4D HCL TRANSPORT IN THE MARTIAN ATMOSPHERE WITH COMPARISONS TO TGO OBSERVATIONS.
K. Rajendran, School of Physical Sciences, The Open University, Milton Keynes, UK (kylash.rajendran@open.ac.uk),
S. R. Lewis, P. M. Streeter, J. A. Holmes, School of Physical Sciences, The Open University, Milton Keynes, UK,
M. R. Patel, School of Physical Sciences, The Open University, Milton Keynes, UK, Space Science and Technology
Department, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Oxfordshire, UK.

The recent discovery of hydrogen chloride (HCl) in
the martian atmosphere [1, 2] has renewed interest in
the martian chlorine cycle and its potential impact on
martian atmospheric chemistry. Here we present the
results of a numerical modelling study of the transport
and chemical interactions of chlorine species in the martian atmosphere, in order to better constrain the rates of
creation and destruction of HCl.
Korablev et al. [1] used the Atmospheric Chemistry Suite (ACS) aboard the ExoMars Trace Gas Orbiter
(TGO) and detected HCl in the 1–4 ppbv range in both
hemispheres before and during the Mars Year (MY) 34
global dust storm. Subsequent measurements in MY 35
[2] have confirmed the seasonal nature of observed HCl,
with almost all detections occurring during the dusty
second half of the martian year (LS =180 − 360°). HCl
abundances were found to decrease rapidly at the end of
the dusty season. The seasonal nature of the observed
HCl, with near-simultaneous onset in both hemispheres,
and the non-detection of any sulfur species in the martian
atmosphere [3] suggest that transient volcanic activity is
unlikely to be the source of the observed HCl.
In addition to the correlation with dust, a correlation
between HCl and water vapour has also been observed
[4]. Olsen et al. [2] found that HCl spectral lines were
never found without the presence of water vapour lines,
and reported a Pearson correlation coefficient of 0.73
between vertical profiles of HCl and water vapour over
MYs 34 and 35. These correlations have raised the
possibility of a direct coupling between surface minerals
and atmospheric gases, with heterogeneous processes
likely playing a key role [1, 2, 5].
Studies of martian chlorine chemistry thus far have
focused on studying chemical reactions using simplified 1-dimensional column models of the martian atmosphere. Whilst these models have advantages, they cannot evaluate the crucial impact of atmospheric dynamics
on the chlorine cycle. In this work we present results of
an investigation of the chlorine cycle on Mars using a full
Global Climate Model (GCM). The GCM used for this
study is the Open University Mars GCM, which shares
physics packages with the Laboratoire de Météorologie
Dynamique (LMD) Mars GCM [6] but utilises a spectral
dynamical core [7] and a semi-Lagrangian scheme to advect aerosols and chemical species [8]. The model uses
a modified version of the LMD photochemistry scheme
[9] that has been augmented with a chlorine sub-model

Figure 1: The evolution of HCl released from 15 km above the
surface at 45°S (left column) and 45°N (right column) respectively. HCl is first introduced at LS =180° and is continuously
injected for 60 sols. Panels show the zonal mean HCl volume
mixing ratio at multiples of 40 sols (+1) after initial release.

REFERENCES

with 11 new gas species, 47 gas-phase reactions and
5 photolysis reactions in order to simulate the major
chemical pathways of atmospheric chlorine [10, 11].
By varying the locations, times and rates at which HCl
is introduced and removed from the model (e.g. Figure
1), we quantify the impact of both chemical and transport processes on the subsequent horizontal and vertical
distribution of HCl. These results are then compared
to the reported observations of chlorine, allowing us to
derive constraints on the creation and destruction rates
of HCl.
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