Variations in vertical CO/CO2 profiles in the Martian mesosphere and lower
thermosphere measured by ExoMars TGO/NOMAD: Implications of variations
in eddy diffusion coefficient
N. Yoshida, Graduate School of Science, Tohoku University, Sendai, Japan (nao.yoshida.q7@dc.tohoku.ac.jp),
H. Nakagawa, Graduate School of Science, Tohoku University, Sendai, Japan, S. Aoki, Graduate School of
Frontier Sciences, The University of Tokyo, Kashiwa, Japan, J. Erwin, Royal Belgian Institute for Space
Aeronomy, BIRA-IASB, Brussels, Belgium, A. C. Vandaele, Royal Belgian Institute for Space Aeronomy, BIRAIASB, Brussels, Belgium, F. Daerden, Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium, I. Thomas, Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium, L. Trompet,
Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium, S. Koyama, Graduate School of
Science, Tohoku University, Sendai, Japan, N. Terada, Graduate School of Science, Tohoku University, Sendai,
Japan, L. Neary, Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium, I. Murata,
Graduate School of Science, Tohoku University, Sendai, Japan, G. Villanueva, NASA Goddard Space Flight
Center, MD, USA, G. Liuzzi, NASA Goddard Space Flight Center, MD, USA, M. A. Lopez-Valverde, Instituto
de Astrofisica de Andalucia (IAA/CSIC), Granada, Spain, A. Brines, Instituto de Astrofisica de Andalucia
(IAA/CSIC), Granada, Spain, A. Modak, Instituto de Astrofisica de Andalucia (IAA/CSIC), Granada, Spain, Y.
Kasaba, Graduate School of Science, Tohoku University, Sendai, Japan, B. Ristic, Royal Belgian Institute for
Space Aeronomy, BIRA-IASB, Brussels, Belgium G. Bellucci, Institute di Astrofisica e Planetologia Spaziali
(IAPS/INAF), Rome, Italy, J. J. López-Moreno, Instituto de Astrofisica de Andalucia (IAA/CSIC), Granada,
Spain, and M. Patel, School of Physical Sciences, The Open University, UK

Introduction:
CO is produced by the photodissociation of CO2
and recycled to CO2 by the catalytic cycle involving
HOx in the Martian atmosphere [e.g., McElroy &
Donahue, 1972]. The photochemical lifetime of CO
is ~6 years in the lower atmosphere [Krasnopolsky,
2007]. While, in the middle and upper atmosphere (>
~50 km), the photochemical lifetime of CO becomes
even much longer due to the decrease in the HOx
species density and longer than the characteristic
times of production and eddy diffusion. It suggests
that CO profiles are determined by the production
and eddy diffusion at those regions. The eddy diffusion coefficient is used for parameterizing the efficiency of vertical diffusion, however, estimated values have a large scatter between 40 and 90 km altitude [Rodrigo et al., 1990]. Recently, a substantial
variation in the eddy diffusion coefficient at the
homopause altitude has been suggested [Slipski et
al., 2018]. It implies that CO profiles in the middle
and upper atmosphere vary with variations in the
eddy diffusion coefficient. ExoMars Trace Gas Orbiter (TGO) can measure the vertical profiles of CO
in the mesosphere and thermosphere. Olsen et al.
(2021) reported the vertical distribution of CO and
its variation during a dust storm, however, the effects
of change in the eddy diffusion coefficient on the
profile of CO mixing ratio have not been investigated. In this study, we use Nadir and Occultation for
MArs Discovery (NOMAD, Vandaele et al., 2018)
aboard TGO to retrieve the vertical CO/CO2 profile
and to investigate the variability of the eddy diffusion coefficient.

Dataset and analysis:
Here, we applied the equivalent width technique
[Chamberlain & Hunten, 1987; Krasnopolsky, 1986]
to derive CO and CO2 column densities. In the case
that the optical depth of the absorption line is not
saturated, the slant column density is given by W =
SN, where W is the area of absorption, S is the line
intensity, and N is the slant column density in the line
of sight. We derived the slant column density using
4288.2 and 4291.5 cm-1 for CO and 3355.7, 3357.2,
3358.7, and 3360.3 cm-1 for CO2. The CO/CO2 ratio
is derived between 70 and ~105 km altitudes. We use
only the orbits which measure CO spectra (in order
190, 4269.95 – 4303.99 cm-1) and CO2 spectra (in
order 149, 3348.54 – 3375.23 cm-1) simultaneously
in MY 35, corresponding from 25th March 2019 to
6th February 2021. The total number of orbits used
in this study is 649.
Results and discussion:
We found that the retrieved CO/CO2 ratio between 70 and ~105 km shows a significant seasonal
variation in the southern hemisphere, which decreases near perihelion and increases near aphelion between ~1500 and ~5000 ppm at 85 km. The slope of
CO/CO2 profiles becomes steep near perihelion in
the southern hemisphere (Figure 1). To investigate
the contribution of the variability of the eddy diffusion coefficient in each hemisphere and season, we
estimated the best CO/CO2 profile by a 1D photochemical model [Koyama et al., 2021] using a chisquare test with two cases: (1) the eddy diffusion
coefficients are uniform in vertical; (2) the vertical

profile of eddy diffusion coefficient is given by K =
An-1/2, where A is constant, and n is total number
density [cf. Lindzen, 1971]. Our estimation shows
that the altitude-dependent eddy diffusion coefficient
is better than the vertically-uniform eddy diffusion
coefficients to reproduce the observed profiles (Figure 2). In addition, our observation firstly suggested
the variation of the eddy diffusion coefficient. In the
southern hemisphere, K = 4.25×1013n-1/2 for Ls = 90
– 120 and K = 1.5×1014n-1/2 for 240 – 270. Throughout the altitude range, the eddy diffusion coefficient
in Ls = 240 – 270 is larger by a factor of ~2 than that
in Ls = 90 – 120 in the southern hemisphere. On the
other hand, the estimated eddy diffusion coefficient
in the northern hemisphere is comparable between
both Ls ranges; K = 7×1013n-1/2 for Ls = 90 – 120 and
K = 1014n-1/2 for 240 – 270. That would suggest the
efficiency of the vertical diffusion varies with season
and latitude.
Figures:

Figure 1 Vertical profiles of the CO/CO2 ratio in Ls
= 90 – 120, 180 – 210, 240 – 270, and 330 – 360 in
MY 35. Horizontal lines are error bars. Color represents Ls. Profiles are separated into the northern (a, c,
and e) and southern (b, d, and f) hemispheres. To
distinguish the enhancement in polar regions, profiles
are separated into two latitudinal bins: from the equator to 70 degrees (c and d) and 70 to 90 degrees (e
and f).

Figure 2 (a, c, e, and g) Vertical profiles of the
CO/CO2 ratio estimated with the 1D model and observed by NOMAD SO. The broken lines represent
the initial CO/CO2 profiles in the model. For the
northern (southern) hemisphere, the observed
CO/CO2 profiles in Ls = 240 – 270 is shown in light
blue (blue), and that in Ls = 90 – 120 is in magenta
(red). (b, d, f, and h) The determined eddy diffusion
coefficients used for the 1D model. Ls are distinguished by colors. Profiles are divided into two hemispheres, those in the northern hemisphere are shown
in panels (a, b, e, and f), and those in the southern
hemisphere in panels (c, d, g, and h).
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