1. Ozone monitoring with GOMOS-ENVISAT experiment version 6
Philippe Keckhut(1), Alain Hauchecorne(1), Laurent Blanot(3), Klemens Hocke(2), Sophie Godin-Beekmann(1), Jean-Loup Bertaux(1), Gilbert Barrot (3), Erkki Kyrölä (4) and Marielle Guirlet (3)
LATMOS-IPSL, CNRS/INSU, UVSQ, UPMC, Verrières le Buisson and Paris, France
Institute of Applied Physics, University of Bern, Switzerland
(3) ACRI-ST, Sophia Antipolis, France
(4) FMI, Helsinki, Finland

2. Comparison between GOMOS v6 and the Ozone OHP lidar (courtesy S
Comparison between GOMOS v6 and the Ozone OHP lidar (courtesy S. Godin-Beekmann)
Data selection
±10°in latitude and ±20° longitude
Stars id 1-70
Best illumination condition in black
Best stars (Spectrum) in red
Bias observed at 20 and 40 km of 10-20% since 2006

3. Comparisons between GOMOS v6 and the Ozone Bern Microwave spectrometer GROMOS (courtesy K. Hocke)
Data selection
±10°in latitude and ±20° longitude
Stars id 1-70
Obvious bias since mid 2005
Positive bias (20%) at 26 km
Small bias in km range
Large negative bias in km range (20%)

4. CCD dark charge evolution

5. Dark charge bias correctionUV IR
No bias correction V6.0cf
After bias correction V7.0ab

6. O3 line density profiles S143 in 2006
Level 1b updates
Reference star spectrum flagged in UV part
High DC noise + low signal + threshold (0) = flagged columns
nm discarded
O3 line density profiles
S143 in 2006
… but noise increases above 70 km – O3 retrieval driven by low UV part

7. O3 line density profiles S143 in 2006
Level 1b updates
Reference star spectrum flagged in UV part
So, no specific correction for cold stars but new baseline
improves the DC correction and…
O3 line density profiles
S143 in 2006
We observe a decrease of bad profiles !

8. Conclusion
Biases are detected in GOMOS ozone V6.0cf data compared to NDACC lidar and microwave profiles
These biases may be due to a bad dark charge correction of the CCD coupled with the regular dark charge increase
A new dark charge correction algorithm has been implanted in V7.0ab which improves greatly the quality of GOMOS ozone profiles. The comparison of V7.0ab data with validation instruments will be done after the reprocessing.
This work points out the importance of long-term validation of satellite data