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Remaining problems with the correction for the interference

The correction for the ozone interference described in the main background correction section is working quite well, much better than the previous correction and the number of problem cases is reduced dramatically. But the problems are not complete over yet, as described below. For the moment we do not know how we could set up a better correction of the interference problem and thus avoid problems like those mentioned here.
 

Problems at high SZA

SO2 near Iceland on 5 August 2007

SO2 near Iceland on 5 August 2007 with lower SZA

For SZA well above 80 degrees there can still be a problem, as the SO2 vertical column observed on the North of Scandinavia (top graphique): the nadir state in the middle near Scandinavia shows a few ground pixels with vertical column values of more than 10 DU. There are no reports of volcanic activity on Iceland around that time, nor is there any anthropogenic source nearby. Clearly, the seemingly high SO2 values in this graph are artifacts. For this reason we have decided to use an "adaptative" SZA according to the day of year and to the hemisphere (North or South).

Our study of an optimal SZA gives the falling empiric values of the limitation of SZA for the North hemisphere (SZAnorth) and the South hemisphere SZAsouth:

      SCIAMACHY      
Day of year SZAnorth SZAsouth
21/03 75� 80�
21/06 70� 80�
21/09 75� 80�
21/12 80� 70�

        GOME-2        
Day of year SZAnorth SZAsouth
21/03 75� 80�
21/06 70� 75�
21/09 75� 80�
21/12 75� 70�

The SO2 peak values shown in the top image (SZA=80�) satisfy the criteria for exceptional SO2 concentration and would thus trigger a notification, but clearly this is a "false notification". For this reason we use an empirical "adaptative" seasonal SZA to avoid such false notification.
 

Apparent band of higher SO2 at mid-latitudes

Monthly average SO2 for March 2007

The monthly average SO2 slant column plots show in most cases a kind of band of higher and noisier SO2 signal between about 30 and 60 degrees North. See for example the plot for March 2007 shown here, which shows a particularly clear band. A few months show no or a very weak band at the same latitude. A similar band exists on the Southern hemisphere, but there it is much less pronounced. All in all the SO2 slant column lies for low and mid latitudes mostly between +0.3 and -0.3 DU, which can be considered as the noise level on the zero-SO2 background.

Note from the plot shown here that the South Atlantic Anomaly is clearly visible as enhanced noise in the SO2 slant column.
 

Monthly average ozone for March 2007

It is unclear what the origin (reason) for this band is. Most likely it has to do with not entirely adaquate correction of the ozone interference. But it is strange that the band lies at slightly lower latitudes than where the largest variation of ozone along latitudes is observed, as can be seen by comparing the monthly average total ozone column with the SO2 plot above.
 
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