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Content Benchmark E.12.A.2
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Earth Science
Atmospheric Process and Water Cycle
Solar System and Universe
Earths Composition and Structure
Content Areas
Nature of Science (NOS)
Life Science
Earth Science
Physical Science

Students know the composition of Earth’s atmosphere has changed in the past and is changing today. I/S

Currently, Earth’s atmosphere consists primarily of just three gases. Argon, nitrogen and oxygen make up just over 99.9% of our present day atmosphere (Table 1). At first blush, the atmosphere appears to be of relatively constant composition. In fact, argon, nitrogen and oxygen, together with helium, neon and krypton, are often referred to as the “non-variable” gases of our atmosphere. Water vapor is the most variable component, ranging from 0.01% to 3% of the atmosphere. Carbon dioxide (CO2) has slowly, but steadily, increased in abundance over the past several decades (Figure 1), but still constitutes less than 0.04% of the atmosphere.

Table 1. Composition of the Atmosphere

Major Permanent Gases in the Atmosphere


78.1 %


Other Permanent Gases in the Atmosphere


Variable Gases in the Atmosphere
Water Vapor
0 to 4%
Carbon Dioxide

Figure 1. Historic atmospheric carbon dioxide concentrations of carbon dioxide at the Mauna Loa Observatory (from

Matter cycles through the atmosphere just as it does through other natural reservoirs. Gases might be added or removed during the biological processes of photosynthesis and respiration. Minerals in Earth’s crust can chemically react with gases of the atmosphere (the oxidation of iron-bearing minerals removes O2). Bodies of water dissolve certain atmospheric gases, such as CO2, only to release them at some later time. Further, the dissociated ions in ocean waters can react with atmospheric gases during the formation of chemically precipitated minerals, tying up those gases in solid rock. Combustion of organic matter adds to atmospheric gases. Volcanic eruptions and hydrothermal events can return to the atmosphere gases which have been stored for eons.

Concern exists over the quantity of greenhouse gases (e.g., CO2) emitted into the atmosphere as the result of burning fossil fuels. Also, significant quantities of methane (another strong greenhouse gas) released into the atmosphere via livestock flatulence, decomposition of animal waste, and melting of the permafrost in the artic regions, particularly in Siberia and Alaska.

To learn more about methane entering the atmosphere and atmospheric composition, go to

Geochemists attempt to estimate how long the various gases will remain in the atmosphere as they cycle in and out of other parts of the environment. The concept of “residence time” has been developed to address these studies. Residence time, by definition, is: the average length of time a substance persists in a system; the capacity per rate of influx.

The residence time of nitrogen (N2) is about 44 million years; that of oxygen (O2) is 7 million years. It is easy to see why we think of these gases as non-variable components of the atmosphere. Conversely, the residence time of carbon dioxide (CO2) is 4 years while that of methane (CH4) is 3.6 years. It does not take long to affect the atmospheric concentration of these two gases.

The discussion on our evolving atmosphere is not complete without addressing the composition of Earth’s earliest atmosphere. Evidence suggests our ancient atmosphere was chemically reducing (as opposed to the oxidizing atmosphere of today). Hydrogen (H2) and helium (He) were the most abundant gases in the universe at the time of Earth’s accretion, thus it is likely they helped form an atmosphere. Their molecular masses were too low to be easily held by Earth’s gravity and the majority of H2 and He were lost to space. Volcanic outgassing is believed to have contributed water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen sulfide, chlorine and nitrogen. As gases in the primordial atmosphere reacted, ammonia and methane formed.

To learn more about Earth's early years: differentiation, water and early atmosphere, go to

Over time, two processes were working to add oxygen to Earth’s atmosphere. Beginning over 4 billion years ago, during the Hadean, ultraviolet radiation caused the photochemical dissociation of water vapor in the upper atmosphere. Hydrogen and oxygen were both released. Some of the oxygen remained as O2, while other oxygen was converted to ozone (O3). Ultimately, photochemical dissociation may have contributed as much as 2% of the oxygen in our current atmosphere. Atmospheric oxygen increased as primitive photosynthetic organisms began filling the warm oceans. Oxygen was produced as CO2 was consumed. Ancient cyanobacteria produced so much O2 during the Proterozoic that by its end, around 545 million years ago, the atmosphere became oxidizing, as evidenced by the prevalence of deposits of red, iron-rich sediments.

Figure 2. Most of the early Earth gases were released through land volcanism (from

Figure 3. The Earth's atmosphere for at least the first two billion
years was very oxygen-poor and hence reducing

To learn more about the evolution of Earth’s atmosphere, go to

As Earth matured and cooled, its rate of geothermal outgassing decreased. While Earth’s atmosphere does still change, it does so at a slower pace than early in our history. Natural events are still of consequence, and more recently, human activities have made significant changes to the composition of the atmosphere. In the last 200 years, human combustion of wood and fossil fuels has greatly increased the concentrations of CO2 and other greenhouse gases in the atmosphere.

In the past 500,000 years, atmospheric concentrations of CO2 have been significantly greater than current levels. Evidence from ice coring in Greenland and Antarctica shows a strong correlation between greater concentrations of atmospheric CO2 and significantly higher atmospheric temperatures.

To learn more about the evidence supporting Earth’s changing atmosphere, go to

Ice core data have provided a very compelling line of scientific evidence suggesting that humans maybe causing global warming and climate change because their activities are increasing atmospheric concentrations of CO2 and other greenhouse gases.

An excellent site discussing global climate change is found at

Humans have also significantly altered the atmosphere through emissions of chlorofluorocarbons (CFCs). During much of the 20th Century, CFCs were used to propel aerosols, and also in refrigeration. Through these uses, CFCs were released to the atmosphere. While CFCs themselves are not harmful, they have a very long residence time in the atmosphere and will eventually migrate to the Earth’s stratosphere. In the stratosphere, CFCs react with O3 (ozone) found naturally in this atmosphere layer. In particular, these reactions cause stratospheric O3 concentrations to be significantly reduced over the Earth’s poles, resulting in what are commonly called “ozone holes.” Because stratospheric O3 absorbs ultraviolet light from space that is harmful to biota, these ozone holes present a significant danger to humans, animals, and plants in the arctic and antarctic regions. In 1993, worldwide production and use of CFCs was virtually eliminated through international agreements. However, due to the long atmospheric residence times of CFCs, the ozone hole phenomenon persists.

To learn more about stratospheric O3 depletion, go to

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Content Benchmark E.12.A.2

Students know the composition of Earth’s atmosphere has changed in the past and is changing today. I/S

Common misconceptions associated with this benchmark:

1. Students incorrectly believe Earth’s atmosphere has always been a constant composition or that changes to Earth’s atmosphere occurred only during prehistoric times.

Students may have been instructed previously that a specific compositional percentage of nitrogen and oxygen exist in the Earth’s atmosphere and that this percentage has not varied from the earliest to the most recent point of their education. This may lead students to incorrectly believe that the composition of the atmosphere is unchanging.

Students may also understand Earth of the past was up to three times hotter than it is today. They grasp the concept of early Earth as a much more geologically dynamic planet then than now and most have learned how the original atmosphere formed during outgassing as countless volcanoes erupted across Earth’s surface. But, their Earth seems a much less active place, with volcanic activity relatively infrequent. If the atmosphere is to change at all, they may incorrectly believe it will be far in the future as Earth cools even more.

A research article discussing student misconceptions about weather, including student misconceptions of atmospheric composition is found at

2. Students incorrectly think the Earth’s atmosphere is a reservoir so vast that it cannot be significantly altered by natural events or by human industry.

Students correctly visualize the atmosphere as enveloping the Earth and extending hundreds of kilometers towards space. It seems reasonable to them that pollution from an automobile would no more alter the atmospheric composition than a 1 pound container of table salt would affect an ocean’s salinity. Students have observed first hand the exhaust from a car, or smoke from a stack, simply dissipating throughout the air with no noticeable effects. Even in cities with early morning smog, they will see as the day warms and the winds blow, the smog will disappear, its components distributed across the county, the state, and eventually across the world.

An excellent site to learn about human impact on the atmosphere is found at

3. Students incorrectly believe Earth’s atmosphere is of a strictly terrestrial origin.

Students have generally been taught that a little under 5 billion years ago, volcanic outgassing began to create our atmosphere and subsequently filled our oceans. It is their belief that all the gases in Earth’s atmosphere were originally trapped within magma, and have been released over time, first rapidly and then now more slowly. The students do not acknowledge evidence that our atmosphere has been added to by influx of cometary gases contributed as comets burn up in the atmosphere.

To learn more about comets and the evolution of Earth’s atmosphere, go to

4. Students confuse the enhanced greenhouse effect with issues regarding the ozone hole.

The enhanced greenhouse effect and ozone hole are often discussed simultaneously, and therefore, students incorrectly think that these two result in global climate change. In fact, the two phenomena are not closely related. The enhanced greenhouse effect is predicted to result from increased emission of CO2 and other greenhouse gases due to human activity. This enhanced greenhouse effect would increase global surface temperatures resulting in global climate change. On the other hand, stratospheric O3 depletion does not result in significant increases in global temperatures and climate change. Although ozone holes allow more ultraviolet light from the Sun to reach the Earth’s surface, the amount of the Sun’s energy in this frequency is so much smaller than that received in visible and infrared.

To learn more about this misconception, go to

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Content Benchmark E.12.A.2

Students know the composition of Earth’s atmosphere has changed in the past and is changing today. I/S

Sample Test Questions

1st Item Specification:  Explain how variations in the ozone layer affect the amount of ultraviolet radiation entering the Earth’s atmosphere.

Depth of Knowledge Level 1

  1. Ozone helps prevent ultraviolet radiation (UV) from reaching Earth’s surface by
    1. reflecting UV back into space.
    2. diffracting UV into the ionosphere.
    3. absorbing UV into its molecular structure.
    4. converting UV into gamma rays.
  1. Products containing halocarbons can deplete the ozone layer and result in increased levels of ultraviolet radiation reaching Earth’s surface.  Which of the following has NOT been a source of halocarbons?
    1. Yard wastes
    2. Automobiles
    3. Air conditioners
    4. Refrigerators

Depth of Knowledge Level 2

  1. Use the diagram to answer the following question.


    CFCs increase the amount of ultraviolet radiation reaching Earth because they
    1. prevent ozone from forming near Earth’s surface.
    2. deplete the amount of ozone in the stratosphere.
    3. are absorbed into the Sun, where they change into ultraviolet radiation.
    4. form a layer around Earth, acting like a lens which increases radiation.
  1. Use the diagram to answer the following question.


    Which statement most accurately interprets the data presented on the graph?
    1. Ozone levels steadily decrease while UV levels steadily increase.
    2. Ozone levels steadily increase while UV levels steadily decrease.
    3. Minor increments in percent of ozone and UV occur over time.
    4. Sharp decreases in ozone are mirrored by sharp increases in UV.

2nd Item Specification:  Describe how life forms have affected the composition of the atmosphere over time.

Depth of Knowledge Level 1

  1. Earth’s early atmosphere gained oxygen through photosynthesis and
    1. cellular respiration.
    2. photochemical dissociation.
    3. organic decomposition.
    4. nitrogen fixation.
  1. The two most abundant greenhouse gases in Earth’s atmosphere are
    1. water vapor (H2O) and carbon dioxide (CO2).
    2. carbon dioxide (CO2) and methane (CH4).
    3. ozone (O3) and carbon monoxide (CO).
    4. nitrogen (N2) and oxygen (O2).

Depth of Knowledge Level 2

  1. The term “anthropogenic emissions” refers to greenhouse gases released into the atmosphere as a result of human activities.  Use the diagram to answer the following question.


    Which statement is an accurate interpretation of the graph?
    1. Atmospheric concentrations of CO2 have steadily decreased as anthropogenic emissions have steadily increased.
    2. Before 1850, humans were releasing so much CO2 into Earth’s atmosphere that the values won’t even fit on the scale of this graph.
    3. There is absolutely no correlation between anthropogenic emissions and atmospheric concentrations of CO2.
    4. Since the 1960s, atmospheric concentrations of CO2 have risen at a rate approximately equal to that of anthropogenic emissions.
  1. Cyanobacteria first appeared in Earth’s oceans over 2 billion years ago, and were extremely abundant by 545 million years ago.  Cyanobacteria affected Earth’s atmosphere by
    1. rapidly increasing carbon dioxide levels in the atmosphere.
    2. gradually adding to the amount of oxygen in the atmosphere.
    3. slowly consuming all the ozone from the atmosphere.
    4. progressively using up the nitrogen in the atmosphere.

3rd Item Specification:  Describe how natural events have affected the composition of the atmosphere over time (e.g., volcanoes and meteorites).

Depth of Knowledge Level 1

  1. The two sources which contributed significant quantities of gases to Earth’s early atmosphere were volcanic outgassing and
    1. sublimation and thawing of Earth’s ice caps.
    2. widespread burning of the plant matter in Earth’s forests.
    3. ejection of gaseous matter from a nearby supernova.
    4. comets impacting Earth, releasing gases as they vaporized.
  1. Certain atmospheric gases are thought of as “non-variable” because their abundance remains unchanged over vast periods of time.  The two gases with the longest residence time in Earth’s atmosphere are
    1. nitrogen (N2) and oxygen (O2).
    2. carbon dioxide (CO2) and methane (CH4).
    3. carbon monoxide (CO) and chlorine (Cl2).
    4. ozone (O3) and hydrogen (H2).

Depth of Knowledge Level 2

  1. Of the following, which provides evidence that Earth’s atmosphere was altered from that of a reducing atmosphere to an oxidizing atmosphere?
    1. Volcanic outgassing poured vast quantities of water vapor into the atmosphere.
    2. Levels of carbon dioxide have steadily increased over the past several decades.
    3. Sediments rich in iron (III) oxide have been layered in thick deposits across Earth.
    4. The majority of hydrogen and helium has escaped Earth’s atmosphere into space.
  1. Use the diagram to answer the following question.


    Which statement most accurately explains how frozen water in comets is added to Earth’s atmosphere?
    1. The comets strike the oceans and melt, then evaporate into the atmosphere.
    2. As comets vaporize, they cause precipitation which falls into the atmosphere.
    3. Descending water vapor from the comets is mixed by wind into the atmosphere.
    4. Ice crystals from comet break up are scattered throughout the atmosphere as snow.

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Content Benchmark E.12.A.2

Students know the composition of Earth’s atmosphere has changed in the past and is changing today. I/S

Answers to Sample Test Questions

  1. C, DOK level 1
  2. A, DOK level 1
  3. B, DOK level 2
  4. D, DOK level 2
  5. B, DOK level 1
  6. A, DOK level 1
  7. D, DOK level 2
  8. B, DOK level 2
  9. D, DOK level 1
  10. A, DOK level 1
  11. C, DOK level 2
  12. C, DOK level 2

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Content Benchmark E.12.A.2

Students know the composition of Earth’s atmosphere has changed in the past and is changing today. I/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. The Goldilocks Principle: A Model of Atmospheric Gases.
The National Center for Atmospheric Research developed a content site with lessons useful to the understanding of Earth’s atmosphere. Activity 1, referenced below, specifically addresses topics in Performance Benchmark E.12.A.2.

This content and lessons can be accessed at


2. Ozone Information and Lessons
NOAA Research provides information and lessons on many aspects of Earth’s oceans and atmosphere. This site addresses ozone concentrations, sources and importance.

To access this site, go to


3. Student Investigations of the Atmosphere
Penn State’s College of Education has prepared a series of lessons regarding Earth’s atmosphere. Lesson 1 deals with the structure, composition and function of the atmosphere. Lesson 3 further addressed ozone depletion as a change in composition of the atmosphere.

• Lesson 1
• Lesson 3


4. WebQuest: The Nitrogen Cycle
ACCENT (Atmospheric Composition Change the European Network of Excellence) contains lessons tracing nitrogen, in its various forms, through the atmosphere and biosphere.

The WebQuest is found at


5. Atmosphere, Climate, and Environment Information Program Lessons
Encyclopedia of the Atmospheric Environment has a very comprehensive set of lessons on all aspects of the atmosphere. Many of the lessons are offered at two levels of difficulty.

To access their lessons on the atmosphere, go to


6. History and Composition of the Atmosphere Lessons
GirlTECH has created a set of lessons suitable for middle school and early high school students to teach about the history and composition of the atmosphere.

These lessons are found at


7. Chemistry of the Atmosphere Activity
Chemistry NOW produced an activity examining how Earth’s present atmosphere might have evolved from possible earlier atmospheres.

To download this activity, go to

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