Chlorine Dioxide – OClO
One key question related to the expected recovery of stratospheric ozone is the degree of chlorine activation observed in polar winter and spring in both hemispheres. This effect depends not only on the total available inorganic chlorine amount but also on the presence of PSC for the activation of the chlorine reservoirs. The latter is a function of temperature and polar vortex stability and therefore is impacted by changes of the stratospheric dynamics and temperatures in response to increased concentrations of greenhouse gases. One good indicator for chlorine activation is the presence of OClO which is formed by reaction of BrO and ClO.
While SCIAMACHY observations in nadir (Kühl et al. 2006) continue the global measurements of total columns of OClO started with GOME (Wagner et al. 2001, Kühl et al. 2004, Richter et al. 2005b), vertical profiles of OClO can also be derived from the limb observations (Kühl et al. 2008). By applying a tomographic 2D approach (Pu??te et al. 2008), the retrieval can take into account horizontal gradients in the distribution of OClO, which is particularly important at the edge of the polar vortex. Fig. 3-18 shows the OClO number density at 19 km altitude above the northern hemisphere derived from the SCIAMACHY limb observations (top), and the corresponding total column derived from nadir view (bottom) for mid of February in the Arctic winters 2002/03 to 2008/09. The data displays strong stratospheric chlorine activation only inside the polar vortex for cold winters, reflecting the strong dependence of the degree of chlorine activation on the meteorological conditions of the respective winter, such as temperature and potential vorticity. While almost no OClO is found for the warm winters 2003, 2004, 2006 and 2009, much stronger activation of chlorine is observed for the cold winters 2005, 2007 and 2008.