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OClO (chlorine dioxide) is an important indicator for stratospheric chlorine activation, the prerequisite for massive ozone destruction in polar spring.
In the polar winter, heterogeneous reactions on PSC particles (or background aerosols) lead to conversion of ozone-inert chlorine reservoirs (mainly HCl, ClONO2) into ozone destroying ("active") chlorine species (mainly Cl, ClO, ClOOCl). This chlorine activation causes the massive ozone destruction in polar spring, proceeding by the catalytic ClO dimer (Molina and Molina, 1987) and ClO-BrO cycles (Mc Elroy et al., 1986).
One path of the reaction between ClO and BrO leads to the formation of chlorine dioxide, OClO (Sander and Friedl, 1989):

ClO + BrO-->OClO + Br[1a]
-->ClOO + Br                [1b]
-->BrCl   + O2[1c]
Reaction [1a] is the only significant source of OClO (Toumi, 1994). Therefore, OClO is often applied as an indicator for chlorine activation (Solomon et al., 1990; Schiller et al., 1996; Wagner et al., 2001; Kühl et al., 2004; Richter et al., 2005).
While ground based observations of OClO (Solomon et al., 1987) were among the first observations demonstrating the responsibility of the anthropogenic chlorine (CFCs) for the ozone hole (Farman et al., 1985), GOME measurements of OClO were used for monitoring of chlorine activation (Wagner et al., 1999; Kühl et al., 2002), as well as for case studies on stratospheric chemistry (Kühl et al., 2004; Richter et al., 2005).
In our group an algorithm was developed to retrieve vertical concentration profiles of OClO from the SCIAMACHY limb observations. The algorithm is based on a two step approach:

1) Retrieval of SCDs by DOAS       2) Inversion to vertical profiles

More information on the retrieval approach can be found here. For a detailed description please refer to Kühl et al., 2008 and Pukite et al., 2008. Fig. 1 shows the retrieval of the OClO absorptions by DOAS for a measurement in the cold Arctic winter 2002/2003 at a SZA of 91 degrees. For warmer regions outside the polar winter vortex and for lower SZAs the OClO absorptions are much weaker.

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Figure 1: Example for a DOAS-Analysis of SCIAMACHY limb spectra for OClO. The black curve is the retrieved optical depth of OClO, i.e. the absorption cross section scaled by the SCD retrieved. The red curve is the black curve plus the residual, i.e. noise and structures that are not related to the spectra included in the fit.

Examples for profile retrievals are given in Fig. 2, showing a retrieval for a measurement inside the polar vortex (upper panel) with high chlorine activation and one for outside the polar vortex (lower panel) with practically no chlorine activation.
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Figure 2: Examples for OClO Profile retrievals inside the polar vortex (upper panel) with high chlorine activation and outside the polar vortex (lower panel) with practically no chlorine activation. Panel a: Measured SCDs and standard deviation in blue, SCDs of the a priori in green and SCDs of the retrieved profile in red. Panel b: Profile obtained by inversion constrained by a priori (optimal estimation) in red, profile obtained by direct (LSQ) inversion in blue and a priori profile in green. Panel c: Averaging kernels and measurement response profile. Shown are results for state no. 5 (65 °N) of orbit no. 15066 from January 16th, 2005 (upper panel) and for state no. 8 (51° N) of orbit no. 15066 (lower panel).

Unfortunately, for OClO there are still no correlated observations available to validate the profiles derived from SCIAMACHY measurements. However, all verification studies performed show that the obtained profiles are in good agreement with correlated and comparable measurements as well as with model simulations and expectations. As an example Fig. 3 shows the OClO number density at 19 km altitude for selected days in the Arctic winter 2004/05 in comparison to the ClO observations at 475 K derived from MLS on AURA.
For or all of these days a good agreement was found regarding the location where active chlorine is observed. Also the degree of chlorine activation shows a good qualitative agreement. Similar comparisons were also performed for other Arctic winters and also to the ClO observations from ODIN-SMR, also there a very good agreement was found (Kühl et al., 2008).

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Figure 3: Northpole projection of the OClO number density at 19 km altitude derived from SCIAMACHY measurements for selected days in January, 2005 in comparison to measurements of ClO by AURA-MLS (obtained from the MLS website:

Our retrievals were also compared to published OClO profiles from earlier balloon measurements, see Figure 4. Of course, this is only a verification check, since the actual level of chlorine activation strongly depends on the present meteorological conditions. Nevertheless, these comparisons show that for cold Arctic winters, the magnitude and the altitude of the profile peak agree.

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Figure 4: Vertical OClO profiles measured by SCIAMACHY for selected days in the Arctic winters 2005 and 2006 (see legend) compared to earlier balloon measurements (Pommereau and Piquard, 1994; Fitzenberger, 2000).

In a first study, the OClO number densities during the northern polar winter 2004/05 were correlated to the PSC threshold temperatures (TNAT and TICE). Figure 5 shows the largest retrieved OClO number density at 19 km altitude in comparison to the lowest temperature at approx. 55 hPa. It can be seen that as soon as the temperature drops below TNAT (beginning of December), the OClO number density increases (due to chlorine activation on PSCs or aerosols). In the middle of March, when the temperature rises again, the OClO rapidly drops.
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Figure 5: Maximum OClO number density at 19 km altitude (blue) and the minimum temperature at the 54.62 hPa level (red). The dotted line indicates TNAT at this altitude (196,2 K). For the 4th and 7th of January the number of orbits is very limited (= 1), which causes lower OClO maxima.

The dataset studied so far (2002 to 2008) reveals that the amount of OClO in the Artic polar vortex varies considerably from year to year, which is expected due to the large differences in the stratospheric temperatures (in particular regarding TNAT). The OClO number densities strongly reflect the coldness of the particular winter:

Large OClO values are observed only for the cold Arctic winters 2002/03, 2004/05 and 2006/07. Vice versa, for the warmer winters much less or almost no OClO is observed. Also the locations where enhanced chlorine activation is measured, are in very good agreement with the respective meteorological situations. Contrary, for the southern hemisphere the variability is much less due to the more stable polar vortex.