Student The Dual Nature of Ozone:
Materials TROPOSPHERIC OZONE
SM-1
SAVE SMOG CITY 2 FROM OZONE WORKSHEET
Worksheet Courtesy of the U.S. Environmental Protection Agency and Sacramento
Air Quality Management District
http://www.smogcity2.org/smogcity.cfm?preset=ozone
Direction: Follow the instructions listed below. Be sure to answer the questions associated to some
instructions in the activity.
1. Access the Smog City 2 web site at www.smogcity2.org.
2. Select “Save Smog City 2 from Ozone.”
3. Once Smog City 2 loads to your computer, take note of the areas of Smog City 2, including Weather
Conditions, Emissions Levels and Population. All areas have “clickable” choices. Mouse-over or click on
the choices.
• NOTE: in the information box at the bottom of the screen, there is information about
each choice.
4. Notice how each of the choices are pre-set to a certain level. These are called the default settings. You
can use the reset button at any time to return to the default settings. In the chart below, circle or
highlight the default setting for each choice. The first setting, Sunlight, has already been completed for
you.
A. Weather Conditions
Weather Conditions
Choices Included in the Area
Sunlight
Clear
Inversion Layer
No inversion
Wind Speed
Calm
Maximum Daily Temperature
30ºF
Partly Cloudy
Low inversion
Light Breeze
40ºF
Cloudy
50ºF
High Inversion
Breezy
80ºF
90ºF
Windy
100ºF
110ºF
B. Emission
Emission
Choices Included in the Area
Energy Sources
Some energy sources produce more smog-producing
emissions than others.(level 1 is cleaner sources like a
wind or solar technology, level 3 produces more smog
like a coal-fired power plant)
Levels: 1 2 3
Cars and Trucks
This includes Passenger vehicles
(all sizes), large and medium trucks, motorcycles
Levels: 1 2 3 4 5
Off Road Vehicles
This includes airplanes, trains, power
boats, earth movers, tractors, harvesters, forklifts,
bulldozers, backhoes
Levels: 1 2 3 4 5
Consumer Products
This includes paint thinner, charcoal lighter fluid, glue or
other adhesives, gasoline
Levels: 1 2 3 4 5
Industry
This includes manufacturing facilities, power plants, oil
refineries/storage/distribution centers, food and
agricultural processing
Levels: 1 2 3 4 5
C. Population
Population
Population in Smog City 2 affects air quality.
Changing population, as shown by the “total emissions”
chart and the emission sources in the cityscape, affects
VOCs, NOx and SO2. The compounds react to form
ground-level ozone and particle pollution. When
temperatures are cool, changing population also
changes the usage of wood-burning stoves, which emit
particle pollution.
Choices Included in the Area
In Smog City 2, you can increase the
population from near-zero to about two
million people.
Levels: 1 2 3 4 5
5. Observe the AQI (Air Quality Index) box in the lower right corner. The default settings, which are
circled above, result in a “red”, or “Unhealthy” AQI for ground level ozone. The health message is:
“Active children and adults, and people with respiratory disease, such as asthma, should avoid
prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor
exertion.”
Scenario 1: Emission Sources
1. Minimize the “Save Smog City 2 from Ozone!” instructions at the top of the screen.
2. Turn only Cars and Trucks control to 1. Leave all other choices at the default settings. Record what
happens on the Student Worksheet in the table below. Use the reset button to return the Cars and
Trucks control to 4, so all controls are in default position.
3. Turn only Off Road down to 1. Leave all other settings alone. Record what happens on the Student
Worksheet. Use the reset button to return the Off Road control to the middle setting, so all controls are
in default position.
4. Adjust each of the remaining controls noted in red and record the result in Data Table 1.
Data Table 1
default
Change Cars &
truck only
Change Off
Road only
Change
consumer
products only
Change
Industry only
Change
Energy
Sources only
Energy
Sources
2
Cars &
Trucks
4
Off
Road
3
Consumer
Products
3
Industry
2
1
3
3
3
2
4
1
3
3
2
4
3
1
3
2
4
3
3
1
1
4
3
3
3
3
Air Quality Index (AQI)
Color | message | value
Red – Unhealthy- 175
5. Turn all Emission controls to level 1. What is the AQI? Why?
6. Using the reset button, return all Emission controls to the default setting. Turn the Population control
to level 1. What is the AQI? Why? (Hint: Click the Population icon and read the information under “What
Is This?” in the “Information” box.)
Scenario 2: Weather
1. Reset all Emission controls to the default setting. What is the AQI level?
2. Increase only the temperature control to 110 ºF. Check the black sign in the cityscape for the
temperature. How does this affect the AQI? Why?
3. Now move the cloud cover to Cloudy (level 3). How does sunlight affect ozone formation? Why?
Concluding Questions:
Answer the following questions during class discussion;
1. Was there any one variable that seemed to have a greater increase in ozone than others tested?
Which one?
2. What steps could be taken to control emissions levels?
3. Can you think of ways to reduce ozone levels?
SM-2
EXPERT GROUP CARDS
GROUP 1
What is Tropospheric Ozone?
Tropospheric ozone (O3), also known as low-level ozone, is an air pollutant that is harmful to
humans and the natural environment. It is the same O3 molecule as stratospheric ozone. However, since
tropospheric ozone occurs where it can be inhaled by humans or enter plant stomata, it can cause
damage to our health and vegetation. That is why the phrase “Good up high, bad nearby” is sometimes
used to distinguish between the natural stratospheric ozone that protects us from ultraviolet radiation,
and the tropospheric ozone that can be a harmful air pollutant. Only about 10% of all atmospheric
ozone is in the troposphere, and there is usually no more than 1 molecule of ozone for every 10 million
molecules of air near the ground. However, ozone is such a strong pollutant that even those small
amounts can be harmful when breathed in to our lungs.
Figure 1. Good and bad ozone in the atmosphere. Credit: U.S. EPA : Ozone: Good Up High, Bad Nearby
In the previous lesson, we learned how stratospheric ozone forms naturally from oxygen (O 2)
and UV-C sunlight. However, in the troposphere, ozone forms as a pollutant when manmade and natural
emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of
sunlight. Unlike other air pollutants, tropospheric ozone is not emitted directly into the atmosphere but
instead forms from these other compounds. Details on how those compounds form ground-level ozone
are discussed in another expert card.
In Texas, the Houston and Dallas regions have long struggled to meet federal air quality
standards for ground-level ozone. Large amounts of emissions from vehicles and industries (including
numerous petrochemical facilities in Houston) and hot weather conditions contribute to ozone
pollution. Ozone levels in Houston and Dallas have greatly improved as control devices have been
developed to reduce air pollutant emissions from cars and factories. However in spite of the
improvement, both cities continue to exceed federal limits. The U.S. Environmental Protection Agency
recently lowered the ozone limit from 85 ppb (parts per billion) to 75 ppb, because scientists found that
health impacts can occur even at these low levels. This lower limit will make it more challenging for
states to reduce emissions sufficiently to meet the ozone standard.
SM-2
EXPERT GROUP CARDS
GROUP 2
What are the sources and controls of nitrogen oxide (NOx) emissions?
Nitrogen Oxides: Nitrogen oxides (NOx) is a term used to describe two highly reactive gases: nitric oxide
(NO) and nitrogen dioxide (NO2). These gases, together with VOCs, contribute to the formation of
tropospheric ozone by chemical reactions that are described in a separate group card.
Emission Sources: Nitrogen oxides are formed from the combustion of fossil fuels or biomass, and by
natural processes such as volcanic eruptions, forest fires, soils, and lightning. It can be seen from Figure
2 that the two leading sources of NOx emissions in the United States are on-road vehicles (i.e., cars,
trucks and buses) and off-road vehicles and equipment such as construction or agricultural equipment,
trains, airplanes, and boats. Electric utilities (“power plants”) and other industries that combust fossil
fuels such as coal, oil, and natural gas are also important sources of NOx emissions. Natural emissions of
NOx come from lightning, wildfires, and soil.
U.S. Nitrogen Oxide Emissions (2008)
Biogenics (Natural)
6%
Industrial Fuel
Combustion
8%
Other
10%
On-road Vehicles
39%
Electric Utilities
17%
Off-road Vehicles &
Equipment
20%
Figure 2. U.S. nitrogen oxide emissions by source category in the US EPA 2008 National Emissions Inventory.
Controls: Since most NOx in the atmosphere comes from burning fossil fuels in vehicles, power plants
and other industries, one way to control NOx emissions is to use less fossil fuel. This can be done by
driving less, using renewable or cleaner burning fuels, driving within the speed limit, maintaining
vehicles, and using energy efficient lights and appliances.
In addition, government regulations require that new vehicles and power plants emit far less
NOx than they did in the past. This has been made possible by the development of many control
technologies that strongly reduce NOx emissions from vehicles and other sources. For example, catalytic
converters in cars can convert most NOx into harmless nitrogen gas (N2) before the exhaust leaves the
tailpipe. Similarly, control technologies for power plants and other industries can sharply reduce the
amount of NOx that leaves their smokestacks. Together, these policies have allowed the U.S. to cut NOx
emissions by more than 50% since 1980, even as vehicle and electricity use have grown.
SM-2
EXPERT GROUP CARDS
GROUP 3
What are the sources and controls of volatile organic compound (VOC) emissions?
Volatile Organic Compounds: VOCs are unstable organic compounds that contain carbon. The
atmosphere contains hundreds of types of VOCs. When you smell a forest, baking bread, perfume, or
many other scents, what you are smelling is VOCs. Some VOCs are harmful to human health, but many
others are harmless at typical concentrations in the atmosphere. Together with NOx, VOCs contribute to
the formation of tropospheric ozone by chemical reactions that are described in a separate group card.
Emission Sources: Figure 3 shows the emission of VOCs to the atmosphere in the United States. The
majority of VOC emissions originate from natural biogenic sources, especially trees which emit a VOC
called isoprene. However, manmade VOCs can be important contributors to ozone formation in some
cities. The biggest category of manmade emissions is evaporation of VOCs from solvents, including
paints, cleaning supplies, and other products. VOCs are also emitted when fossil fuels are combusted by
on-road vehicles (i.e., cars, trucks, and buses), off-road vehicles or equipment (e.g., construction or
agricultural equipment, trains, airplanes, and boats), or for other purposes. Some VOCs are also released
in the extraction and processing of petroleum and natural gas.
U.S. VOC Emissions (2008)
Petroleum &
Related Industries
4%
Other
14%
Off-road Vehicles &
Equipment
5%
On-road Vehicles
6%
Solvents
7%
Biogenics (Natural)
64%
Figure 3. U.S. Emissions of VOCs in the US EPA 2008 National Emissions Inventory.
Controls: Regulations and improved technologies have reduced the VOC emissions from paints and
other solvents. Catalytic converters and cleaner burning fuels have reduced VOC emissions from
vehicles, and other control technologies can reduce emissions from petroleum refineries and other
industries. On an individual level, anything we do to reduce our driving and use of solvents can reduce
VOC emissions. Biogenic (natural) sources from plants are largely beyond our control; however, trees
tend to emit more VOCs on warm and sunny days, so emissions could increase if the climate warms.
SM-2
EXPERT GROUP CARDS
GROUP 4
The Chemistry of Tropospheric Ozone Formation
How does ozone form in the troposphere?
In the previous lesson, we learned how ozone forms naturally in the stratosphere from oxygen
and UV-C sunlight. In the troposphere, ozone instead forms as a pollutant from chemical reactions
involving nitrogen oxides (NO and NO2, known collectively as “NOx”), volatile organic compounds
(VOCs), and sunlight. Other expert cards describe the emission sources and control technologies for NOx
and VOCs. Notice that unlike most other air pollutants, tropospheric ozone is not emitted directly into
the atmosphere, but instead forms from these other compounds.
Figure 4. Tropospheric ozone (‘bad ozone). Credit: http://www.airnow.gov/index.cfm?action=aqibasics.ozone
Since virtually all UV-C sunlight is absorbed by oxygen molecules (O2) in the stratosphere, almost
none of the UV-C can reach the troposphere to help form ozone. However, most visible sunlight (the
portion of the electromagnetic spectrum that our eyes can see) does reach the troposphere. Visible
light can break apart nitrogen dioxide (NO2) to form ozone (O3) and nitric oxide (NO), as shown in
reaction 1.
#1
NO2
+
visible light +

O2
NO + O3
The NO formed by Equation 1 can react with ozone (O3) or with another radical:
#2a
#2b
NO
NO
+
+
O3
radical (e.g., HO2)


NO2
NO2
+
+
O2
other products
Note that if Reaction 1 is followed by Reaction 2a, all of the reactants and products cancel out;
in other words, each gas is produced and destroyed one time. Thus, if only Reactions 1 and 2a occurred,
ozone could not build up to very high levels in the troposphere.
However, if Reaction 1 is followed by Reaction 2b, O3 levels could build up in the troposphere,
since the NO2 is replaced without destroying an O3. Notice that these reactions change the form of NOx
between NO and NO2, but the overall amount of NOx stays the same. In other words, the NO2 that is
lost in Reaction 1 is reproduced by Reaction 2b. This means that NOx serves as a “catalyst” for forming
O3 in the troposphere.
How do we get the radical needed for Reaction 2b? These radicals form when VOCs react with
oxidant gases such as OH (hydroxyl radical) as shown in Reaction 3:
#3
VOC +
OH 
radical (e.g. HO2)
+ other product
We can summarize how ozone (O3) forms in the troposphere by a single equation:
SUMMARY:
NOx
+
VOC
+
sunlight

O3
Even though ozone formation involves many complicated reactions rather than the simplified
equation shown here, this summary reminds us of the three (3) ingredients needed to form ozone (O3)
in the troposphere: nitrogen oxides (NOx), volatile organic compounds (VOCs), and sunlight. If any of
these ingredients are missing, very little ozone can form. To reduce ozone pollution, we must reduce
our emissions of NOx, VOCs or both.
.
SM-2
EXPERT GROUP CARDS
GROUP 5
How do weather conditions affect tropospheric ozone formation?
In addition to NOx and VOCs, weather conditions or meteorological factors are known
to have some influence in the formation of tropospheric ozone. These factors include
temperature, humidity, cloud cover and wind direction and speed. Combination of these
factors can increase or decrease the amount of ozone in the troposphere.
1. Temperature. Since ozone formation is a photochemical reaction, sunlight has to be
present. The heat and light from the sun drive the reactions involved in the formation of ozone
molecules. Higher temperatures also increases the evaporation of VOCs into the atmosphere,
and causes trees to release more VOCs. For all of these reasons, observational monitors usually
measure more ground-level ozone when temperatures are high.
2. Humidity. Water vapor in the atmosphere helps to form hydroxyl radical (OH), a
highly reactive gas that reacts with VOCs to accelerate the formation of tropospheric ozone.
However, very humid days may also be cloudy and rainy which can slow down ozone formation.
Thus, O3 levels may increase or decrease as humidity changes.
3. Cloud cover. The presence of clouds in the atmosphere blocks some of the sun’s UV
and visible radiation. When there are more clouds, less radiation from the sun can reach the
ground level. Thus, tropospheric O3 levels tend to be lower on cloudy days.
4. Wind direction and speed. Ozone formation is also influenced by the circulation of
air. Wind carries the ingredients necessary for ozone formation from one area of town to
another. Fast wind speeds tend to dilute pollution. When air is very calm under an inversion
layer, pollution levels can build up.
SM-2
EXPERT GROUP CARDS
GROUP 6
What are the negative impacts of tropospheric ozone?
Tropospheric or low-level ozone can have the following health effects, even at concentrations of
less than 100 parts per biilion (i.e. 1 ozone molecule for every 10 million air molecules):
1.
2.
3.
4.
Make people more sensitive to allergens.
Aggravate asthma.
Damage and inflame the lungs, making it harder to breathe.
Irritate the respiratory system – coughing and irritation in the chest.
All of the above conditions can prevent people from taking part in vigorous activities outdoors.
Children and the elderly are especially more sensitive to these health effects.
In plants, tropospheric or low-level ozone has been known to enter the plants through their
stomata (opening in the leaves for carbon dioxide to enter and oxygen to exit) and damage the cells.
This can cause chlorosis in plants (a condition when leaves produce less chlorophyll) and slows down
photosynthesis as well as plant growth. In plants that are sensitive to O3, the damage can be seen as
brown flecks on the leaves. Ozone damage can reduce crop yields on farms and slow the growth of
forests.
Figure 5. Snapbeans in clean air (L) and Snapbeans damaged by ozone (R).
Credit: Fitzgerald Booker, USDA-ARS Plant Science Research Unit, North Carolina State University
SM-3
METEOROLOGICAL FACTORS AND OZONE CONCENTRATION
Lab Worksheet
Directions: Use this worksheet to plan for your investigation.
Fill out page 1 and turn it in for
approval before conducting the experiment. Use the elements (title, question, hypothesis, etc) in this
worksheet in writing the final lab report. Only 1 lab report is required per group.
Title of the Investigation:
_____________________________________________________________________________________
_____________________________________________________________________________________
Question:
_____________________________________________________________________________________
_____________________________________________________________________________________
Hypothesis:
_____________________________________________________________________________________
_____________________________________________________________________________________
Variables:
Independent variable
__________________
Dependent variable
________________
Controlled variables
____________________ __
_______________________
_______________________
Materials:
_____________________________________________________________________________________
_____________________________________________________________________________________
Procedure:
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
Experimental Site: ___________________________________
Experimental design reviewed and approved on : _______________________
-page 1-
Results and Discussion:
Use tables and graphs here.
Sample Data Table
Meteorological Factor (example: air temperature)
Ozone Concentration (ppb)
Conclusion:
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
Group Names
_______________________________
_______________________________
_______________________________
_______________________________
- page 2-
SM-4
CLOUD GUIDE
(To help students fill in “Cloud Types” and “Cloud Cover” on Rice Air Curriculum Data Sheets)
Cloud Types
High in the Sky:
Cirrus
Middle of the Sky:
Cirrocumulus
Altocumulus
Cirrostratus
Altostratus
Low in the Sky:
Stratus
Rain or Snow Producing Clouds:
Stratocumulus
Nimbostratus
Cumulus
Cumulonimbus
Cloud Cover
No Clouds
0% - No clouds
Clear
<10% Clouds
Isolated
10-25% Clouds
Scattered
25-50% Clouds
Broken
50-90% Clouds
Overcast
Images:http://www.globe.gov/tctg/atmo_chap.pdf?sectionid=1&lang=EN
SM-5
The Daily Maximum Eight-Hour Ozone Averages
Summary of Data
Make a summary of the ozone values in terms of air quality condition or levels of health concern.
Good
Moderate
Unhealthy
for Sensitive
Groups
Unhealthy
Very
Unhealthy
Hazardous
Days
For the days, write the date or dates of the month where these AQI are observed.
Answer the following questions:
1. Do you see any variation of the ozone level in the area you selected?
2. Which date has the highest ozone level? _______ and which one has the lowest? ________
3. Explain the possible cause or causes of this variation.
4. What should you do before going out for any outdoor activity?
SM-6
Rice Air Curriculum
Ozone and Meteorology Data Sheet
Group’s Name : ___________________________________________________________
Measurement location : ____________________________________________________
Please choose three places where you will measure surface temperature (circle
one for each):
Surface Temperature #1: Grass Barren Land Shrubs Concrete Asphalt Other: ____________________
Surface Temperature #2: Grass Barren Land Shrubs Concrete Asphalt Other: ____________________
Surface Temperature #3: Grass Barren Land Shrubs Concrete Asphalt Other: ____________________
Day 1
Date
Day of Week
Beginning of class (ozone strip exposed)
Time (hour:min)
Wind direction
Air temperature (°C)
Surface temperature
(°C)
Relative Humidity (%)
Cloud Types (see
Cloud Guide; ; List all
types seen)
Cloud Cover
End of class (ozone strip read)
Time (hour:min)
Wind direction
Air temperature (°C)
Surface temperature
(°C)
Relative Humidity (%)
Ozone concentration
(parts per billion)
AQI
Day2
Day 3
Day 4
Day 5
SM-7
EVALUATION
Direction: Write at least a 1-page essay on the following prompt:
Houston is one of the cities in the U.S. that exceeded the
ozone standard. As an individual and resident of Houston, how
can you help reduce the ozone concentration around the area?
What steps should you take and how would these steps impact
the environment?
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