SEASONAL AND GLOBAL OZONE VARIATIONS WITH HETEROGENEOUS
CHEMISTRY IN THE MARTIAN ATMOSPHERE.
M. A. J. Brown, School of Physical Sciences, The Open University, Milton Keynes, U.K. (megan.brown@open.ac.uk),
M. R. Patel, S. R. Lewis, J. A. Holmes, J. Mason, A. Bennaceur, The Open University, Milton Keynes, U.K., A. C.
Vandaele, Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.

Introduction
This study investigates the temporal and spatial effects
of heterogeneous chemistry on ozone and hydroxyl radicals (OH and HO2 , collectively known as HOx ) using
observed total column abundances (TCAs) of ozone, water ice, and water vapour, combined with Mars global
climate model (MGCM) simulations with and without
heterogeneous chemistry. We analyse the seasonal and
latitudinal distribution of ozone and HOx to assess where
heterogeneous chemistry influences trace gases, and the
extent of this chemical impact. Analysing how the inclusion of heterogeneous chemistry affects ozone and HOx
will help us understand the variation of HOx species,
their sensitivity to water ice, and potentially improve
simulations of ozone to be in better agreement with observations. Preliminary results show that the addition
of heterogeneous chemistry increase the ozone TCA, although it does not account for all ozone deficit when
compared to nadir observations.
Martian atmospheric chemistry is a key component
to understanding crucial features on Mars, such as the
water cycle, composition of the atmosphere, and the
stability of carbon dioxide (CO2 ) [1]. These features
can be investigated using trace gases in the atmosphere.
HOx are highly reactive trace species, and act as a
catalyst in the recombination of carbon monoxide (CO)
and molecular oxygen (O) to form CO2 . As they are responsible for the stability of CO2 in the atmosphere, the
reactions HOx species undergo are important to clarify
since they have the potential to impact multiple atmospheric species through trace amounts. Such investigations into the sensitivity of HOx on other species can be
conducted through modelling. HOx are formed by the
photolysis of water vapour;
λ ≈,180nm

H2 O + hν −−−−−−−→ H2 + OH

(1)

where h is Planck’s constant and ν is the velocity of
light at the wavelength given by λ [2]. Due to the high
reactivity and very short lifetime, HOx cannot be easily
measured directly. Instead, a combination of climate
modelling and proxies are used to understand the temporal and spatial variation [3, 4, 5].
Ozone is often used as a proxy for HOx species, as its
dayside lifetime is relatively short (2–3 hours), and it is
sensitive both to the presence of HOx and to photochemistry. HOx react with ozone and destroy it, producing

more HOx species, which can then further react and destroy ozone, causing a set of chain reactions. Because of
the direct production of HOx species from water vapour,
there is a known photochemical anti-correlation between
ozone and water vapour [1, 2, 6]. This relationship between ozone and water has been used in previous studies
to investigate various chemical reactions which involve
HOx species.
Comparing observed ozone to modelled ozone is
often used to verify chemical reactions in models. Currently, ozone is underpredicted in GCMs, indicating
missing or inaccurate chemical processes in models [3,
6, 7, 8]. One set of chemical reactions which have been
offered as an explanation for this ozone deficit are heterogeneous chemical processes [5].
In the martian atmosphere, heterogeneous chemistry
consists of trace species such as HOx , adsorbing onto
the surface of dust and water-ice particles. In this study,
we investigate the adsorption of HOx and hydrogen peroxide (H2 O2 ) onto water ice. The water ice acts as a
sink, preventing HOx from reacting with other species;
the reduction in HOx results in an increase in ozone
abundance as there are fewer HOx species to destroy
ozone [7]. A proxy for detecting heterogeneous chemistry is via a positive relationship between ozone and
water ice. In addition, heterogeneous chemistry can be
investigated by comparing simulated ozone from global
climate models (GCMs) with observations [5, 8].

Nadir Observations
Ozone and water ice TCA data used in this study are
from nadir observations from the the Ultravoilet and
VISible channel (UVIS) [?] and spectrometer suite [?]
of the NOMAD (Nadir and Occultation for MArs Discovery) spectrometer suite aboard the ExoMars Trace
Gas Orbiter (TGO), and cover LS = 0◦ − 360◦ , Mars
Year 35 (Mason et al. in prep). Compared to vertical
profiles derived from occultations measurements[6, ?],
nadir observations have a wider spatial and temporal
coverage and are thus more suitable for studying global
and seasonal variations. Data are filtered by < 70◦ SZA
(solar zenith angle) and a relative error < 70%. Observations are divided into bins of 60◦ LS to capture the
seasonal trend, and latitudinal bins of 30◦ . As there is
little variation of ozone longitudinally, data are zonally
averaged.

Model Setup
The MGCM used for this study is that has been developed by the Open University Mars Modelling group,
arising from a collaboration between the Laboratoire de
Météorologie Dynamique (LMD), the Open University,
the University of Oxford, and Instituto de Astrofı́sica de
Andalućia [9]. The MGCM is run with 1920 dynamical
timesteps per day, roughly 80 timesteps per hour, and
uses a T31 spectral resolution, 70 vertical layers (scaled
by pressure), 26 tracers, and is run both with and without
heterogeneous chemistry.

Cloud Microphysics
In the MGCM, there exists a ‘simple clouds’ scheme
which uses the saturation water vapour limit to condense water vapour and form water ice. However, this
scheme often overpredicts water ice, which, when used
in conjunction with the heterogeneous scheme, tends to
lead to an overprediction of ozone by a factor of two.
This overprediction results from the heterogeneous
reactions use of the water ice surface area to calculate
the rate of adsorption. Consequently, the overprediction of water ice produces higher adsorption reaction
rates, leading to a greater abundance of HOx adsorbed
by waterice and, hence, a lower ozone destruction rate.
In order to account for this, a cloud scheme containing
water ice microphysics parametrizations is used in all
MGCM simulations. The formation of water ice in this
scheme is dependent on the cloud condensation nuclei
(CCN) available, and allows for the supersaturation of
water vapour to occur [10]. The cloud microphysics
scheme produces a lower water ice abundance, which
should decrease the impact of heterogeneous reactions
on ozone.

Chemistry
The chemical scheme used, ASIS, is taken from [11],
using offline photochemical rates. It consists of 22
chemical tracers, and 60 chemical and photochemical
reactions. The heterogeneous chemistry scheme used
in the MGCM is taken from [8], and has been adapted
by Brown et al. (in prep). Previously, studies have
incorporated water ice only for calculating the adsorption rates of heterogeneous reactions [5, 8]. This work
expands on Brown et al. (in prep) by improving the
desorption of HOx and H2 O2 by directly relating the
desorption rate constant to the sublimation of water-ice
clouds, as well as adapting the work from a 1-D model
to a full GCM. We use the change in water ice due to
sublimation to calculate the rate of desorption, which
makes the heterogeneous reactions more sensitive to the
vertical and temporal distribution of water ice. As a result, rapid changes in water ice abundance, such as the

Figure 1: Difference in ozone TCA between simulations from
the MGCM with and without heterogeneous chemistry (heterogeneous – gas-phase) between LS = 0◦ − 25◦ . Data are
zonally averaged. Note the non-linearity of the colourscale.

diurnal variations from night to day, are likely to have a
greater impact on the chemical processes.

Results
By comparing the ozone TCA simulated with and without heterogeneous chemistry, we study the global impact of heterogeneous chemistry on ozone and HOx .
This analysis expands on the investigation conducted
by Brown et al. (in prep), by discussing the vertical
distribution of ozone with heterogeneous chemistry and
assessing the global impact of heterogeneous chemistry
on ozone. We present results which show the global and
temporal differences in ozone when simulated with and
without heterogeneous chemistry, as well as the effects
this has on HOx species.
Observed and modelled TCA of ozone, and the observed relationship between ozone and water ice are used
as a proxy for the occurrence of heterogeneous chemistry. Analysing the ozone and water ice TCA observations in conjunction with the simulated ozone and water
ice gives an opportunity to show the strengths and weaknesses of including heterogeneous chemistry in a GCM.
The addition of heterogeneous chemistry changes the
HOx distribution, which can be studied by the seasonal
and local time variations in HOx abundance.
Seasonal Variation
Water-ice clouds have a seasonal variation, with the
greatest abundances occurring at high latitudes during
the local winter [12, 13]. As a result, HOx species are
expected to differ seasonally from the heterogeneous
simulation to the gas-phase-only simulation.
Figure 1 shows the preliminary ozone difference
between heterogeneous and gas-phase-only simulations
across all latitudes, zonally averaged, from LS = 0◦ −
25◦ . Orange indicates an increase in ozone abundance
in the heterogeneous scheme with respect to the gas-

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phase-only scheme, while purple indicates a decrease.
Ozone TCA increases at higher latitudes, likely due to
the formation of polar hood clouds during this time of
the year.
From preliminary results, ozone TCA is still underpredicted at low latitudes during the aphelion season
( LS = 0◦ − 90◦ ). This is likely due to water vapour
being overpredicted, which decreases ozone TCA as a
result of increased HOx abundance.
Local Time
Water ice abundance varies with local time, with a higher
abundance during the morning due to the condensation
of water vapour overnight [14]. With the inclusion of
heterogeneous chemistry, this variation is likely to impact the HOx vertical distribution. As water ice sublimates, HOx are released from the water-ice clouds.
Combined with the photolysis of water vapour at the
beginning of the day, the HOx abundance may therefore
be larger earlier in the sol than later in the evening. The
change in the vertical distribution of HOx species with
heterogeneous chemistry could impact the ozone abun-

dance, which is typically lower during the morning than
the evening [6].
Conclusion
This work builds on the analysis from Brown et al. (in
prep) to analyse the impact of heterogeneous chemistry
on ozone in areas of high and low water vapour TCA.
Nadir observations from TGO/NOMAD allow for a wide
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processes in the atmosphere.
This investigation studies the global and temporal effects of heterogeneous reactions through observed and
simulated ozone, and assesses the variation in HOx with
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ozone underprediction in GCMs [5, 8], and explore the
sensitivity of HOx in the presence of water ice on a local
and seasonal timescale. From preliminary results, the
addition of heterogeneous chemistry increases the ozone
TCA at high latitudes (> 60◦ N/S) at the northern spring
equinox, due to the presence of the polar hoods.
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