and minimum occurred at those times
and not at other times during the day?