# Investigating the discrepancy between observed and modelled ozone on Mars using ACS and NOMAD data

Benjamin Benne, Paul Palmer, Kevin Olsen, Franck Lefèvre

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Benjamin Benne, Paul Palmer, Kevin Olsen, Franck Lefèvre. Investigating the discrepancy between observed and modelled ozone on Mars using ACS and NOMAD data. EPSC-DPS Joint Meeting 2025, Sep 2025, Helsinki, Finland. 10.5194/epsc-dps2025-1341. hal-05279574

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Submitted on 24 Sep 2025

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# Investigating the discrepancy between observed and modelled ozone on Mars using ACS and NOMAD data

**Benjamin Benne**<fnref n="1" />,<fnref n="2" />, **Paul Palmer**<fnref n="1" />,<fnref n="2" />, **Kevin Olsen**<fnref n="3" />,<fnref n="4" />, and **Franck Lefèvre**<fnref n="5" />

The photochemistry of ozone in the Martian atmosphere is generally considered to be well understood. Ozone forms through a three-body reaction involving O and O₂, both products of CO₂ photolysis, and it is destroyed by odd-hydrogen species (HO⁻) generated from water vapour photolysis, which helps to explain the observed anticorrelation between ozone and water vapour [1,2]. However, current photochemical models cannot reproduce ozone observations from various missions (e.g., Trace Gas Orbiter (TGO), Mars Express), with models generally suffering from a negative bias [2,3]. This discrepancy highlights gaps in our knowledge of the photochemical links between odd-oxygen (O₃), odd-hydrogen, and water vapour in the Martian atmosphere. A recent study investigated different factors that could influence the ozone content and concluded that the underestimation of ozone in the MPCM might be due to heterogeneous uptake of HO⁻ species on water ice clouds or an overestimation of HO⁻ photochemistry efficiency in the model [2].

We build on that explorative study and use the latest configuration of the Mars Planetary Climate Model (MPCM) with initial conditions from the Mars Climate Database (MCD) v6.1 [4] to investigate how different parameters could influence the ozone vertical profiles. We study data collected in MYs 34 and 35, including Ox, HOx, CO, and water vapour retrievals from the ACS (Atmospheric Chemistry Suite) and NOMAD (Nadir and Occultation for MArS Discovery) instruments aboard TGO. This approach allows us to examine any altitude-dependent changes in chemistry. We will present the results from a systematic investigation into the impact of various assumed model parameters, e.g., absorption cross sections, reaction rates, and heterogeneous chemistry, on these species. We will also consider the impact of introducing new chemistry into the model, e.g., chlorine photochemistry that was recently implemented in the MPCM by Benne et al. (2025) (in review). We will conclude our presentation by highlighting the parameters with the greatest impact on model ozone, the interactions and variations of ozone and its precursors across the two MYs, and prioritising the future research required to bridge the gap between model and observed Martian ozone.

## References:

[1] Lefèvre, F., and Krasnopolsky V. "Atmospheric Photochemistry." In *The Atmosphere and Climate of Mars*, 1st ed., 405-32. Cambridge University Press, (2017).

[2] Lefèvre, F., A. Trokhimovskiy, A. Fedorova, L. Baggio, G. Lacombe, A. Määttänen, J. L. Bertaux, et al., JGR: Planets 126, no. 4 (2021)

<footnote n="1">¹University of Edinburgh, School of Geosciences, Edinburgh, United Kingdom (benjamin.benne@ed.ac.uk)</footnote>

<footnote n="2">²Centre for Exoplanet Science, University of Edinburgh, Edinburgh, United Kingdom</footnote>

<footnote n="3">³Department of Physics, University of Oxford, Oxford, United Kingdom</footnote>

<footnote n="4">⁴School of Physical Sciences, The Open University, Milton Keynes, United Kingdom</footnote>

<footnote n="5">⁵Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS/CNRS), Paris, France</footnote>
[3] Olsen, K. S., A. A. Fedorova, A. Trokhimovskiy, F. Montmessin, F. Lefèvre, O. Korablev, L. Baggio, et al., JGR: Planets 127, no. 10 (2022)

[4] Millour, E, Forget F., Spiga A., Pierron T., Bierjon A., Montabone L., Vals M., et al., Copernicus Meetings, (2022).