1a. What are the sources of most gases currently being introduced into Earth's atmosphere?

1b. Describe three loss processes for atmospheric gases.

1c. What important property do gases that are eventually transported to the stratosphere have? Explain.


2a. In which Earth system do we find most of the nitrogen?

2b. What nitrogen oxide species supplies reactive nitrogen to the stratosphere? How is it transformed in the stratosphere?

2c. Explain how inert molecular nitrogen (N2) can be transformed into reactive forms of nitrogen.

2d. Give an example of how the nitrogen cycle balance has been disturbed through human processes.


3a. Has the amount of nitrous oxide (N2O) increased or decreased over the last two hundred years? Explain.

3b. What are the anthropogenic sources of N2O in our atmosphere?


4a. What products does methane form that impact stratospheric ozone?

4b. What are natural sources of methane?

4c. Is the methane concentration in the atmosphere increasing? Explain.


5a. What compounds are considered most reactive when considering stratospheric ozone? Why?

5b. What are natural sources of halogens to the atmosphere? Are these more or less important than industrial sources? Explain.


6a. What are the characteristics of chlorofluorocarbons (CFCs) that give them long atmospheric lifetimes?

6b. Using the naming convention for CFCs, determine the formulas for CFC-13 and CFC-122.

6c. List some of the uses of CFCs. How have these usages changed from 1974 to the 1990s?


7. When you introduce a gas to the atmosphere at a constant rate its concentration will build until it reaches a steady-state where it will remain constant. Although production and use of CFCs decreased rapidly in the 1980s and 1990s, atmospheric levels continued to increase. Why?


8a. Why are halon concentrations in the atmosphere considered so important for stratospheric ozone levels?

8b. HCFCs and HFCs have been used as replacements for CFCs. Why are HCFCs and HFCs considered less of a hazard to stratospheric ozone?

8c. Are there atmospheric disadvantages to using HFCs in place of CFCs? Explain.

8d. Why is methyl bromide considered as less of a threat to stratospheric ozone in spite of its potentially hazardous bromine atom?


9a. What international steps have been taken to limit the production and potential release of CFCs to Earth's environment?

9b. What effects have the steps had on concentrations of atmospheric CFCs?


10a. What species are emitted from volcanos that have a potential impact on stratospheric ozone levels?

10b. What role does sulfur dioxide (SO2) play in polar stratospheric clouds?



1a. The source of virtually all gases to the atmosphere is the surface of the Earth. The existing atmosphere accumulated over billions of years from the escape of gases from volcanic vents and fumaroles. Gases are being continually emitted from such vents as well as from vegetation, the oceans, and industrial processes.

1b. Three loss processes for atmospheric gases are (1) surface deposition, (2) rainout of soluble gases, and (3) oxidation. In surface deposition gases come into contact with Earth's surface. They are then removed in chemical reactions, dissolving in surface waters, or being taken up by plants and converted into another form. An example of a gas taken up by plants during photosynthesis is carbon dioxide (CO2). Rainout of soluble material occurs when gases are dissolved in raindrops that fall to the surface. The dissolved gases are then washed away in streams into lakes or the ocean. Noxious pollutants emitted in industrial processes are removed in this way from the atmosphere. Oxidation occurs when atmospheric gases react with various oxidizing agents, such as the hydroxyl radical (OH). Oxidation removes hydrocarbons from the atmosphere, but is creates other pollutants in the process.

1c. An important property of gases that are eventually transported to the stratosphere is their long atmospheric lifetime, or residence time. Such gases are not water soluble and are nonreactive in the lower atmosphere. They also don't absorb visible or near-ultraviolet radiation such as reaches the troposphere.

2a. The bulk of the nitrogen in the Earth system is in the atmosphere (4 x 109 teragrams (Tg)).

2b. N2O acts as a carrier of reactive nitrogen to the stratosphere. N2O is particularly important because it is an unreactive gas which is not water soluble and does not absorb visible radiation. The atmospheric lifetime of N2O is estimated to be about 150 years. Its primary destruction path is transport to the stratosphere where it absorbs UV radiation forming N2 and an oxygen atom (O). A minor destruction pathway (about 3.5%) is reaction with O(1D) to form two NO molecules.

2c. The process of conversion from atmospheric N2 to more reactive forms of nitrogen occurs through nitrogen fixation. Fixation can be accomplished in a number of ways. One of these is biological fixation. Certain bacteria fix nitrogen by conversion from N2 to NH4+. These include the blue-green algae. Bacteria which live in symbiosis with legumes (beans, peas, clover, etc.) also fix nitrogen. The manufacture of nitrogen fertilizer uses a chemical process to fix nitrogen as NH4+ or NO3-.

2d. Industrial fertilizer production has introduced an imbalance into the nitrogen cycling system. The present imbalance in the system is due to the recent rapid increases in the amount of fertilizer production, which is driving an upward trend in N2O at a rate of about 0.2% per year.

3a. The measured N2O concentration was steady at about 275 to 280 ppbv from 1750 to the late 1800s as measured from air trapped in ice cores. It then began to rise so that a concentration of about 290 ppbv was reached by 1950. Since that time the increase has been about 0.6 ppbv/year or about 0.2%/year. The present concentration is a little over 310 ppbv.

3b. Anthropogenic sources of nitrous oxide include nitrogen fertilizer denitrification and combustion.

4a. The complete oxidation of methane leads to one CO2 and two H2O molecules. The oxidation process consists of many reactions, some of which produce hydrogen oxides such as OH and HO2 (HOx). Thus, methane oxidation consumes an OH molecule in its initial step but can produce additional HOx during its oxidation. Another by-product of methane (and other hydrocarbons) oxidations is ozone. This occurs in the presence of nitrogen oxides (NOx).

4b. Methane is produced from carbon in oxygen-free (anaerobic) conditions. These conditions occur in swamps, rice paddies, and in the stomachs of ruminant animals such as cows and sheep. Methane also forms in deposits from decay of very old plant and animal material.

4c. CH4 concentrations are increasing in the atmosphere. The current concentration is about 1.8 ppmv which is more than twice that deduced to have existed thousands of years ago from analyses of air trapped in Arctic and Antarctic ice. The rate of increase over the last couple of decades has been in the range of 0.5 to 1 %/year.

5a. The most active elements to stratospheric ozone are chlorine and bromine, members of the chemical family called the halogens. Free radical chlorine or bromine atoms can extract an oxygen from ozone producing oxidized forms of chlorine (ClOx) and bromine (BrOx) oxides, which then provide a means to produce oxygen, preventing the reformation of ozone.

5b. Natural sources of halogens to the atmosphere include sea salt spray, ocean produced methyl chloride and methyl bromide, and volcanic emissions. They are less important than industrial sources because the lifetimes of the industrial sources are longer and their concentrations greater.

6a. CFCs are tightly bound, nonreactive molecules. They are not soluble in water. They don't absorb visible or near-ultraviolet radiation. As a result of these properties CFCs have long atmospheric residence times and are able to provide their constituent atoms to the stratosphere.

6b. CFC-13 = CFC-013 so C=1, H=0, F=3, Cl=1 CF3Cl

CFC-122 so C=2, H=1, F=2, Cl=3 C2HF2Cl3

6c. CFCs are used as propellants in aerosol cans, as refrigerants, as cleaning agents, and in foam blowing.













foam blowing



7. Most CFCs such as CFC-11 and CFC-12 emission rates into the atmosphere increased exponentially during the 1960s and 1970s. This means that the steady-state value (towards which their atmospheric concentrations were increasing) continually changed to higher values. When production was first slowed in the late 1970s and then decreased rapidly in the late 1980s and 1990s, the atmospheric concentrations continued to increase because they were still well below the steady-state concentrations consistent with the new source rates.

8a. Halons contain bromine which can be delivered to the stratosphere. Bromine is a stronger catalyst for ozone loss than chlorine.

8b. The presence of a hydrogen atom allows the reaction of the HCFC with hydroxyl radicals (OH). This reaction extracts the hydrogen atom and combines it with OH to form water. The resulting fragment is a reactive radical and rapidly reacts to convert the chlorine and fluorine atoms to oxides and then to the acids HCl and HF. The reaction with OH occurs rapidly enough that much of the HCFC is mostly destroyed in the troposphere where the HCl and HF can be dissolved in water and rained out of the atmosphere. Only a small fraction of the HCFC makes it to the stratosphere. The HFCs have had all the chlorine removed, so they have no potential for delivering chlorine to the stratosphere.

8c. While HFCs deliver no chlorine to the stratosphere, they are absorbers of infrared radiation and can contribute to global warming.

8d. Once CH3Br is released to the atmosphere it reacts with OH with a lifetime of a little more than 1.5 years. Thus much of it is destroyed in the troposphere. The ocean can also serve as a sink for CH3Br.

9a. In 1987 the governments of the world, through the United Nations Environment Program (UNEP), agreed to a protocol to limit the production and release of a variety of CFCs. This protocol was put forward at a meeting in Montreal, Canada and has become known as the Montreal Protocol. It has been ratified or accepted by 165 countries. Since the original protocol, its provisions have been amended; in 1990 at a London conference and in 1992 at a Copenhagen conference. These amendments have been ratified or accepted by 120 and 78 countries respectively. In 1997, a further amendment was adopted at a conference in Montreal. The Montreal Protocol put forward schedules for phaseouts of various CFCs and other ozone depleting substances (e.g. halons) based on their calculated "ozone depletion potential (ODP)". These calculations compared the projected ozone loss for a given release of a CFC compared to that for the same release of CFC-11. Substances with an ODP close to that for CFC-11 were scheduled for rapid phaseout. Those with lower ODPs were scheduled for slower phaseout. Replacement substances such as HCFCs were scheduled to buildup over a period of time as they served as replacements and then to later be phased out in favor of substances with even lesser (or zero) ODPs.

9b. The effects of the protocol can be seen in the slowing down and reduction of the atmospheric concentrations of some CFCs such as CFC-11 (illustrated in Figure 10.11).

10a. SO2 and HCl are gases that may be emitted from volcanoes and may impact the stratosphere. Since HCl is rapidly rained out in the troposphere it would have to be injected into the stratosphere through an explosive eruption.

10b. SO2 is oxidized in the stratosphere to sulfuric acid (H2SO4) which coalesces into small particles or aerosols. These particles are generally much smaller than those originally emitted by the volcano. They act more like gases in that their fall velocity is small and turbulent motions are sufficient to keep them in the same air mass with any remaining gas from the volcano. These aerosol particles can enhance the formation of polar stratospheric clouds, key players in the formation of the Antarctic ozone hole.