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It is well known that the Antarctic ozone hole is returning every polar spring, which is in October for the southern hemisphere. In the northern hemisphere, strong ozone depletion in the Arctic stratosphere was observed during several years in March. However, the building up of the ozone destroying halogen compounds starts already months before: In the polar winters, stratospheric temperature can fall below the threshold for formation of polar stratospheric clouds (PSCs), which is e.g. 195 K at an altitude of approx 19 km.

By heterogeneous reactions on the PSC-particles, the ozone-inert chlorine reservoirs (mainly ClONO2 and HCl) are converted into ozone destroying species (active chlorine, mainly Cl, ClO and ClOOCl). This chlorine activation is the prerequisite for the depletion of ozone which, after the return of sunlight in the polar spring, goes along with the ClO-Dimer cycle (Molina, 1987) and the ClO/BrO Cycle (McElroy, 1986). More information about the development of the ozone hole:

[The Ozone Hole]
[The Ozone Hole Tour]
[Stratospheric Ozone: An electronic textbook]

GOME observations of OClO
OClO (Chlorinedioxide) is exclusively formed as a product of the reaction from ClO with BrO (Sanders 1989, Toumi 1994). The amount of OClO in an airmass therefore gives a good indication of the degree of the chlorine activation (Schiller, Solomon, Tornkvist). Since OClO shows strong differential absorption features in the UV spectral range it can be detected by means of Differential Optical Absorption Spectroscopy (DOAS), see Platt 1994 and Wagner et al. 1999.

The GOME instrument onboard the ERS-2 satellite (Burrows et al., 1999) observes the light scattered back from the atmosphere with moderate spectral resolution. By applying the DOAS method to the GOME measurements, the integrated concentration of several trace gases along the light path, the so called Slant Column Densities (SCDs), can be derived.

Since GOME has a coverage of the polar regions every day, it gives a good overview of the intensity and the extension of the chlorine activation. Therefore, GOME OClO measurements can be used to monitor stratospheric chlorine activation (Wagner et al. 2001, Wilms-Grabe et al. 2002) and to study its dependency on meteorological parameters like the occurrence of PSCs, the location of the ploar vortex, etc.

In addition, the correlation to other trace gas species like for example BrO, NO2 and O3 can be studied. As OClO indicates the amount of ozone destroying chlorine compounds, the OClO SCDs correlate with the chemical ozone depletion for the respective Arctic winters.

Comparison between different winters
Wagner et al. (2001) have shown that the level of OClO anti-correlates with the stratospheric temperature inside the Arctic vortex and therefore, like these temperatures, shows a strong annual variability. The highest OClO SCDs above the Arctic since 1995 (the beginning of the GOME measurements) were found for the winter 1999/2000 (Wagner et al., 2002). For the Antarctic winters, GOME OClO SCDs are approx. 30% higher and are similar every year (Wagner et al. 2001, Wilms-Grabe et al. 2002).

For the Arctic winter 1999/2000, Kühl et al. (2002) have shown that the maximum SCD of OClO is in good agreement with the maximum ClO mixing ratio derived from the stratospheric chemistry transport model SLIMCAT. Also, it was found that in the Arctic stratosphere the time for deactivation of chlorine, which goes along by the reaction of ClO with NO2, depends on the degree of denitrification (raining out of nitrogen compounds) in the respective winter.

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Temporal evolution of OClO above the Arctic in all consecutive winters from 1995–2003. Shown are daily maximum SCDs, measured by GOME at a SZA of 90°. Minimum and maximum values for the Antarctic winters 1996-2001, shifted by 6 months for better comparison, are displayed as red envelope. Updated from Kühl et al. (2004a).

Mountain wave-induced chlorine activation

The efficiency of stratospheric mountain-waves for the activation of chlorine species is also seen in the GOME OClO measurements, which show a rapid increase during days with strong mountain wave activity.

Adiabatic cooling in mountain waves is especially important for the activation of chlorine species in the Arctic stratosphere (Carslaw et al. 1998, Dörnbrack et al. 2001), where the temperature is often near the threshold for formation of PSCs. An extraordinarily strong mountain wave event, leading to record low Arctic stratospheric temperatures, was reported for January 20-22, 1997 (Dörnbrack 1999, 2001)

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The figure above shows maps of OClO Slant Column Densities (SCDs) derived from GOME measurements for 20 to 22 January 1997. The solar zenith angle (SZA) ranges from 66 to 92 degrees from south to north. Due to the polar night, trace gases can not be observed at SZAs > 95 ° by GOME.

On 21 January, a substantial increase in the slant column densities (SCDs) of OClO is seen in the lee of the Scandinavian mountain ridge. On 22nd, the activated air masses have been transported to the east, in good agreement with trajectories.

Chemical ozone depletion in the Arctic stratosphere

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GOME Total O3 in March (63° to 90° N) in Dobson Units, taken from the official DLR level 2 data, plotted against the sum of the daily OClO maximum SCDs at SZA = 90° from February to March, for all Arctic winters observed by GOME.

In spring 2000, the peak of the vertical OClO (and ClO) profile was located at lower altitudes, leading to smaller O3 loss than expected (Wagner et al., 2002a, and references therein).
Wagner, T., C. Leue, K. Pfeilsticker and U. Platt, Monitoring of the stratospheric chlorine activation by Global Ozone Monitoring Experiment (GOME) OClO measurements in the austral and boreal winters 1995 through 1999, J. Geophys. Res., Vol. 106, 4971-4986,

2001 Wagner, T., F.Wittrock, A. Richter, M.Wenig, J.P. Burrows, and U. Platt, Continuous monitoring of the high and persistent chlorine activation during the Arctic winter 1999/2000 by the GOME instrument on ERS-2, J. Geophys. Res., Vol. 107, No. D20, 8267, doi : 10.1029/2001JD000466, 2002a