These findings are in good agreement with observations of the main active ozone depleting chlorine species ClO, the determining meteorological parameters, results from atmospheric chemistry models, and the related chemical ozone loss (compare Figs. in the section on ozone). The strong inter-annual variability in the degree of chlorine activation reveals that despite the decrease of the stratospheric inorganic chlorine loading, large enhancements of OClO are still observed in the Arctic stratosphere during cold winters. Therefore, monitoring of stratospheric chlorine activation and detailed investigation of the relation to meteorological parameters is necessary to broaden the understanding of the related processes.
Bromine Oxide – BrO
Bromine compounds play an important role in the catalytic destruction of stratospheric ozone. Despite their importance, however, there are only few measurements of bromine compounds in the stratosphere. For the first time, SCIAMACHY provided global observations of stratospheric BrO profiles down to the lower stratosphere (Rozanov et al. 2005, Sioris et al. 2006, Kühl et al. 2008). The long-term changes of BrO as observed from SCIAMACHY agree well with ground-based observations at mid-latitudes (Hendrick et al. 2009). The seasonal cycle, including the activation during winter and long-term changes, are consistently seen by both instruments. When comparing zonal mean BrO at selected stratospheric altitudes obtained from limb observations and from model simulations, an additional source of stratospheric bromine from very short-lived substances (VSLS) is required (fig. 3-19) to explain the difference. This component amounts to about 3 to 6 parts per trillion by volume (pptv), or about 20% (WMO 2007, Sinnhuber et al. 2005). In an opposite analysis approach, Theys et al. (2009) used the SCIAMACHY limb stratospheric BrO observations to demonstrate the validity of a new stratospheric BrO profile climatology.
Polar Stratospheric Clouds – PSC
Polar Stratospheric Clouds play a key role in the chemical processes which lead to severe ozone depletion in the polar stratosphere. These clouds are necessary for transferring inactive chlorine compound reservoirs such as HCl and ClONO2 to active Cl that participates in different catalytic O3 destruction cycles. PSC form at altitudes of about 15-25 km and exist as different types. Type Ia consists of crystalline NAT (nitric acid tri-hydrate) particles, the liquid type Ib PSC consist of ternary solutions of nitric acid, sulphuric acid and water. Type II PSCs are made of water ice. A common feature of all types is that they only form at very low temperatures of less than about -78°C (195 K). PSC scatter solar radiation and thus affect the measured limb radiance spectra. Since PSC are rather Mie-scatterers than Rayleigh-scatterers in the UV-SWIR spectral range, the spectral dependence – although highly variable – of their scattering cross section differs from the l-4 spectral dependence of the molecular Rayleigh scatterering. This spectral difference can be exploited in a colour-index approach to detect PSC (von Savigny et al. 2005b).