Chlorine photochemistry in the Mars Planetary Climate Model

Introduction

A few years ago, two instruments on the Mars Trace Gas Orbiter (TGO) revealed the presence of hydrogen chloride (HCl) at ppbv levels in the Martian atmosphere during Martian Year (MY) 34 (Korablev et al. 2021) — the first detection of a halogenated gas on Mars. It has since been detected during MY 35 and 36 (Olsen et al. 2021, Aoki et al. 2021, Olsen et al. 2024).  There is still a lot of debate about the production and loss of HCl on Mars but there are some clues associated with its seasonal behaviour, linked with changes in atmospheric dust and water vapour. Specifically, HCl appears to be linked with dust activity – it appears at the start of the dust season and quickly disappears soon afterwards.

Chlorine gas-phase chemistry, as described in current photochemical models, is not sufficient to reproduce observed levels and geographical variations of Martian HCl. The atmospheric e-folding lifetime of atmospheric HCl is 90 to 1000 sols (Aoki et al. 2021) but the variations in the HCl abundance observed by Korablev et al. (2021) correspond to a lifetime  shorter than 75 sols (Krasnopolsky 2022). New studies show that heterogeneous chemistry – including the release Cl from airborne dust and the uptake of HCl on dust and water ice – is more consistent with observed variations of HCl (Taysum et al. 2024, Streeter et al. 2024). The 1-D photochemistry model of Taysum et al. (2024) reproduced HCl observations from the TGO ACS MIR instrument in the southern hemisphere but had more difficulty in the northern hemisphere. The authors attribute these model errors partly to the absence of horizontal transport. We address this shortcoming of the 1-D model by including the Taysum et al. (2024) chemical network into the 3-D LMD Mars Planetary Climate Model (MPCM).

Implementing the chlorine chemistry in the MPCM

The Taysum et al. (2024) chemical chlorine network includes 14 chlorinated species, 7 photodissociation reactions, 50 gas-phase reactions, and 5 heterogeneous reactions. Chlorine is produced via a heterogeneous reaction between water vapour and dust, based on experimental work from Zhang et al. (2022), while HCl is lost to dust and water ice.

The MPCM is a Global Circulation Model developed collaboratively by the Laboratoire de Météorologie Dynamique (LMD), the Laboratoire Atmospheres et Observations Spatiales (LATMOS) and the Atmospheric and Oceanic Planetary Physics sub-department in Oxford. It couples a dynamic and a physical part, including atmospheric photochemistry, allowing us to compute the composition of the Martian atmosphere, and therefore the HCl abundances as observed by the ExoMars TGO instruments.

We present results from control and sensitivity numerical experiments to highlight the importance of the heterogeneous chemistry on HCl abundances. These experiments use input data – atmospheric species abundances, aerosol profiles and dust scenario – for MY 34 from the Mars Climate Database (MCD) v6.1 (Millour et al. 2018).

Results

Our model produces layered HCl structures with maximum abundances at ppbv level in both hemispheres, broadly consistent with ACS MIR data collected during MY 34 (Korablev et al. 2021). It also reproduces the seasonal variations of HCl in the Martian atmosphere –  HCl levels increase at the beginning of the dust season and drop rapidly at the end of the season. In agreement with Taysum et al. (2024), we find that this rapid loss is mainly due to HCl uptake on water ice, followed by uptake on dust, with integrated column rates orders of magnitude higher than gas-phase reactions. In other words, the heterogeneous chemistry appears to play an important role in reproducing HCl in the Martian atmosphere. Consequently, HCl abundances in our model are very sensitive to our assumed heterogeneous uptake coefficients. This points to a need for new measurements of these coefficients in conditions representative of the Martian atmosphere to challenge the robustness of our results.