Earth-orbiting satellites have been monitoring the total ozone contained in the atmosphere for nearly three decades in order to diagnose any potential long term changes in the ozone layer and assess the variability and changes in polar ozone loss. The nature of the viewing geometry typically results in the derivation of a Total Ozone Column (TOC), which contains information from both the stratosphere and the troposphere contributions. The uneven distribution of ozone throughout the column (as a result of chemical production) means that ~90% is contained in the stratosphere. Therefore, to partition the Tropospheric Total Ozone Column (TTOC) as a fraction of the total ozone column requires the subtraction of the stratospheric contribution (commonly known as the ‘residual’ method). The derivation of the TTOC is of interest due to high spatial and seasonal variability in tropospheric ozone caused by chemistry, stratospheric intrusions and long-range transport. Further, the global coverage of satellites could potentially provide values where ground-based measurements are scarce.

Researchers Jos de Laat (AK) and Michiel van Weele (CK) at KNMI are utilizing assimilated vertical ozone profiles from the GOME instruments on ERS-2 in order to calculate the stratospheric contribution to the total column accurately by an optimal combination of satellite observations and model stratosphere dynamics from ECMWF. Also the exact height of the tropopause as a boundary between the troposphere and stratosphere is defined using ECMWF model data. By subtracting the stratosphere contribution from the GOME TOC observations, a preliminary GOME TTOC data set has been constructed for the period 1996-2002.

Figure 1 shows a comparison of GOME-derived TTOCs (blue: collocated with sondes; grey: all derived columns) with TTOCs calculated from ozone sondes (red) for Payerne (stn156; 46.5N, 6.6E) and Ascension Island (stn328; 8.0S, 14.4W) over the full observations period (upper panels). The lower panels show the corresponding monthly means. The uncertainties in the GOME-derived TTOCs are still quite large and the TTOCs show too much variability at mid-latitudes, where negative TTOCs values can occur. These observations point to a larger assimilated stratosphere-integrated vertical profile than the co-located total ozone column which is observed. The comparison at Ascension located at tropical latitude is much more favorable and the derived TTOCs largely follow the TTOCs based on the ozone sondes. Finally, Figure 2 shows the global distribution of the GOME-derived TTOCs for the year 2000. Results of this study have been presented at the Quadrennial Ozone Symposium held in Trømso, Norway in July 2008.