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

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 in
MIPAS 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. We
discuss various candidate explanations, among them forward model and
retrieval algorithm errors, interchannel 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 an 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 interchannel bias. Thus, spectroscopic
inconsistencies in the MIPAS database used for ozone retrieval are
practically the only reason left. To further investigate this issue, we
performed retrievals using additional spectroscopic databases. Various
versions of the HITRAN database generally produced rather similar channel
AB–A differences. Use of a different database, namely GEISA-2015, led to
similar results in channel AB, but to even higher ozone volume mixing ratios
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 channel AB–A 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 database. The interchannel bias in retrieved
ozone amounts can be reduced by increasing the air-broadening
coefficients of the lines in MIPAS channel AB in the HITRAN-2008 database by
6 %–8 %.
