Ozone and the Ozone Hole

Heather Raven and Stefanie Spayd

Dr. Lisa Goddard and Dr. Mark Cane

December 10, 2006

Ozone and the Ozone Hole

The abundance of ozone (O3) in the atmosphere has been a hot topic ever since it was confirmed in 1985 (Weatherhead and Andersen, 2006) that Chlorofluorocarbons (CFCs) played a part in depleting the ozone layer. Many factors play into ozone depletion and formation and will be discussed in this paper: location of ozone depletion, the role of greenhouse gases, chemistry, sunlight, and temperature.

The Arctic and Antarctic vary slightly concerning their abundance of ozone. The Arctic experiences warmer temperatures than the Antarctic with more variation between ‘warm’ and ‘cold’ winter seasons (Tilmes et al., 2006). The North Pole is affected more by atmospheric dynamics such as strong planetary waves, transport of ozone poleward, Arctic and North Atlantic oscillations,and an unstable and small polar vortex in winter (Weatherhead and Andersen, 2006). A smaller polar vortex volume of air contains less cold air and less polar stratospheric clouds, where ozone depletion is promoted. The attributes of the Arctic result in higher column ozone concentrations than in the Antarctic and faster re-supply of ozone.The Antarctic is the topic of ozone debate[g1]because: the temperature is generally cooler than the Arctic during winter (greater volume of air exists below the polar stratospheric cloud threshold temperature explained in a subsequent section) promoting the loss of ozone, variability from year-to-year is small, the polar vortex is more stable and larger than in the Arctic (Tilmes et al., 2006). It is therefore ‘easier’ to study ozone changes in the more stable environment of the Antarctic where great change in ozone is already expected due to CFC emission and global warming.

Greenhouse Gases and Chemistry

Greenhouse gases, such as chlorofluorocarbons (CFCs) and carbon dioxide, are released into the atmosphere at an excessive rate [g2]by humans. CFCs have stopped being released since 1989, due to the Montreal Protocol which many nations signed, saying that they would stop producing products with CFCs in them. The United Nations created this protocol because of scientific evidence, particularly a study conducted by Molina and Rowland (1974), that suggested the CFCs were exacerbating a depletion of atmospheric ozone in the stratospheric ozone layer, leaving an “ozone hole” over Antarctica. CFCs and other greenhouse gases affect the ozone layer in several different ways. Carbon dioxide, for example, is a greenhouse gas that blocks outgoing longwave radiation from escaping out of the atmosphere. When this happens, less radiation reaches the upper parts of the atmosphere, like the stratosphere, because it is trapped [g3]below the layer of greenhouse gases in the troposphere. So, while they are warming the surface of the Earth and the troposphere, they are effectively cooling the lower stratosphere. This affects ozone depletion by helping to create a perfect environment for the formation of polar stratospheric clouds (PSCs). These clouds form only under very cold conditions below a particular threshold, 195 Kelvin.Theycontain high levels of nitric acid (HONO2) and frozen water (H2O) when they are trapped in the polar vortex of the atmosphere over Antarctica during the cold, dark winter monthswhen the southern jet streams circle around Antarctica (Rowland, 2006). Other molecules, like hydrochloric acid (HCl) or chlorine nitrate (ClONO2), can also be present in these clouds. The reservoirs of HCl are converted to ClO by reactions on the surfaces of PSCs (Salawitch, 1998). Here, throughout the winter, while there is no sunlight for months, chemical reactions quickly take place, separating the chlorine out into chlorine molecules (Cl2) or combining it with water to form HOCl and more nitric acid, but not letting the chlorine molecules themselves separate into chlorine atoms. Sunlightis required for catalytic removal of O3, butsunlight also leads to the suppression ofincreased concentrations of ClO (Salawitch, 1998). Ultraviolet rays from the sun, in the Antarctic springtime, can break up the Cl2 molecules, leaving two separate chlorine atoms. Chlorine is highly reactive and can workto break up ozone molecules or attach onto oxygen atoms, preventing the O2 and O from creating ozone, and thus producing ClO andO2 (Rowland, 2006).[g4]

Sunlight

Sunlight plays another very important role. In the same way that Ultraviolet (UV) rays from the sun break up Cl2 molecules, they also break up O2 and O3 molecules (Rowland, 2006). If there are O2 molecules present, and sunlight to make the split, oxygen molecules [g5]can come together to create ozone. If there is no sunlight, then ozone cannot be split, neither can oxygen molecules. This means that only preexisting oxygen atoms or those that are released due to other chemical reactions when there is no sunlight or UV rays present, can combine to form ozone during the dark winter months. There is generally an increase in ozone over the winter months [g6]and a severe and rapid decrease as soon as the sunlight shows over the horizon in the spring.

In conclusion, sunlight plays a very important role in the chemical feedbacks associated with ozone: ozone depletion, and ozone creation. Without UV [g7]radiation at the poles, there is a net gain in ozone in the upper stratosphere. However, when the sun first comes over the horizon in the springtime, ozone depletion begins as chlorine atoms are released, attacking oxygen molecules and atoms to effectively slow the replenishing of ozone and the ozone layer.

The Figure shows expected chlorine and O3 trends from model outputs. (Weatherhead and Andersen, 2006).

Temperature

Greenhouse gases (GHGs) as well as natural and anthropogenic-related chemical reactions contribute greatly to the temperature of the stratosphere. We are mainly concerned with the lower stratosphere, which is cooling at a rate of about 0.25 K/decade (IPCC, 2001; as referenced in Newman et al., 2006). The cooling is a result of greenhouse gas emissions preventing longwave radiation from traveling through the stratosphere and out to space.[g8] The radiation is absorbed by the GHGs as mentioned in the section above, and cools the lower stratosphere. Because of the increased incidence of PSCs in cooler lower-stratospheric regions, ozone depletion is enhanced (as mentioned above, through depletion of ozone by chlorine in numerous chemical reactions). The cooling not only contributes to ozone depletion but will likely delay detection of a decrease in the ozone hole in the future [g9](Newman et al., 2006). The detection[g10] of the diminishing ozone hole and reduced ozone amounts [g11]are affected by the magnitude of cooling, impact of climate change on stratospheric circulation, whether the “past represents the future” in terms of ozone loss rates, and the affect of aerosols in the atmosphere (Newman et al., 2006). In the study by Newman et al. (2006) they state that temperature, along with chlorine abundance, primarily controls the area of the ozone hole; it can be deduced that temperature is then one of the main factors in the recovery of the ozone hole.

Temperature in the stratosphere is also influenced by the stability of the wintertime polar vortex circulation over the Arctic and Antarctic (Weatherhead and Andersen, 2006). The polar vortex, when stable, allows cold temperatures to persist for long periods of time and can lead to the formation of PSCs and further decrease ozone concentrations. When the polar vortex is less stable, higher levels of ozone can be seen over the poles (Weatherhead and Andersen, 2006), likely due to less persistent cold temperatures.

Recovery

It is estimated, in the current literature using model outputs, that full ozone hole recovery will take place by about 2068 (Newman et al., 2006). This best-guess is 18 years over the estimate of the WMO in 2003 that the ozone hole will recover in 2050, likely because the current studies use new estimates of equivalent effective Antarctic stratospheric chlorine in their model runs. The Newman et al. (2006) study also predicts that ozone hole recovery will first be statistically detectable in about 2024.In contradiction to studies that predict a date for recovery, Weatherhead and Andersen, 2006, acknowledge that ozone levels may never stabilize at pre-1980’s levels of ozone (prior to CFC production) because of changing influences on ozone formation/depletion such as temperature. It is unsure how temperature and atmospheric dynamics will change with global warming, so it is difficult to determine the year of ozone hole recovery or rate of recovery.

Short term influences on ozone abundance, such as solar activity, affect the detection of recovery of the ozone layer (Weatherhead and Andersen, 2006). This variability is difficult to account for in model predictions that use long timescales. Models must attempt to account for all feedbacks evident in ozone formation and depletion(chemical, radiative, and dynamical)[g12]in order to successfully predict future impacts on ozone abundance. These three feedback types are known to affect ozone in the short and long term but the extent of some of the affects are still unknown (Weatherhead and Andersen, 2006). The fate of ozone recovery lies in anthropogenic impacts from greenhouse gas output and inevitable climate change from the damage we have already done.

• The authors of this paper highly recommend reading a scientific paper by Stolarski et al., 2006, (reference below) concerning the current increase in summer ozone seen over the Antarctic as a result of the spring ozone hole. It is a very interesting study on a seemingly beneficial outcome of the ozone hole, and is also very current since it was released in November, 2006.
• The authors also highly recommend the paper by Rowland, 2006, (reference below) since it has a very detailed yet ‘user friendly’ description of all the chemical reactions and factors that affect stratospheric ozone. It is an interesting read and very beneficial when an overview of ozone is needed. It is also a new and up-to-date paper as it was released in February, 2006.

[g13]

References

Newman, P.A., Nash, E.R., Kawa, S.R., Montzka, S.A., Schauffler, S.M. (2006) When will the Antarctic ozone hole recover? Geophysical Research Letters, 33, L12814: 1-5.

Rowland, F.S. (2006) Stratospheric ozone depletion. Philosophical Transactions of the Royal Society B, 361: 769-790.

Salawitch, R.J. (1998) A greenhouse warming connection. Nature, 392: 551-552.

Stolarski, R.S., Douglass, A.R., Gupta, M., Newman, P.A., Pawson, S., Schoeberl, M.R., Nielsen, J.E. (2006) An ozone increase in the Antarctic summer stratosphere: A dynamical response to the ozone hole. Geophysical Research Letters, 33, L21805: 1-4.

Tilmes, S., Müller, R., Engel, A., Rex, M., Russell III, J.M. (2006) Chemical ozone loss in the Arctic and Antarctic stratosphere between 1992 and 2005. Geophysical Research Letters, 33, L20812: 1-5.

Weatherhead, E.C., Andersen, S.B. (2006) The search for signs of recovery of the ozone layer. Nature, 441: 39-45.

We, Heather Raven and Stefanie Spayd, do hereby claim that we are the two authors of the written material contained in the ozone document and all references have been awarded due credit.

Signature: ______Heather Raven______

Signature: ______Stefanie Spayd______

1

[g1]Concern?

[g2]This is a judgemental term. How about “unnatural”?

[g3]This is NOT the cooling effect on the stratosphere. GHG cooling occurs IN the stratosphere.

[g4]Good

[g5]An oxygen molecukke can combine with an oxygen atom. Oxygen molecules do not comine with each other.

[g6]This is dye to the continual production of ozone in the tropics and subsequent transport to high latitudes.

[g7]Without UV, there is no ozone.

[g8]NO – see 1st page

[g9]Why?

[g10]Why do these items listed in this sentence impact “detection”?

[g11]Aren’t these two phrases contradictory?

[g12]Radiative feedback has not been mentioned here. Also, not that these feedbacks relate to cold temperatures not O3.

[g13]I like the recommendations.