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Ozone O3

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Ozone – O3

The important role of ozone in the Earth’s atmosphere is attributed to the fact that it absorbs solar UV radiation which would otherwise reach the surface where it can cause damage to the biosphere. In the wavelength range below 290 nm, UV photons are almost completely blocked. Radiation from 290 nm to 320 nm is strongly attenuated so that dose levels on ground become harmless. Ozone does not only impact conditions at the bottom of the troposphere but also in the upper atmosphere through the effects of absorption of UV to IR radiation and subsequent heating. The heating produces a temperature profile which makes the stratosphere vertically stable. Even transport mechanisms in the layers above – the mesosphere and thermosphere – were found to be influenced by the energy content of the stratosphere.

 

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fig. 3-14

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Slices of the polar southern hemisphere ozone field at altitudes of approximately 20, 24 and 28 km on 27 September 2002 and 27 September 2005 as measured by SCIAMACHY. The observed split of the ozone hole in 2002 is not so obvious in the lower stratosphere around 20 km, but clearly visible at 24 and 28 km. In 2005, an ozone hole of 'normal shape' existed at all altitudes. (Graphics: C. von Savigny, IUP-IFE, University of Bremen)
   
 

In the year of ENVISAT’s launch, the ozone hole over Antarctica differed significantly from what had been observed before and after. Its extent was reduced in 2002 by 40% as compared to previous years. However, this did not indicate a recovery of the ozone layer but was actually caused by peculiar meteorological conditions where an unprecedented major stratospheric warming led to a split-up of the polar vortex, thereby interrupting the heterogeneous processes that usually lead to massive ozone destruction. A more detailed view of this September 2002 event was obtained by retrieving stratospheric profiles over Antarctica (von Savigny et al. 2005a). The vertically resolved SCIAMACHY limb measurements showed that the ozone hole split did not occur throughout the entire stratosphere but only above about 24 km. At 20 km there was still a single elongated area with low O3 values (fig. 3-14).

In normal cold Antarctic winters, however, the O3 profiles display a more regularly shaped ozone hole throughout the altitude range. The anomalous ozone hole in 2002 also developed quickly in time. This ozone hole split-up was already predicted in the 9-day ozone forecast at KNMI (see Eskes et al. 2005). The following years displayed again an ozone hole similar in size to those observed by SCIAMACHY’s predecessor GOME (fig. 3-15).

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fig. 3-15

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Time series of the size of the Antarctic ozone hole from 2000-2009 based on observations of GOME and SCIAMACHY. The area includes ozone column values below 30?S lower than 220 Dobson Units. (Graphics: TEMIS KNMI/ESA)
 


Over the Arctic, stratospheric temperatures are usually higher than over Antarctica, i.e. the polar vortex is less stable and PSC are a rare phenomenon. Thus, ozone depletion is not observed as regularly as in high southern latitudes at the end of the winter. However, unexpected cold northern winters change the situation, as was the case in 2005 (e.g. Bracher et al. 2005).
While most studies of the chemical ozone loss inside the polar vortices focused mainly on the northern hemisphere because of the strong inter-annual variability in the stability of the vortex and the following ozone loss, Sonkaew et al. (2010) also analysed ozone depletion inside the Antarctic polar vortex by using limb ozone profiles. They determined the chemical ozone loss via the difference between the observed vortex-average ozone abundance and the abundance modelled without considering chemical processes, but with including dynamically induced ozone changes.

 

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fig. 3-16

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Relative chemical ozone losses at the 475 K isentropic level (around 18 km) for the period 2002-2009 in the Arctic (dotted lines) and Antarctic (solid lines) polar vortices. (Graphics: IUP-IFE, University of Bremen)
 


Fig. 3-16 depicts the relative chemical ozone losses at the 475 K isentropic level – corresponding to an altitude of about 18 km – for the period 2002-2009 in the Arctic and Antarctic polar vortices. Several obvious differences exist between the two hemispheres. The chemical ozone losses in the Antarctic polar vortex do not vary much from year to year. Even in the anomalous year 2002 (see above), the relative ozone loss inside the vortex is similar to all other years. In the northern hemisphere, however, significant inter-annual variability exists, with some years, e.g. 2005 and 2007, exhibiting relatively strong chemical ozone losses and other years (2004, 2006) showing little or no ozone loss. (fig. 3-16)

Ozone depletion occurring during winter and spring in each hemisphere inside the polar vortices is a more localised phenomenon when compared to the global and continuous effects of anthropogenic halogen emissions on the stratospheric ozone layer. Again, O3 limb profiles have proven a valuable tool for investigating long-term trends in stratospheric ozone. Steinbrecht et al. (2009) determined upper stratospheric ozone trends for several latitude bands from 1979 to 2008 using ground-based LIDAR and microwave as well as satellite observations with SAGE II, HALOE, SBUV, GOMOS and SCIAMACHY. As demonstrated by Steinbrecht et al. (2009), ‘witnessing’ the recovery of stratospheric ozone requires long-term datasets. These are usually not provided by a single instrument but by a series of preferably similar sensors. Since SCIAMACHY is GOME heritage, combining GOME, GOME-2 and SCIAMACHY total columns from nadir measurements generates a unique repository. Loyola et al. (2009) formed a homogeneous dataset by merging O3 columns from June 1995 to August 2009. Measurements from over 70 globally distributed Dobson and Brewer ground stations served as validation reference. Since the GOME data record is very stable, it was used as a transfer standard and SCIAMACHY and GOME-2 data in periods of instrument overlap were adjusted accordingly. Global ozone trends were then derived by applying statistical methods, including the entire 60°N-60°S average serving as a near global mean. Fig. 3-17 illustrates how well the merged GOME/GOME-2/SCIAMACHY dataset of total ozone columns compares with results from the chemistry-climate model (CCM) E39C-A. This figure again displays the so-called ‘O3 anomaly’ which is the residual when subtracting the mean annual cycle from the satellite measurements. Apparently, the phase of minimum stratospheric ozone is just occurring and a recovery can be expected in the next decades.

 

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