Vertical profiles of NO2 are retrieved from OSIRIS limb scattered sunlight spectra using a Differential Optical Absorption Spectroscopy (DOAS) technique in combination with an Optimal Estimation inversion. The retrieval uses a spectral fitting window from 435 to 451 nm and are performed in logarithmic space, partly to provide a positivity constraint and also because the true concentrations are believed to follow a log-normal distribution. OSIRIS provides a unique data set of NO2 with high-latitude coverage and a retrieval range between 10 and 46 km with a vertical resolution of about 2 km and precision of 16% between 15 and 25 km and 6% between 25 and 35 km.
This retrieval is regularly processed at the University of Saskatchewan for all OSIRIS stratospheric limb scan measurements. Data from November, 2001, to the current date are available for download in an OSIRIS Level 2 Product in HDF-EOS5 format. OSIRIS remains fully operational and processing of this algorithm will be maintained into the foreseeable future.
Figure taken from Brohede et al., A stratospheric NO2 climatology from Odin/OSIRIS limb scatter measurements, Can. J. Phys., 85, 1253-1274, doi:10.1139/P07-141, 2007.
Two versions of the NO2 product have been produced and distributed to the community: V3.0 and V5.0. Production of version 3.0 started in 2006 and was superceded by version 5.0 in December 2010. The only difference between version 3.0 and version 5.0 algorithms is that version 3.0 was generated with a mixture of older and newer Level 1 radiance data while version 5.0 exclusively uses new Level 1 radiance data. The new Level 1 radiance data is tagged with a version number of 55092 or higher (this is the second number in the audit field). The main improvement in the new Level 1 data is that the dark current removal is significantly better than in the original version data. The improved dark current removal does have a small impact on the NO2 retrieval.
Users who have invested significant effort into trend analysis with version 3.0 may continue that analysis with the new version 5.0 product as it becomes available. All version 3.0 users should be aware that the product is derived from a mixture of new and old level 1 radiance data versions. The version of Level 1 data used in the retrieval of version 3.0 (or 5.0) Level 2 product is available in the level 2 data files via the L1 field of the OSIRIS\Odin NO2 DOAS OE swath. We shall re-process the entire mission with the Level 2, version 5 algorithm but do not expect the back-log to be completed until January 2012.