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Abstract. Discrepancies in ozone retrievals in MIPAS channels A (685–970 cm −1 ) and AB (1020–1170 cm −1 ) have been a long-standing problem inMIPAS data analysis, amounting to an interchannel bias (AB–A) of up to 8 %between ozone volume mixing ratios in the altitude range 30–40  km . Wediscuss various candidate explanations, among them forward model andretrieval algorithm errors, interchannel calibration inconsistencies andspectroscopic data inconsistencies. We show that forward-modelling errors aswell as errors in the retrieval algorithm can be ruled out as an explanationbecause the bias can be reproduced with an entirely independent retrievalalgorithm (GEOFIT), relying on a different forward radiative transfer model.Instrumental and calibration issues can also be refuted as an explanationbecause ozone retrievals based on balloon-borne measurements with a differentinstrument (MIPAS-B) and an independent level-1 data processing schemeproduce a rather similar interchannel bias. Thus, spectroscopicinconsistencies in the MIPAS database used for ozone retrieval arepractically the only reason left. To further investigate this issue, weperformed retrievals using additional spectroscopic databases. Variousversions of the HITRAN database generally produced rather similar channelAB–A differences. Use of a different database, namely GEISA-2015, led tosimilar results in channel AB, but to even higher ozone volume mixing ratiosfor channel A retrievals, i.e. to a reversal of the bias. We show that thedifferences in MIPAS channel A retrievals result from about 13 % lowerair-broadening coefficients of the strongest lines in the GEISA-2015database. Since the errors in line intensity of the major lines used in MIPASchannels A and AB are reported to be considerably lower than the observedbias, we posit that a major part of the channel AB–A differences can beattributed to inconsistent air-broadening coefficients as well. Tocorroborate this assumption we show some clearly inconsistent air-broadeningcoefficients in the HITRAN-2008 database. The interchannel bias in retrievedozone amounts can be reduced by increasing the air-broadeningcoefficients of the lines in MIPAS channel AB in the HITRAN-2008 database by6 %–8 %.

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