OZONE DEPLETION AND THE ARCTIC Please see this previous post about the ozone hole over Antarctica and its effect on the countries bordering it (including increased skin cancer rates):http://on.fb.me/SMU6uC To recap somewhat, ozone is a gas made up of three oxygen atoms (O3), which occurs naturally in trace amounts in the stratosphere (the upper atmosphere) and protects life on Earth from the Sun’s ultraviolet (UV) radiation. The stratospheric ozone layer screens all of the most energetic, UV-c, radiation, and most of the UV-b radiation. Ozone only screens about half of the UV-a radiation. Stratospheric ozone is typically measured in Dobson Units (DU), the number of molecules needed to create a layer of pure ozone 0.01 millimetres thick at a temperature of 0 degrees Celsius and an air pressure of 1 atmosphere. Earth’s atmosphere has an average amount of ozone of 300 Dobson Units. Ozone depletion is caused by chemicals called chlorofluorocarbon compounds (CFCs) known as "freons" and bromofluorocarbon compounds known as Halons, which escape into the atmosphere from refrigeration and propellant devices and processes. These chemicals are so stable that they can persist for decades within the lower atmosphere, but in the stratosphere ultraviolet light breaks the chemical bond holding chlorine to the CFC molecule. This destruction of the ozone does not happen immediately; these roaming chlorine molecules become part of two chemicals that are so stable that scientists consider them to be long-term reservoirs for chlorine, under normal atmospheric conditions. The atmospheric conditions over Antarctica during winter are unusual however; there is an endlessly circling whirlpool of stratospheric winds called the polar vortex that isolates the air in the centre. As there is no sunlight in Antarctica over winter, the air in the vortex gets so cold that clouds form and chemical reactions take place which could not take place anywhere else. These reactions can occur only on the surface of polar stratospheric cloud particles, as their frozen crystals provide a surface for the chemical reactions that free chlorine atoms in the Antarctic stratosphere. The inactive chlorine chemicals are converted into more active forms like chlorine gas (Cl2). When sunlight returns to the South Pole, the UV light breaks the bond between the two chlorine atoms which releases free chlorine into the stratosphere. The free chlorine molecules are then responsible for a series of chemical reactions that destroy ozone molecules but return the free chlorine molecule unchanged and free to do more damage (a catalytic reaction). There is also a second catalytic reaction with chlorine that contributes a large fraction of ozone loss, which involves bromine. The ozone hole grows until temperatures warm enough that the polar vortex weakens, enabling air from the surrounding latitudes to mix with the air in the polar vortex. The ozone-destroying forms of chlorine are then dispersed, until the following spring. The Montreal Protocol on Substances that Deplete the Ozone Layer, which was signed in 1987, limited production of ozone-depleting substances. All non-essential products containing CFCs were banned; these included all aerosol products, pressurised dispensers and foam products. All CFC containing air conditioning and refrigeration appliances were also banned in 2001. Hydrofluorocarbons (HFCs) were used to replace chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) for many uses, for example solvents or refrigerating agents. This in turn was partly responsible for the increased concentration of HCFCs in the atmosphere. HFCs were used as they do not pose any harm to the ozone layer, having no chlorine. HCFCs and HFCs are now thought to contribute to anthropogenic climate change, as these compounds are capable of trapping enormous amounts of infrared radiations in the atmosphere. They are thought to be up to 10,000 times more potent greenhouse gases than carbon dioxide on a molecule-for-molecule basis. The Montreal Protocol currently calls for a complete phase-out of HCFCs by 2030, but does not place any restriction on HFCs. In the first three months of 2011, a large hole appeared in the ozone layer above the Arctic. Every year the Arctic ozone layer had suffered some damage over the winter period but the effect was normally short-lived, as there are generally warmer stratospheric conditions over the Arctic than over the Antarctic. The Arctic polar vortex is also about 40 percent smaller than a typical Antarctic vortex, but is more mobile. In 2011, between 18 and 20 kilometres above the ground, over 80 per cent of the existing ozone was destroyed. The ozone loss in 2011 over the Arctic was twice the levels seen in 1996 and 2011, previously the highest records. The hole was similar in size to the holes seen over Antarctica in the 1980’s. Please read the previous post for more information on the Antarctic ozone hole (http://on.fb.me/SMU6uC). Though the Arctic ozone hole would have allowed in more UV radiation than before, it is unlikely this would have added much risk to the underlying population's risk of UV-related cancer. Scientists are now examining why the hole over the Arctic grew so large. It is possible this occurred because the stratosphere remained cold for several months longer than usual. This cold air then allowed water vapour and nitric acid to condense into polar stratospheric clouds, which as explained above, allow chlorine to convert into chemically active forms. It is unknown why the stratosphere remained cold for so long, though climate change could be responsible. Global warming occurs only at the bottom of the atmosphere and warms the surface but cools the stratosphere. The Intergovernmental Panel on Climate Change (IPCC) concluded in 2007 that there has been global stratospheric cooling since 1979, but it is not yet clear whether this is a result of climate change. Without the 1987 Montreal Protocol however, chlorine levels would be so high that an Arctic ozone hole would form every spring. The ozone-depleting chemicals that are already in the atmosphere mean that the ozone hole over Antarctica and possible future Arctic ozone loss will continue for decades. If an ozone-depletion area over the Arctic forms which is similar in size to the one over the South Pole, over 700+ million people, wildlife and plants could be exposed to dangerous UV ray levels. You can watch a video showing Arctic ozone loss 2010-2011 here:http://www.youtube.com/watch?v=aNNRjbBK1Ns. The maps used of ozone concentrations over the Arctic come from the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite. The left image shows January 1 to March 23, 2010, and the right shows the same dates in 2011. March 2010 had relatively high ozone, while March 2011 has low levels. The image consists of maps of ozone concentrations over the Arctic from the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite. The left image shows March 19, 2010, and the right shows the same date in 2011. March 2010 had relatively high ozone (the red colour denotes high DU), while March 2011 has low levels (blue and purple show low DU). -TEL Arctic ozone hole: http://www.nature.com/nature/journal/v478/n7370/full/nature10556.html; http://www.newscientist.com/article/dn20988-arctic-ozone-hole-breaks-all-records.html; http://www.theozonehole.com/arcticozone.htm; http://www.theozonehole.com/nasaarctic.htm; http://earthobservatory.nasa.gov/IOTD/view.php?id=49874 http://ozonewatch.gsfc.nasa.gov/facts/hole.html More on ozone layer protection: http://www.epa.gov/ozone/strathome.html You can read the Montreal Protocol here: http://ozone.unep.org/pdfs/Montreal-Protocol2000.pdf Image: Rob Simmon, with data courtesy of Ozone Hole Watch.