Ozone observed by TGO/NOMAD-UVIS solar occultation: an intercomparison of three retrieval methods
A. Piccialli, A. C. Vandaele, Y. Willame, Royal Belgian Institute for Space Aeronomy, Belgium, A.
Määattäanen, LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, Paris, France, L. Trompet1, J.T. Erwin1,
F. Daerden1, L. Neary1, Royal Belgian Institute for Space Aeronomy, Belgium, S. Aoki, Graduate School of
Frontier Sciences, The University of Tokyo, Kashiwa, Japan, S. Viscardy, I. R. Thomas, C. Depiesse, B.
Ristic, Royal Belgian Institute for Space Aeronomy, Belgium, J. P. Mason, M. R. Patel, School of Physical
Sciences, The Open University, Milton Keynes, U.K., M.J. Wolff, Space Science Institute, Boulder, CO, USA,
A.S.J. Khayat, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; Center for Research and
Exploration in Space Science and Technology II, University of Maryland, USA; G. Bellucci, Istituto di Astrofisica e Planetologia Spaziali, IAPS-INAF, Rome, Italy, J.-J. Lopez-Moreno, Instituto de Astrofisica de Andalucia, IAA-CSIC, Granada, Spain.

Introduction:
The NOMAD-UVIS instrument on board the
ExoMars Trace Gas Orbiter has been investigating
the Martian atmosphere with the occultation technique since April 2018 (Vandaele et al. 2015). In the
solar occultation mode, it is mainly devoted to study
the climatology of ozone and aerosols content (Patel
et al. 2021; Khayat et al. 2021). Here, we will present an inter-comparison of retrieval codes for ozone
observed with NOMAD-UVIS.
NOMAD-UVIS solar occultations:
We analyzed almost two Mars Years of ozone
vertical distributions acquired at the day-night terminator, corresponding to more than 8300 solar
occultations, acquired between April 2018 (MY 34,
LS=163°) and November 2021 (MY 36, LS=132°).
Inter-comparison of three retrieval methods:
As in the work of Määttänen et al. (submitted to
Icarus), the NOMAD-UVIS ozone retrievals proved
more difficult than expected due to the presence of
spurious detections of ozone caused by instrumental
effects, high dust content, and very low values of
ozone. This lead us to compare the results from three
different retrieval approaches:
1) an onion peeling method (OP);
2) a full occultation Optimal Estimation Method (FOEM), and
3) a direct onion peeling method (DOP).
The OP method is similar to that used for Mars
and Venus stellar occultations (Quémerais et al.,
2006; Piccialli et al., 2015). The FOEM and DOP
approaches are based on ASIMUT-ALVL, the
BIRA-IASB radiative code (Vandaele et al., 2008,
Piccialli et al. 2021).
The main challenge was to find reliable criteria to
exclude spurious detections of O3, and we finally
adopted two criteria for filtering: i) a detection limit,
and ii) the Δχ2 criterion. Both criteria exclude spurious O3 values especially near the perihelion, where

based on the simulations from a general circulation
model, we do expect very low values of ozone.
Comparison of filtering methods between
UVIS and SPICAM:
We compared the results of filtering with
SPICAM-UVIS observations. The SPICAM team
applied very similar criteria for filtering their data to
the ones implemented here (Määttänen et al. submitted to Icarus). Even if the two instruments observed
during different Martian Years, the agreement on the
filtered O3 retrievals is very good, and both filtering
approaches lead to very similar results (see Figure 1).

Figure 1: An example of the effect of the selection
criteria for SPICAM-UV and NOMAD-UVIS observations for a latitude band (-60°;-30°). Left panels
show the ozone vertical distribution before applying
the filtering; right panels shows O3 profiles after the
selection criteria.
References: Khayat et al. (2021) https://doi.org/
10.1029/2021JE006834; Määttänen et al. Icarus, in
review; Patel et al. (2021) https://doi.org/
10.1029/2021JE006837; Piccialli, A. et al., (2015)
https://doi.org/10.1016/j.pss.2014.12.009; Piccialli et
al.
(2021)
https://doi.org/10.1016/
j.icarus.2019.113598; Quémerais et al., (2008)
https://doi.org/10.1029/2005JE002604; Vandaele et
al. (2006) https://doi.org/10.1029/2008JE003140;
Vandaele et al. (2015)
https://doi.org/
10.1364/OE.23 .030028