1a. How is the Antarctic ozone hole defined?
1b. How does it compare to the annual spring low total ozone amounts over Antarctica?
1c. Where in altitude is most of the ozone over Antarctica located? Hint: see Figure 11.08.
2a. Based on Figure 11.01, explain how ground measurements of total ozone compare to satellite measurements.
2b. Why is this comparison important to research scientists?
3a. What explanations did Farman et al. use to explain ozone loss over the south polar region? Were Farman et al. correct? Explain.
3b. What did the "dynamical theory" propose? What evidence disproved this explanation for ozone loss?
3c. How did the "nitrogen oxide theory" explain ozone loss? What evidence disproved this theory?
1. Describe the characteristics of the polar night jet and polar vortex.
2a. What is the impact of the polar night jet on stratospheric polar ozone concentrations?
2b. How does the longevity of the northern polar night jet compare to the southern jet?
3a. Why do extremely cold temperatures develop in the polar vortex during the winter season?
3b. Do the coldest temperatures appear at all altitudes at the same time during the winter season? Explain.
3c. Characterize the differences between northern and southern polar temperatures. Will these temperature differences affect polar ozone losses?
4a. Why would you expect the Antarctic polar vortex to be highly contained?
4b. What evidence shows this containment?
4c. What impact does the potential vorticity have on the ozone hole?
5a. Explain how and why nitrous oxide (N2O) serves as an important tracer of stratospheric motion?
5b. How has the nitrous oxide been measured from space?
5c. What characteristics of atmospheric dynamics did the CLAES N2O measurements provide evidence for?
6a. What is Brewer-Dobson circulation?
6b. What causes this basic circulation pattern?
Describe the following characteristics of the Antarctic ozone hole.
1a. longitudinal and latitudinal symmetry
1b. October amounts as a function of time
1c. mobility with respect to the pole
Use Figure 11.31 to answer the following questions.
2a. What are the maximum partial pressure and total ozone amounts (in Dobson units) for September 2, 1994?
2b. How do these amounts compare to October 5 and 8 of the same year?
2c. What altitude (in km) in the atmosphere shows an ozone hole for October?
Refer to Figure 11.32 to answer the following questions.
3a. What do the white areas represent and what is the cause?
3b. What variables have the data been averaged over?
3c. What can you say about the average amounts over time in the tropical latitudes (-30° to 30°) compared to the polar amounts?
3d. What is the feature with amounts between 350 and 400 DU centered at approximately 60°S in October? What causes this feature?
Use Figures 11.33 and 11.34 to answer the following questions.
4a. Over what period during 1992 is an ozone low present?
4b. When is the hole at a maximum (in size and ozone low amount)?
4c. Are the 1992 amounts for August through December typical of the 1978-94 average? Explain.
In assessing the severity of the Antarctic ozone loss surface areas with ozone amounts less than 220 DU can be monitored.
5a. Why is the 220 DU contour a reasonable delineation of the ozone hole?
5b. Based on Figure 11.35, what time period shows the maximum surface area under 220 DU for 1992?
5c. When does the maximum surface area under 220 DU occur for 1996?
5d. Quantitatively, how do the maxima for 1992 and 1996 compare to the 1978-94 average amounts for the southern hemisphere surface area under the 220 DU contour?
6. Describe four sources of data that have been used to characterize the Antarctic ozone hole.
7a. What is the primary reason for the seasonal breakup of the Antarctic ozone hole?
7b. Do all vertical regions of the ozone hole recover at the same time? Explain why or why not.
1a. What is a polar stratospheric cloud?
1b. Discuss the basic characteristics of Type I and II PSCs
1c. What causes the more rapid sedimentation velocities of Type II PSCs and what consequences does this have for the southern hemisphere polar vortex?
1d. What months show the greatest frequency in Type II PSCs in the Antarctic? Why is this possible?
1e. How is the formation and growth of PSCs monitored?
1f. Is the duration and impact of Type II PSCs the same in the northern hemisphere? Explain.
2a. What is a fundamental difference between homogeneous and heterogeneous stratospheric processes?
2b. Why are heterogeneous processes so important to understand ozone loss?
2c. What are "denoxification" and "denitrification"? What part do they play in the development of the ozone hole?
3a. What chemical species accounts for the greatest ozone loss during the hole period?
3b. How is this species formed?
3c. What percentage of ozone loss is attributable to this species?
4. What two components are necessary to produce photochemical destruction of ozone through ClO-ClO catalytic processes? What role does each play?
Explain the role of each of the following on ozone loss:
5c. reservoir species, ClONO2 and HCl
6. What is the significance of enhanced ClO values in the southern hemisphere polar vortex measured during July, August, and September 1992.
6b. Why do the ClONO2 amounts in the polar vortex decrease from July to September in the same year?
7a. List the conditions necessary to form the ozone hole and what part each plays.
7b. Can an ozone hole form in the north polar regions? Explain.
1a. What did aircraft and ozonesonde data indicate about ozone loss in the Arctic from 1988 to 1995?
1b. Was an ozone hole comparable to that seen in the Antarctic present over the Arctic from 1988 to 1995? Explain.
2a. When were large ozone losses first recorded over the Arctic?
2b. What is the proposed explanation for the large Arctic ozone loss in 1996-97?
3. Explain the climatic differences between the Arctic and Antarctic that favor ozone loss in the southern hemisphere.
4. Explain what factors may dictate Arctic stratospheric ozone levels in the future and what impact each is likely to have.
5. What role did laboratory work play in developing a theory of ozone hole formation?
6. Why is there optimism about the disappearance of the ozone hole during the next century?
1a. The Antarctic ozone hole is a region of extreme ozone loss that has been appearing annually since the 1970s. The hole begins to develop each August and culminates by early October, subsequently disappearing by early December.
1b. October column ozone amounts on the hole region are now at least 50% lower than values seen in the 1970s
1c. Ozone profiles showed an ozone decrease occurring in approximately the 15-24 km region, i.e. the lower stratosphere during the August and September period.
2a. The minimum values of the October averages from these satellite instruments are included show excellent agreement with the Halley Bay ground-based measurements.
2b. Satellite instruments need to be "ground-truthed" to be sure the measurements correspond to ground-based measurements. This gives the values more reliability.
3a. Farman et al. suggested that these large depletions were a result of the uniquely cold temperatures over Antarctica combined with the increasing burden of chlorine in the stratosphere. While the suggestion that the ozone hole results from the cold temperatures and the increasing chlorine was eventually shown to be correct, the proposed mechanism was not correct.
3b. The dynamical theory of the ozone hole proposed that the Antarctic circulation associated with the Brewer-Dobson circulation was changing, and that ozone-poor air from the troposphere was being transported upward into the lower stratosphere. Evidence that disproved the dynamical theory came from N2O (and other long-lived trace gas) observations. The AAOE findings clearly demonstrated that air inside the lower stratosphere of the Antarctic polar vortex had indeed descended from the middle and upper stratosphere, in line with the Brewer-Dobson circulation. This meant that ozone amounts should have been higher.
3c. The nitrogen oxide theory of the ozone hole (proposed by Callis and Natarajan (1986)), suggested that large amounts of NOx were being produced during the solar maximum in 1979. The theory was disproved by satellite observations of total ozone that revealed no solar cycle variation in the depth of the ozone hole, such as would be expected if NOx amounts were varying with the solar cycle.
The Polar Vortex
1. The polar stratospheric regions of both hemispheres are surrounded by a narrow band or stream of fast-moving winds very high up blowing from west to east. Similar to the upper tropospheric jet stream, this stream develops along a zone of sharp temperature contrasts (gradients) that sets up along the line of polar night. This divides the region of the 6-month-long polar night from sunlight. This high altitude jet stream is referred to as the polar night jet, and it can reach speeds of 100 mph at altitudes of 70,000 feet. The polar vortex is the stratospheric region poleward of the polar night jet inside the region of polar night darkness. Temperatures become extremely cold inside this region, which is effectively isolated from the rest of the atmosphere. Temperatures inside the more isolated Antarctic polar vortex can drop to -90C or -130F at 20 km altitude. These exceptionally cold temperatures permit the formation of polar stratospheric clouds (PSCs), which play a key role in the heterogeneous chemical reactions of reactive chlorine that destroy ozone.
2a. The polar night jet acts as a barrier to transport between the polar stratosphere and the middle latitudes stratosphere, especially in the case of the southern hemisphere. Little mixing of air occurs between the inside and outside of the vortex. Ozone-rich air from the middle latitudes is unable to enter the polar vortex. Ozone loss reactions are able to occur unimpeded without replenishment of ozone-rich air.
2b. The southern polar night jet breaks up in October-December period, nearly 2 full months later in the spring period than the northern polar night jet.
3a. Such extremely cold temperatures develop in the polar vortex during the winter season because of the lack of sunlight, which causes the air to cool radiatively toware its equilibrium temperature, which without sunlight is quite cold. The lack of north-south mixing caused by the isolation of the vortex region allows this cooling to occur all winter long without any intrusions of warmer air.
3b. No. The coldest temperatures (under 192K or -81C) first appears at approximately 24 km altitude and gradually move downward as the winter season progresses. Hence, at higher stratospheric altitudes, the coldcst period is in early winter, while at lower stratospheric altitudes, the coldest temperatures are in late winter.
3c. Temperatures in the southern polar vortex are colder than in the northern polar vortex. The cold temperatures inside of the polar vortex are crucial to the large polar ozone losses. Polar stratospheric clouds (PSCs) form at extremely low temperatures (below approximately 195 K). Polar stratospheric clouds provide efficient sites for heterogeneous chemical reactions that ultimately result in ozone loss.
4a. Antarctic vortex ought to be highly contained because of the barrier set up by the sharp transition in potential vorticity values between the polar and mid-latitude regions. The containment occurs because a northward displacement of the very negative Antarctic potential vorticity will act to create a flow field that will displace this air back towards the south.
4b. The containment of the Antarctic vortex has been demonstrated with both trace gas observations (e.g., Schoeberl et al., 1989) and with modeling studies (e.g., Bowman, 1993).
4c. Potential vorticity generates a barrier to transport that prevents intrusions of ozone-rich midlatitude air into the ozone depleted region over Antarctica. In addition, nitrogen compounds that might inhibit ozone loss by reacting with chlorine are not transported over Antarctica. Polar ozone loss processes can occur undisturbed until the polar vortex breakup in late November.
5a. N2O has a very long lifetime in the lower stratosphere. Because N2O amounts fall off with altitude, the presence of air with high amounts of N2O in the lower stratosphere is an indication that this air has tropospheric characteristics and origins. Air with low amounts of N2O in the lower stratosphere is an indication that this air has upper stratospheric characteristics and origins.
5b. The CLAES (Cryogenic Limb Array Etalon Spectrometer) instrument which flies on the Upper Atmosphere Research Satellite (UARS) measures N2O. See Lecture 7 for details on techniques.
5c. The CLAES N2O observations provide clear evidence for: 1) strong downward motion into the polar vortex, 2) isolation of the polar vortex from mid-latitude influence, 3) vertical uplift in the tropics, and 4) confirmation that potential vorticity is an extremely important diagnostic of the ozone hole.
6a. Brewer-Dobson circulation consists of a large atmospheric cell that involves rising motion in the tropics and sinking motion in the polar region.
6b. The Brewer-Dobson cell is a result of weather systems ("waves") that propagate into the stratosphere from the troposphere. These waves deposit energy that act to decelerate the polar night jet, the existence of which is due to the tight temperature gradient at the edge of the polar night. A very weak poleward circulation develops to balance this wave driven deceleration of the polar night jet, thus maintaining the jet and producing the gentle uplift in the tropids and sinking motion in the polar region. An alternative way to think about it is to recognize that the deceleration of the polar night jet results in a thermodynamical and radiatifve imbalance in the polar region. Warmer air that intrudes inside the vortex quickly cools in the polar night. Cooling air sinks. Because of mass continuity requirements, air rises in the tropics. The result is the Brewer-Dobson circulation cell.
Structure and Dynamics of the Antarctic Ozone Hole
1a. The Antarctic ozone hole is generally longitudinally and latitudinally symmetric about the pole although the hole does elongate over time. While the wave is generally symmetric about the South Pole, there is an underlying north-south or latitudinal wave structure in the total field that becomes apparent when you subtract out the zonal mean field.
1b. Prior to 1980, the October average amounts in the polar region were greater than 280 DU, but these amounts have also decreased into the late 1980's and 1990's to amounts as low as 150 DU.
1c. The ozone hole tends to be highly mobile. A typical pattern that develops is the elongation of the ozone hole that slowly rotates eastward.
2a. The maximum partial pressure is16.5 mPa and the total ozone amount is 282 DU for Sept. 2, 1994.
2b. Both October 5 and October 8 show maximum partial pressure and total ozone amounts much lower. October 5 maximum partial pressure is about 7.0 mPa and total ozone is 102 DU. October 8 maximum partial pressure is about 9.0 mPa and total ozone is134 DU.
2c. The ozone hole is largely confined to the 14-22 km region over Antarctica.
3a. The white regions represents areas of no data because of polar night. Since there is no incoming sunlight it is impossible to measure backscattered UV light from ozone.
3b. The data are averaged over both time (1979-92) and longitude.
3c. From the figure, we can see that ozone amounts vary little over the course of the year in the tropics, while they vary considerably over both polar regions.
3d. This high ozone feature is the "collar" region. It reaches its largest amounts in late October as a result of the continual accumulation of ozone in the lower stratosphere that is driven by the poleward and downward transport of the Brewer-Dobson cell. This air is barred from moving into the polar vortex by the strong polar night jet.
4a. An ozone low is present from late August to late November 1992.
4b. The hole is at a maximum in size and low ozone amount in early October 1992.
4c. The 1992 amounts are similar to the 1978-1994 average, though they are somewhat lower than the white curve, which represents the average of daily minimum amounts for each date in the 1978-1994 period. However, the 1992 amounts are not at the bottom of the gray range, indicating that the all-time lowest ozone amounts did not occur during that year.
5a. This 220 DU contour is a reasonable representation of the ozone hole, since: 1) it cleanly separates the low total ozone from the high total ozone, 2) it is an amount of total ozone that was not observed over Antarctica prior to 1979, hence represents a region of real ozone loss with respect to the historic record, and 3) it is relatively insensitive to variations in absolute instrument calibration.
5b. For 1992, the September-October time period shows the maximum surface area under the 220DU contour.
5c. For 1996, the August to mid-October time period shows the maximum surface area under the 220 DU contour.
5d. Amounts in 1992 are more than double the average for 1978-1994.
6. Four sources of data used to characterize the Antarctic ozone hole include 1) the TOMS instrument, 2) balloon ozonesonde observations, 3) the POAM satellite instrument, and 4) the MLS satellite instrument. TOMS provides global coverage of total column ozone data, but it provides little information on the vertical structure of ozone, i.e., how ozone concentration varies through the atmosphere. Balloon ozonesonde observations provide information on the vertical structure for a particular location to about 20 km altitude. POAM satellite instruments provide ozone number density data in the 16 to 35 km region. MLS provides information on the vertical and horizontal structure of the ozone hole.
7a. The Antarctic ozone hole breaks up when the polar vortex breaks down, which in turn is driven by the reappearance of sunlight, hence warmer temperatures in the Antarctic stratosphere. Wind speeds drop steadily as the polar night jet disappears.
7b. The breakdown of the vortex begins first at higher altitudes (36 km in August), and gradually moves downward, with the breakdown at 16 km occurring in early December. The period of ozone recovery first appears at the higher altitudes, and gradually descends to lower altitudes.
Ozone Hole Theory
1a. A PSC is a cloud that forms in extremely cold and dry conditions that exist in polar night regions of the stratosphere.
1c. Type II PSCs are much larger than Type 1 PSCs (greater than 10 microns), hence they have a more rapid sedimentation velocity. This rapid sedimentation leads to the transport of reactive nitrogen and water that settle on the PSC particles out of the lower stratosphere. The removal of reactive nitrogen is known as denitrification and the removal of water is known as dehydration. Denitrification of the southern hemisphere polar vortex means that there is no NO2 to ract with ClO to form the reservoir species ClONO2 (chlorine nitrate), which means that the level of reactive chlorine remains high and more ozone destruction can occur.
1d. From Figure 11.43 we see that the months with greatest frequency in Type II PSCs in the Antarctic are in the middle to late southern winter, corresponding to August and September. This corresponds to the period of coldest temperatures inside the Antarctic polar vortex when temperatures fall below the 188K frost point required for Type II formation.
1e. The threshold formation and growth of PSC particles have been observed by balloon-borne measurements over Antarctica and by aircraft measurements. Satellite observations from the POAM II instrument provide evidence of PSCs inside the Antarctic polar vortex in winter, primarily in the intensely cold lower stratosphere during the depth of winter.
1f. No. While there is about a 2.5-month period during which temperatures are below the frost point required for Type II PSC formation, there are typically only a few days in the northern hemisphere during which time temperatures are below this frost point (188K). Thus, the duration and impact of Type II PSCs is much less in the northern hemisphere than in the southern hemisphere.
2a. Homogeneous processes involve chemical reactions in the gas phase only, while heterogeneous processes involve chemical reactions that occur on solid surfaces (such as PSC particles).
2b. Heterogeneous processes are so important to understand ozone loss because they convert reservoir species such as HCl and ClONO2 into active chorine species such as CLO that can destroy ozone.
2c. Denoxification refers to the process of sequestering reactive nitrogen. Denitrification refers to the process of removing reactive nitrogen from the stratosphere through sedimentation of PSC particles (that carry off the reactive nitrogen). Both denoxification and denitrification act to remove reactive nitrogen compounds from the stratosphere (either by sequestering them in nonreactive forms or carrying them completely out of the stratosphere). Without reactive nitrogen to act as a brake on reactive chlorine chemistry, the chlorine compounds are free to destroy ozone. Thus, goth denoxification and denitrification play a key role in the development of the ozone hole.
3a. The reactive chlorine species ClO (chlorine monoxide) accounts for the greatest ozone loss during the hole period.
3b. ClO is formed via the photolysis of Cl2 (molecular chlorine) as the Sun rises over Antarctica. Cl2 is easily photolyzed into two free chlorine atoms by even weak sunlight. The two Cl atoms are then free to engage in catalytic reactions, principally the ClO-ClO reaction, whereby an ozone molecule is destroyed and the Cl atom reforms itself.
3c. Approximately 40% of the ozone loss during the Antarctic ozone hole period is attributable to this species. About 50% of Arctic ozone loss is attributable to this species.
4. The two components necessary to produce photochemical destruction of ozone through the ClO-ClO catalitic process are 1) a source of chlorine, such as Cl2, and 2) sunlight (visible or UV). The Cl2 provides the source of chlorine, and the sunlight provides the necessary light that photolyzes Cl2 (molecular chlorine) into free chlorine atoms.
5a. PSCs provide the solid surfaces on which heterogeneous chemical reactions occur that lead to the denitirification and denoxification of the stratosphere. They also convert reservoir species of chlorine into reactive forms that can destroy ozone.
5b. Sunlight provides the energy source to photolyze chlorine or bromine containing species to release chlorine or bromine atoms.
5c. Reservoir species bind the chlorine in less reactive states so that it is not available to attack ozone as the sunlight returns to the polar regions in spring.
6a. These enhanced values indicate that chlorine was being released from reservoir species as a result of heterogeneous reactions on PSC particles. This led to the development of an ozone hole as increasing spring sunlight photolyzed the resulting chlorine and allowed destruction of ozone.
6b. ClONO2 amounts in the polar vortex decrease from July to September in the same year because this compound is a reservoir species that is being destroyed by heterogeneous chemical reactions on the PSC particle surfaces and its chlorine converted into more reactive forms.
7a. The necessary conditions for the formation of the ozone hole are: 1) temperatures cold enough to form PSCs, 2) vortex isolation to prevent intrusions ofozone rich air into the polar regions, 3) persistence of cold temperatures late into spring so that there is complete denitrification or denoxification of the stratosphere, both of which remove reactive nitrogen, and 4) sufficient levels of chlorine to generate catalytic destruction of ozone.
7b. Yes, but the extremely cold conditions required for PSC formation often do not occur, so the heterogeneous reactions are unable to occur.
1a. Total ozone amounts have been decreasing over the Arctic during the last two decades. The March observations show a clear downward trend between the 1970s and the 1990s. The March 1997 values are the lowest values on record for the 20 years of observations. The polar averaged March 1997 value of 354 DU is 21% lower than the pre-1990s value of 450 DU. Before 1996, most of the ozone loss observations and calculations showed a significantly smaller loss of ozone in the Arctic in comparison to the ozone losses observed over Antarctica.
1b. Based on observations, there was no "hole" comparable to the one in Antarctica present over the Arctic.
2a. The winters of 1995-96 and 1996-97 were the first winters to show very large ozone losses and a polar ozone low that was very similar to the one observed over Antarctica.
2b. The Arctic winter of 1996-1997 had the most stable and long-lasting vortex in the data record. A polar-centered vortex lasted in a relatively undisturbed state into late April.
3. In the northern hemisphere, large-scale topographic features, as well as greater land-seas temperature contrasts induce more frequent and intense planetary wave activity. This leads to more meridional mixing, hence temperatures do not get as cold in the Arctic as they do in the more isolated Antarctic. This favors ozone loss in the southern hemisphere since the colder temperatures allow for more frequent formation of PSCs.
4. Increases in greenhouse gases such as CO2 are expected to lead to warming at the surface. However, the opposite occurs in the lower stratosphere, where increased concentrations of greenhouse gases cause greater radiative cooling, hence lower temperatures. Recent work suggests that greenhouse gases may also modify the large-scale wave processes. Any weakening of planetary waves in the northern hemisphere will act to weaken the poleward and downward motion associated with the Brewer-Dobson circulation. It is this circulation that increases Arctic ozone levels and warms the lower polar stratosphere. The effect will exacerbate ozone losses because of the greater resultant cooling of the lower polar stratosphere, which in turn will lead to greater chlorine activation. This potential Arctic cooling is a serious concern, since chlorine levels are forecasted to begin to decrease any time now. As this decrease will occur very slowly, taking decades, the polar cooling may lead to severe ozone losses even as chlorine levels are decreasing.
5. Laboratory work determined key chemical reactions, and established probabilities of gas reactions with particles in PSCs. These lab observations were then combined with meteorological observations to produce simulations of the ozone hole.
6. There is optimism that he ozone hole will disappear sometime during the next century as chlorine concentrations decrease.