The interannual vertical distribution of ozone in the Mars atmosphere and observation-model discrepancies

The Nadir and Occultation for Mars Discovery (NOMAD) instrument aboard the ExoMars Trace Gas Orbiter (TGO) has been conducting observations of the vertical profile and column abundance of ozone in the Mars atmosphere since April 2018.  NOMAD is a three-channel spectrometer suite that includes the Ultraviolet and Visible Spectrometer (UVIS) channel that performs the ozone observations. The presence of ozone provides vital constraints on HO_x chemistry, given the difficulty in directly observing HO_x species in the atmosphere, which in turn affects our understanding of how these chemically-relevant species are distributed in the martian atmosphere. Observing ozone therefore provides an insight into the chemical pathways arising from the photochemical processing of water vapour into odd-hydrogen species, and how those species may then be transported throughout the martian atmosphere.

Here we present observations of the vertical distribution of ozone from L_S = 163° in Mars Year 34 to the present-day. We discuss the interannual variability of how ozone is distributed vertically, as a function of latitude and time of year, and the variability of high-altitude layers of ozone through successive years as a function of dust activity in the atmosphere.  Given the critical role of water vapour in chemically regulating the abundance of ozone, we present an investigation into model predictions of ozone over a full Mars year, to investigate the problem in Global Climate Model (GCM) representation of ozone abundances that has existed for many years.  While, in general, GCMs have been successful in recreating the annual relative distribution of ozone, they have not been able to reproduce the observed abundances leading to periods and areas of over- and under-prediction of ozone. Given the criticality of the role of water in the abundance of ozone, this incorrect prediction of ozone has been suggested to be related to potential incorrect representation of water abundance in GCMs at these times. We present results of GCM outputs for a full Mars year where water vapour and temperature from several instruments have been assimilated into a GCM, providing the best-possible representation of atmospheric state and therefore global water abundance and distribution.  We compare the predicted ozone abundances from this assimilation with those observed by the NOMAD-UVIS instrument on ExoMars TGO and discuss the regions of over- and under-prediction of ozone, and the use of this dataset as a platform for future investigations to resolve the problem of ozone representation in GCM simulations.