Session 6 Why Does The Ozone Hole Form?
Project 1: Written Arguments from Evidence
Response Paper, "The ozone layer over the Arctic shrank by a record amount this year [1994-1995] and the region may soon have a seasonal hole like the one over Antarctica." Quote from The New York Times, April 5, 1995, pC5
Under what conditions could an ozone hole form at the North Pole in the Arctic stratosphere?
Purpose: Communicate with others by a paper
Final, March 2, 1999
Helsinki, Finland, April 3 - "The ozone layer the Arctic shrank by a record amount this year, and the region may soon have a seasonal hole like the one over Antarctica," researchers said today. *Ozone levels in the Arctic are down about a third of normal levels,* said Esko Kyro, one of three Finnish Meteorological Institute researchers participating in the survey. *The situation is becoming increasingly reminiscent of the Antarctic,* he said in a telephone interview from an observatory in northern Finland.
Scientists say pollution has caused a seasonal depletion in the ozone layer over the southern Antarctic, which was detected a decade ago. The Northern Hemisphere had not been as seriously affected. But measurements in the last few months have shown 30 percent less ozone over Finland and Siberia, Dr. Kyro said. The Northern Hemisphere*s ozone layer thinned by 25 to 30 percent in 1991 and 1993.
*There*s no risk to people at the moment because the sun is still at such a high angle (*1), * Dr. Kyro said. *But what*s particularly worrying is that we*re seeing less and less ozone every spring.*
Extreme low temperatures and polar whirlwinds can sometimes cause natural thinning in the ozone layer for short periods of time. But scientists say these factors are not to blame for the recent thinning. The New York Times, Apr 4. 1994 p5
The article ended vaguely that "scientists say these factors are not to blame for the recent thinning", and did not say about the other factors to be blamed. So a reader who is aware of ozone depletion, come to do some research by oneself.
First, we need to know what the ozone layer and the hole is. There is a layer in the lower stratosphere ranged about 15 kilometer to 30 kilometer above the ground, which has a high concentration of ozone. This ozone layer is protecting Earth*s life-forms from a harmful ultra-violet light that, nowadays seemingly, everybody knows that we want to avoid exposure to. In this paper, I will not talk about ultra-violet light because the mission of this paper is to find more information about a possibility of an ozone hole in the Arctic, and to scrutinize the information that are available to us.
Planet Earth has this nice layer of blanket to shield the harmful light. The hole means that this ozone concentration is less or nearly zero (less than 100 Dobson Units ) in the column of atmosphere from top to bottom, so it is like a hole in the thick blanket. This ozone depleted hole is formed by a vortex that is a whirl wind (over 200 miles (320 kilometer) per hour) generated by a very cold temperature like below 200 Kelvin (-70*C). This extremely cold temperature could only happen over the pole, so the vortex is called a polar vortex. At such low temperatures, special high-altitude clouds form, called polar stratospheric clouds (PSCs) [Polar Stratospheric Clouds, University of Cambridge, http//www.atm.ch.cam.ac.uk/tour/psc.html]. The combination of these phenomenon, polar vortex and the clouds, is the biggest cause of ozone depletion. The stratospheric clouds are reservoir of the catalyst, chlorine. The chlorine is the spin-off of CFC*s (chlorofluorocarbons ). The total amount of chlorine in the stratosphere is relatively uniform worldwide at any given time. In the polar stratospheric clouds, the chlorine compounds: Cl2, that is in the gas phase, have been proliferated/propagated. For large scale ozone loss to occur, this chlorine compounds must be converted from inactive reservoir forms into active forms. In the presence of sunlight, this chlorine activation occurs, and produces active forms: chlorine radical, Cl and chlorine monoxide, ClO. Such chlorine compounds will engage in catalytic ozone destruction. The chlorine activation is believed to occur via heterogenious chemical reactions on stratospheric aerosols such as: HNO3, H2O, H2SO4 at temperatures below around 195K, [Kawa et al, 1997]. When sunlight hit these active forms of chlorine compounds, the chemical reaction of destroying ozone will occur. Sunlight plays two very important roles in the ozone depleting chemical dynamics. In general, sunlight heat things up, and please keep that in your mind. Then let*s explore some feature of the zone depleting chemical reactions.
* The chemical reactions occur in the stratospheric clouds:
ClONO2 (gas) + HCl (ice particle) * Cl2 (gas) + HNO3 (ice particle)
ClONO2 (gas) + H2O (ice particle) * HOCl (gas) + HNO3 (ice particle)
* The chlorine activation requires sunlight:
Cl2 + visible sunlight * 2 Cl
HOCl + UV sunlight * OH + Cl
* Finally, the chlorine attack ozone:
Cl + O3 * ClO + O2
In particular conditions such that the occurrence of polar vortex and clouds caused by extreme cold temperatures and presence of sunlight will propagate the reaction to destroy larger amounts of ozone than they generally would in warmer climates. So the major keys that form the ozone depleted hole are: the temperatures below about 195K, the polar vortex and clouds, and the presence of active forms of chlorine compounds, and sunlight (visible and ultra-violet light).
All the conditions above seems to occur in both polar regions. But stratospheric dynamic system is more complex over Arctic than it is in the Antarctica. The Antarctic ozone hole was found in 1985, and the theories are already established explaining the ozone depletion. What about Arctic? I gathered some information about the possibility that a ozone hole is forming in the Arctic.
As for Arctic, conditions in its atmosphere are similar to those in the Antarctic atmosphere, except that the Arctic atmosphere tends to be warmer than the Antarctic and the vortex of cold wind circling the pole holds together for a shorter period each winter. The differences between the two regions result in part from the larger land mass in the Northern Hemisphere, which causes more activity in the atmosphere.
In 1989, NASA*s Airborne Arctic Stratospheric Expedition, the first comprehensive research expedition to explore the Arctic region, found that the Arctic stratosphere in winter has almost as much chlorine monoxide (ClO) as is found in Antarctica, the same destructive chlorine that causes the Antarctic ozone hole.
Rowland commented in the 1990 Ambio article, that "ozone loss can certainly occur in the Arctic stratosphere, but extended time periods of major depletion in a single air mass are not very likely because the Arctic polar vortex does not maintain its integrity as tightly as over Antarctica." Antarctic and Arctic Ozone Holes, Reporting on Climate Change, July 1, 1997, http://www.nsc.org/ehc/guidebks/climbox5.htm.
In Executive Summary of Scientific Assessment of Ozone Depletion 1994, by NOAA (National Oceanic and Atmospheric Administration), I found the following information:
* Ozone losses have been detected in the Arctic winter stratosphere, and their links to halogen chemistry have been established. Studies in the Arctic lower stratosphere have been expanded to include more widespread observations of ozone and key reactive species. (NOAA, Executive Summary)
* Interannual variability in the photochemical and dynamical conditions of the vortices limits reliable predictions of future ozone changes in the polar regions, particularly in the Arctic. (NOAA, Executive Summary)
* If an extremely cold and persistent Arctic winter were to occur, then the ozone losses and UV increases could be larger in individual years. (NOAA, Executive Summary)
The above summary by NOAA was published in 1994. At the time that the summary was published there not much ozone depletion occurrence had been recorded over the Arctic yet. According to some information that I found, a remarkable ozone depletion occurred from 1996 to 1997. I found the article in the magazine Popular Science :
Thinning of ozone layer over the Antarctic in the 1980s led to the ban on ozone-damaging chemical, such as certain refrigerants and spray-can propellants. But those chemicals already released remain in the air for years, and researchers now are reporting potentially serious thinning of the ozone layer over the Arctic. The Antarctic is usually colder than the Arctic, causing more frequent stratospheric clouds of the type that lead to ozone depletion within the vortex. In 1996-1997, however, the Arctic vortex produced low temperatures and ozone depletion as great as that recorded in the Antarctic during the early 1980s.
The magazine closed with "Reductions in the northern ozone layer have also been reported by scientists at the U.S. National Oceanic And Atmospheric Administration (NOAA) and National Aeronautics and Space Administration (NASA).
Now I am sure that the potential of forming ozone hole in the Arctic is exist. Extreme low temperature and polar whirlwinds can occurs naturally and cause thinning in the ozone layer for short periods of time. But these are not the factors to be blamed. I found plentiful of the other factors to be blamed. Let me put the part of the article.
"Buildup of green house gases leads to global warming at the Earth*s surface, but cools the stratosphere. Since ozone chemistry is very sensitive to temperature, this cooling results in more ozone depletion in the polar regions," said Dr. Drew Shindell of Columbia University. Increasing Greenhouse Gases May be Worsening Arctic Ozone Depletion and May Delay Ozone Recovery, Internet cite: http://www.caliz.com/nasamail/Incoming/msg00101.html.
The chemical reactions responsible for stratospheric ozone depletion are extremely sensitive to temperature. Greenhouse gases warm the Earth*s surface but cool the stratosphere radiatively and therefore affect ozone depletion. [Shndell, et al. 1998]
It seems that the ozone concentration in the stratosphere is influenced in two ways by human activity in addition to natural, seasonal variations. The greenhouse gases are the possible cause of the 1996-97 anomaly in temperatures and of the resulted relatively stable Arctic polar vortex. Industrially produced chlorofluorocarbons are banned already, and that the world banning treaty had happened amazingly fast. While reducing the greenhouse gases globally is not happening yet. It is very simple to conclude that the rising temperature in troposphere by the greenhouse gases may continue to influence the dynamics of stratosphere, the more heat, the more energy input. I think that I would be able to say, Earth*s atmosphere have got more than enough energy to maintain the naturally stable climate. I learned the mechanism of hurricane in terms of its energy last year. Huge hurricane may have a great magnitude of energy. Atmosphere is mostly consist of gases and water vapors which has a great heat potential energy that is a microscopic kinetic energy. Convection that is a motion of an atmosphere is generated by the heat flow/transfer. The greater temperature gradient create the greater convection in atmosphere.
The Earth*s atmospheres is a very systematically woven blanket designed to absorb a little more than the right amount of solar energy. Later on the blanket emits that excess energy into space. The source of atmospheric energy is solar energy first, then the energy is absorbed by surfaces and redistributed by radiation, evaporation and condensation. These are energy transfer and vertical movement of air. The atmospheres have an ability to control and maintain energy input and output of whole system of Earth by itself. Contribution of greenhouse gases by human activities is disturbing this natural system. I think that this disturbance by human activities, in other words; energy input is way more than too much that the essential capability of Earth*s atmospheric system can take care of. So my responses to the question; Under what conditions could an ozone hole form at the North Pole in the Arctic stratosphere is: the greenhouse gas emission causes to cool stratosphere, and results creating more stable polar vortex; therefore the ozone depleting hole will occur.
I also think that the ozone hole is not a natural and transitory phenomenon any more because of that, in terms of energy in and out of Earth*s system, the balance of energy in and output is disturbed by human activities for decades. I may say that the Earth*s natural healing capacity is over loaded. Or I may say that the Earth*s system require the drastic change of it*s system and proceeding it by Earth*s natural healing ability. And the system may not include an extension of human being.
"Extreme low temperatures and polar whirlwinds can sometimes cause natural thinning in the ozone layer for short periods of time. But scientists say these factors are not to blame for the recent thinning." The New York Times, Apr 4. 1994 p5
Small amounts of stratospheric cooling can greatly increase ozone depletion. The Arctic stratosphere has gradually cooled over the past few decades. The well-known increase in greenhouse gas emissions is one possible cause of the observed trends. Popular Science, "Are Increasing Greenhouse Gases Creating an Arctic Ozone Hole?"
Theoretical models have predicted the possibility of increased Arctic ozone loss due to climate changes leading to a change in the dynamics and a colder Arctic stratosphere. "Will cilmate change affect Arctic ozone loss?", http://mls.jpl.nasa.gov/joe/cl_chl.html.
* "Increasing Greenhouse Gases May Be Worsening Arctic Ozone Depletion and May Delay Ozone Recovery", Caldwell Internet Express, April 8, 1998
* "Global Temperature Trends, 1996 Summation", NASA Goddard Institute for Space Studies, http://www.giss.nas.gov/research/observe/surftemp/1996.html.
* "The Science of the Ozone Hole", Center for Atmospheric Science, University of Cambridge, http://www.atm.ch.cam.ac.uk/tour/part3.html. (chemical process)
* "Science review highlights severe ozone loss in the Arctic and increasing UV levels over Canada", Environment Canada: http://www.ec.gc.ca/press/ozone5_b_e.htm.
* "Science review highlights severe ozone loss in the Arctic and increasing UV levels over Canada", Environment Canada, http://www.ec.gc.ca/press/ozone5_b_e.htm.
* "Surface Air Temperature Analyses", NASA Goddard Institute for Space Studies, http://www.giss.nasa.gov/data/gistemp/
* "Ozone, column ClO and PSC measurements made at the NDSC Eureka observatory (80*N, 86*W) during the spring of 1997", Geophysical Research Letters, 24, 2709-2721, 1997, http://www.agu.org
* Kawa S.R., et al., "Activation of chlorine in sulfate aersol as inferred from aircraft observations", J. Geophysical Research Letters, 102, 3921-3933, 1997
* Shindell et al. 1998, "Increased polar stratospheric ozone losses and delayed eventual recovery owing to increasing greenhouse-gas concentrations. Nature 392, 589-592
* Hansen et al. 1996, "Global surface air temperature in 1995: Return to pre-Pinatubo level." Geophysical Research Letters, 23, 1665-1668
* "Laser Radar Observations of the Arctic Stratosphere", The Web site of Thomas J. Duck, http://science.yorku.ca/units/phas/people/tduck/index.htm.
* Arctic Science Journeys, Winter Ozone Loss.
* "Measurements of gravity wave activity within and around the Arctic Stratospheric vortex", J.A. Whiteway, T.J. Duck, D.P. Donovan, J.C. Bird, S.R. Pal, and A.I. Carswell, Geophysical Research Letters, 24, 1387-1390, 1997