Ozone loss over the South Pole, as measured in Dobson Units. (Source: KNMI/ESA)
The charted amount of ozone loss, charted per year over the last decade. (Source: KNMI/ESA)
Good news for those living in the southern hemisphere, the ozone hole may be shrinking
After reaching a record high in 2006, scientists at the European Space Agency (ESA) reports that the ozone hole decreased by nearly a third in 2007. The agency also warned that the decrease is not a sign of permanent recovery, based on their research.
Ozone (O3) is a compound found in Earth's upper atmosphere. It is found in high concentrations in the titular ozone layer which is part of the stratosphere. The ozone layer provides the essential function of protecting the Earth from harmful ultraviolet (UV) radiation.
The global ozone layer has been thinning at a rate of 0.3% per year. UV radiation causes genetic mutation in exposed tissues, which has led to an increased risk of cataracts and skin cancer as well as damage to marine life.
The largest region of ozone loss, recognized in 1985, has occurred over the South Pole. The hole over the south pole is defined by two means -- the area of the hole and the depth of the hole. In 2006, the hole reached a record 40 million square kilometers. This year the hole had dropped to 24.7 million square kilometers. It is significant to note that the ozone hole fluctuates greatly each year.
This year's ozone loss peaked at 27.7 million tonnes as opposed to 2006's loss of over 40 million tonnes.
Ozone concentration is measured in Dobson Units. The ESA provides the following definition of Dobson Units:
A Dobson Unit is a unit of measurement that describes the thickness of
the ozone layer in a column directly above the location being measured.
For instance, if an ozone column of 300 Dobson Units is compressed to
0º C and 1 atmosphere (the pressure at the Earth’s surface) and spread
out evenly over the area, it would form a slab of ozone approximately
3mm thick.
A common misconception is that areas that are part of an ozone hole contain no ozone. The atmosphere in ozone holes still contains ozone, though the concentration is greatly reduced. This reduction in ozone concentration leads to a drastic reduction in its ability to protect against UV radiation. Any area where the ozone concentration is less that 220 Dobson Units is considered part of the hole. This standard is used to define the yearly size of the hole over Antarctica. The thinnest area of this year's ozone hole had a concentration of 120 Dobson Units.
Ronald van der A, a senior project
scientist at Royal Dutch Meteorological Institute (KNMI) who helped
conduct the study explained why the results were not the sign of a
permanent recovery:
Although the hole is somewhat smaller than usual, we cannot conclude from this that the ozone layer is recovering already. This year's ozone hole was less centred on the
South Pole as in other years, which allowed it to mix with warmer air,
reducing the growth of the hole because ozone is depleted at
temperatures less than -78 degrees Celsius. The ozone hole is a seasonal phenomena. It begins in the winter months when winds called the Polar Vortext cut Antarctica's air mass off from mid-latitude warming air currents. The isolation of the air, combined with the darkness of winter leads to the temperature dropping dramatically. Dropping temperatures lead to the formation of stratospheric clouds containing chlorine and other halogens.
In the spring temperatures rise and these halogens begin to react with the ozone in destructive radical reactions. The halogens behave as radicals, species with a single valence electron. Radicals are typically highly reactive. The reaction breaks down ozone into O2. It does not consume the chlorine or other halogen, so the chlorine radical is free to initiate another reaction.
Because of this, a single chlorine atom in the atmosphere can destroy thousands of ozone molecules. A chlorine atom will remain in the stratosphere reacting for approximately two years; the time it takes for it to reenter the troposphere. Once it reenters the troposphere, it is sometimes reabsorbed by living organism who convert it to compounds such as hydrochloric acid -- frequently it is not and simply reenters the stratosphere via air currents.
The ozone loss continues annually until November or December, which are the summer months of the Southern Hemisphere. At this time the Polar Vortex is finally overcome by sunlight warming and ozone from the mid-latitude stratosphere flows in, replenishing the ozone layer and patching the ozone hole.
Ozone destroying compounds such as chlorine and bromine originate from man-made products like chlorofluorocarbons (CFCs). CFC production was banned by the Montreal Protocol in September of 1987, but the halogen compounds they generate linger in the atmosphere for decades.
The ESA tracks global ozone levels on a daily basis via its Envisat satellite. The satellite has three atmospheric instruments aboard -- SCIAMACHY, the global ozone monitoring by occultation of stars
(GOMOS) sensor and the Michelson interferometer for passive atmospheric
sounding (MIPAS). These instruments allow it to precisely tack ozone concentrations.
The ESA's data from Envisat is used as the basis of a near-real time ozone monitoring and forecasting service which is part of
the PROMOTE (PROtocol MOniToring for the GMES (Global Monitoring for
Environment and Security) Service Element) consortium, made up of more
than 30 partners from 11 countries, including KNMI.
These orginizations seek to both research and better understand ozone depletion and to provide people worldwide with information on the current state of the ozone.
While the study by KNMI and the ESA indicates that the ozone hole recovery of this year was likely simply a temporary trend, international cooperation and increased public awareness will hopefully eventually lead to recovery such as 2007's being a permanent trend.
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