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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 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 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
e, like the stratosphere, because it is trapped
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. They contain 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 months when 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. Sunlight is
required for catalytic removal of O3, but sunlight
also leads to the suppression of increased
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 work to break up ozone
molecules or attach onto oxygen atoms, preventing the O2 and O from creating ozone,
and thus producing ClO and O2 (Rowland, 2006).
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 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 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
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).
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. 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 (Newman et al., 2006). The detection of the
diminishing ozone hole and reduced ozone amounts 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
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) 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.
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______________________