Differences in ozone retrieval in MIPAS channels A and AB: a spectroscopic issue

Discrepancies in ozone retrievals in MIPAS channels A (685–970 cm −1 ) and AB (1020–1170 cm −1 ) have been a long-standing problem in MIPAS data analysis, amounting to an inter-channel ozone bias (AB-A) of up to 8 % in the altitude range 30–40 km. We discuss various candidate explanations, among them forward model and retrieval algorithm errors, inter-channel calibration inconsistencies, and spectroscopic data inconsistencies. We show that forward modelling errors as well as errors in the retrieval algorithm can be ruled out as an explanation because the bias can be reproduced with an entirely independent retrieval algorithm (GEOFIT) relying on a different forward radiative transfer model. Instrumental and calibration issues can also be refuted as explanation because ozone retrievals based on balloon-borne measurements with a different instrument (MIPAS-B) and an independent level-1 data processing scheme produce a rather similar inter-channel bias. Thus spectroscopic inconsistencies are practically the only reason left. To further investigate this issue, we performed retrievals using various versions of the HITRAN spectroscopic database which, however, generally produce rather similar channel AB-A differences. Use of a different database, namely GEISA-2015, leads to similar results in channel AB, but to even higher ozone amounts for channel A retrievals, i.e. to a reversal of the bias. We show that the differences in MIPAS channel A retrievals result from about 13 % lower air-broadening coefficients of the strongest lines in the GEISA-2015 database. Since the errors in line intensity of the major lines used in MIPAS channels A and AB are reported to be considerably lower than the observed bias, we posit that a major part of the inter-channel differences can be attributed to inconsistent air-broadening coefficients as well. To corroborate this assumption we show some clearly inconsistent air-broadening coefficients in the HITRAN-2008 data base. The inter-band bias in retrieved ozone profiles can be reduced by, e.g., increasing the air-broadening coefficients of the lines in MIPAS band AB in the HITRAN-2008 database by 6–8 %.