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The biggest ozone losses ever recorded over the Arctic have shown measurements carried out by an international network of over 30 ground-based stations and satellite- borne sensors during January-March 2011. We study whether this was an exceptional event or whether it is the first appearance of an ozone hole in the Arctic. The main finding is that this winter's likely record-breaking ozone loss has occurred thanks to the extremely low stratospheric temperatures that are linked to climate change, i.e. the coldest winters at the Arctic region have been getting colder and leading to larger ozone losses there, which have progressively reached to the limit levels of the Antarctic ozone.
In the early days following the discovery of the ozone hole in the Antactic spring relatively little attention was paid to the question of whether a similar phenomenon might exist in the Arctic. The ozone hole is not actually a hole but a region of heavily depleted ozone in the atmosphere that is defined, slightly arbitrarily, as a region where the total ozone column (TOC) is less than 220 DU (Dobson Units, 1 Dobson Unit is defined to be 0.01 mm ozone thickness at stp); the hole only survives for a few months. The Antarctic ozone hole was first discovered from Dobson specrophotometer data (Chubachi 1985, Farman et al. 1985), but its extent and temporal evolution is continuously monitored from satellite data obtained from the Total Ozone Monitoring Spectrometer (TOMS) and its successors, such as Microwave Limb Sounder-MLS, and ozonesonde ascents (Manney et al. 1997, Antón et al. 2009).
After a while serious studies of TOC in the Arctic spring began to be undertaken and a similar, but smaller, ozone hole was found there too. In the spring of 2011 a severe loss of ozone in the Arctic was observed and in this Letter we study the question of whether this was an exceptional event or whether it is part of a trend towards the size and depth of the Antarctic ozone hole.
2. The formation of the ozone holes
Every winter, a vortex of naturally cold air swirls around the South Pole, causing the formation of very tenuous clouds (polar stratospheric clouds or PSCs) that are composed of ice crystals. The chemical reactions that deplete ozone take place at the surface of these clouds and the 'ozone hole' begins to appear. By late spring, when temperatures begin to rise, the ozone layer starts to recover. The ozone hole appeared first over the colder Antarctic because the ozone-destroying chemical process works best in cold conditions (below -78°C). In fact, the stratosphere in the southern hemisphere is about five degrees Celsius colder than in the northern hemisphere and the PSCs persist for longer periods. In addition, the natural ozone levels in the Arctic spring are much higher than in the Antarctic spring. Therefore the ozone depletion over Antarctica is much more pronounced than over the Arctic. A scenario however, about the formation of even a moderate ozone hole above the Arctic region could be a much more pertinent problem for the greater populations in the middle and high latitudes of the Northern Hemisphere. Therefore the question "is Arctic stratospheric ozone presently undergoing severe depletion?" is of great importance.
A crucial issue on the polar stratospheric ozone depletion problem is the quantitative assessment of ozone changes by chemical and dynamical processes, separately. The former involve reactions of chlorine and bromine species that originate in the man-made chlorofluorocarbons and the latter caused by advection and mixing through the vortex boundary.
A number of methods have been developed for separating the chemical and the dynamical influences on the polar ozone loss. One such method is known as ''Match'' (e.g. von der Gathen et al., 1995), which uses pairs of ozone profiles (''matches'') obtained at separated locations but identified by trajectory analysis to have traced the same air mass. In simple words the idea of Match is to probe, i.e. to determine the ozone content of, a lot of air parcels twice during their way through the atmosphere.
By this approach, which was initially applied to ozonesonde ascents over the Arctic region (von der Gathen et al., 1995), the dynamical component in ozone changes can be neglected and only the chemical component remains between the first and the second observations of each matching ozonesonde pair, so that the chemical ozone loss rate and amount can be estimated quantitatively in several polar winters (Schulz et al., 2000).
A few years later, the Match method was also applied to the ozone profile data obtained with the satellite-borne sensor, Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) (Sasano et al., 1999, 2000; Terao et al., 2000).
The advantage in using the satellite than the ozonesonde data over the Match analysis is that satellite can make measurements with the same quality homogeneously over an entire narrow zone at high latitude and its multiple sensors can provide observations not only regarding ozone but also nitric acid, passive tracers, aerosol extinction coefficient and so on.
The results obtained from the Match technique showed that the unexpended persistent cold temperatures during late September and early March 2011 over the Arctic region have caused 50% depletion of the ozone layer, which protects the biosphere of our planet from the harmful solar ultraviolet radiation (SUVR) (Alexandris et al. 1999, Katsambas et al. 1997, Kondratyev and Varotsos 1996, Varotsos et al. 1995). These ozone poor air masses that are now inside the polar vortex will spread later on over the most densely populated mid-latitude regions offering reduced SUVR protection. This will happen in a few weeks when the polar vortex will become enough attenuated and then destructed.
This record ozone loss in the Arctic region has been observed by both the satellite-borne instrumentation and the international network of over 30 ozone sounding stations spread all over the Arctic and Subarctic and coordinated by the Potsdam Research Unit of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association (AWI) in Germany. The characteristic feature of this winter-spring season is the cold and stable circumpolar vortex with persistent polar stratospheric clouds. These clouds provoke further decrease in temperature of the captured circumpolar air, in which the heterogeneous chemical reactions that are taking place lead to the ozone depletion.
In this regard Fig. 1 shows that the TOC averaged over the area covered by the polar vortex (derived from the Ozone Monitoring Instrument or OMI flying aboard the NASA Earth Observing System or EOS -Aura satellite platform), is currently about 300 DU and falling by roughly 2-3 DU per day.
Figure 1. The total ozone averaged over the area covered by the polar vortex over the Arctic is currently about 300 DU, and in some regions almost 220 DU (the boundary of the Antarctic ozone hole) (ftp://toms.gsfc.nasa.gov/pub/omi/images/npole/Y2011/)
3 The relation to previous years
To relate the present situation quantitatively to Arctic ozone depletion in previous years the Fig.2 (top panel) is shown. Inspection of this figure shows that polar ozone during recent Arctic winters remains low compared with values observed during the 1980s. In addition, Arctic winter and spring ozone loss between 2007 and 2010 remained in a range comparable to the values since the early 1990s. Chemical ozone destruction on the order of 100 DU (about 80% of the values derived for the record cold winters of 1999/2000 and 2004/2005) is deduced for both Arctic winters 2006/2007 and 2007/2008 (Rex et al., 2006).
Reliable ozone loss estimates are not possible for the Arctic winter 2008/2009 because a strong midwinter warming in late January led to extensive mixing of air from low latitudes with the polar vortex air (WMO 2010). However, as mentioned above the total ozone in the polar vortex is currently about 300 DU and falling by roughly 2-3DU per day, which means that the total ozone value in 2011 is already the smallest total ozone value ever observed over the Arctic region.
Furthermore, to relate the present situation, more generally, to the Antarctic ozone hole the time series of the minimum total ozone over the polar cap, for October in the Antarctic is depicted in Fig.2 (bottom panel). Over Antarctica the line with a constant value of 220 DU is used as the boundary of the ozone hole area that happens at the beginning of spring (Aug-Oct). This is because total ozone less than 220 DU is a result of the ozone loss from chlorine and bromine compounds.
Figure 2. Time series of minimum total ozone over the polar cap, for March in the Arctic (top panel) and October in the Antarctic (bottom panel) (WMO 2010).
Figure 3. Evolution of vortex air volume with PSC (VPSC) for the Arctic over the past four decades. The blue dots represent the maximum values of VPSC during five-year intervals. The dotted line is based on radiosonde analyses, and the solid line is ECMWF data. The gray shading represents the VPSC error assuming a 1K uncertainty of the long-term temperatures stability (WMO 2010).
Figure 3 illustrates the long term evolution of the vortex air volume at temperatures
below the PSCs threshold (VPSC) over the Arctic region, where a cooling of the "cold" Arctic winters is evident. It is notable that on a statistical basis established over the past four decades, a new maximum occurs about only once in five-year intervals. Thus the winter in 2011 corresponds to an upcoming maximum value of VPSC and perhaps a record one.
4. Conclusion: ozone depletion and climate change
Although the interactions between the ozone layer and climate change have not been completely understood as yet, the already released chlorofluorocarbons (despite the binding controls by the Montreal Protocol on their production and consumption) will continue to deplete the ozone layer until many decades from now (Cracknell and Varotsos 2007). The degree of the Arctic ozone depletion essentially depends on the temperature at an altitude of around 20 km and is thus linked to the development of the Earth's climate. For example, during September 2002, the ozone hole over Antarctica was much smaller than in the previous years. Apart from its smaller size it has split into two separate holes, due to the appearance of major sudden stratospheric warming that has never been observed before in the southern hemisphere (Varotsos 2002).
Current projections based on observations and modeling suggest that climate change may lead to large cooling of the stratosphere (as a result of rising greenhouse gas concentrations), leading to more extensive and more frequent PSC formation and greater ozone loss (directly related to the severity and persistence of the Arctic winter) in the future. Thus, ozone layer recovery may not track the slow decline of industrial halogen compounds in the atmosphere (by-products of CFCs released during prior decades)