The data is calculated on a convective cloud differential (CCD) algorithm and averaged whereby only the position of the central coordinate is considered.
The tropospheric column can then be calculated by the difference between the stratospheric column and the total column. The stratospheric column being estimated as above the high reaching convective clouds (cloud cover >0.8 and >8km in height). To reduce the error above the clouds from up draught of tropospheric pollution a clean reference region 70°E to 170°W representitive of the latitude band.

For the Total column only cloud free observations are considered (<10%). This method assumes the stratospheric ozone is constant throughout each month and for one latitude band limit the CCD algorithm to the tropics (20°S to 20°N).

The Ozone Monitoring Instrument (OMI) is a nadir viewing imaging spectrograph that measures
the solar radiation backscattered by the Earth's atmosphere and surface over the entire
wavelength range from 270 to 500 nm with a spectral resolution of about 0.5 nm. OMI was
launched on-board the NASA satellite AURA in July 2004. In comparison to the GOME and
SCIAMACHY sensors, OMI is characterized by a larger swath width of 2600 km, which enables
measurements with a daily global coverage at all latitudes. The nominal OMI pixel size of 13 ×
6 Product Specification Document
Issue: 4.6 – Date of issue: 3/11/2015
Reference: Ozone_cci_ PSD_4.6
Page 8 of 45
24 km2 at nadir is also significantly smaller. The small pixel size enables OMI to look in between
the clouds, which is important for retrieving tropospheric information. The light entering the
telescope is also depolarised using a scrambler, which avoids polarization-related artefacts. OMI data are
available since 2004 and the instrument is still operational, however in 2007 OMI started to
experience the so-called row anomaly which reduces the amount of useful measurements, despite
correction algorithms being implemented in the level-1 processing chain. In the project, the OMI
instrument is used for ozone profile retrievals. OMI total ozone data are also used for
intercomparison and validation with total ozone data products developed from the GOME,
SCIAMACHY and GOME-2 sensors