A spectroscopic issue was found to explain the differences between ozone profiles retrieved in MIPAS channels A and AB

The figure on the left shows mean ozone profiles of 59 orbits from 3 January, 1 April, 2 July and 2-3 October 2009, retrieved in MIPAS channels A (black) and AB (red) using the HITRAN-2008 spectroscopy. The blue profile results from retrievals in MIPAS channel AB after an increase in the air-broadened half widths of the ozone lines by a value of 0.005 cm^-1 atm^-1 at 296 K, and the green profile from retrieval after increase in the air-broadened half widths of the strongest ozone lines by 0.008 cm^-1 atm^-1 at 296 K. The figure on the right shows the absolute differences between retrievals in channels AB and A.

Use of the HITRAN-2008 spectroscopy leads to a positive bias of up to 8% between ozone profiles retrieved in MIPAS channels AB (1020-1170 cm^-1) and A (685-970 cm^-1) in the altitude range of the ozone vmr peak. Comparison to independent measurements have proven that the profiles retrieved from channel A agree better with most other observations (Laeng et al., 2014), and thus, channel AB retrievals provide a high bias. After exclusion of forward modelling, instrumental and calibration issues, the only remaining explanation for the differences is spectroscopic inconsistencies. However, the uncertainties in intensity of strong ozone lines have been declared to be considerably lower than the observed bias. By alternatively applying a correction to the air-broadened half widths of the MIPAS channel AB lines, the retrievals from MIPAS channels A and AB can be brought into better agreement, removing the previous high bias of channel AB retrievals.

For more information see: https://www.atmos-meas-tech.net/11/4707/2018/.