ROSA - Scientific Background

The stratospheric aerosol layer is a key constituent in the Earth's atmosphere playing an important role in determining the radiation budget and having a major role in the chemically-induced ozone depletion in the polar stratosphere. Its composition has been measured by in situ collection and it appears to be comprised of sulphuric acid (SPARC/WMO, 2006) which is formed by the oxidation of the two major precursors COS and SO2, both having mainly tropospheric sources (SPARC/WMO, 2006). The precursors either reach the stratosphere by troposphere-stratosphere-exchange processes or via direct injection from explosive volcanoes (Robock, 2000; Brühl et al., 2012; Bourassa et al., 2012). On the synoptic-scale, a variety of anthropogenic activities, such as high flying aircraft or deep pyro-cumulus convection, alter the load and composition of aerosol in the lower layers of the stratosphere, as meteor debris does in higher altitudes (SPARC/WMO, 2006). However, the relative contributions of these sources are not well established. Moreover, the drivers for the variability in stratospheric aerosol are not fully understood. Solomon et al. (2011) recently demonstrated that the stratospheric aerosol background is more variable than previously thought. Changes of the stratospheric aerosol load affect the radiative balance and the catalytic cycles forming ozone depleting substances (ODS; WMO, 2011). Those changes occur quickly on time-scales of days to weeks and last for several years, dependent on the rate and relative strength of the perturbation. The likely consequences of a changing climate on the stratospheric aerosol layer – i.e. direct effects such as changing microphysics, leading to changes of the aerosol radiative forcing and possible indirect effects induced by an altered Brewer-Dobson-circulation – however, are not well understood (Anderson et al., 2012). In addition as time runs out with respect to the mitigation of the direct release of greenhouse gases, accurate knowledge of stratospheric aerosol is an essential prerequisite for the discussion of geoengineering options based on the release of sulphur dioxide to mitigate the effect of increasing greenhouse gas abundances, initiated by Paul Crutzen in 2006 (Crutzen 2006; Royal Society, 2009).

As a result of their intrinsic and great importance in many different respects for the Earth system, and the fact that several important aerosol related processes are still poorly understood, the investigation of the dynamics and temporal evolution of stratospheric aerosol during the last decade, as proposed here, will significantly improve the scientific understanding of stratospheric processes for present and future climate conditions. This information will be invaluable for the development of evidence based policy addressing environment and climate change.