4D Transport of Hydrogen Chloride in the Martian Atmosphere

Interest in the martian chlorine cycle has been renewed as a result of the recent discovery of hydrogen chloride (HCl) in the martian atmosphere (Korablev et al. 2021, Olsen et al. 2021). Korablev et al. 2021 used the Atmospheric Chemistry Suite (ACS) aboard the ExoMars Trace Gas Orbiter (TGO) and detected HCl quantities in the 1-4 ppbv range in both hemispheres. Subsequent measurements by Olsen et al. 2021 confirmed that HCl appears seasonally, with almost all positive detections occurring in the dusty second half of the martian year and with abundances decreasing rapidly after the dust season. In addition, a significant correlation between the vertical profiles of HCl and water vapour has also been observed (Olsen et al 2021, Aoki et al 2021). These findings have led to speculation that the source of atmospheric HCl arises from a direct coupling between chloride minerals in martian mineral dust with atmospheric water vapour (Korablev et al. 2021). Studies of martian chlorine chemistry thus far have focused on studying gas-phase and heterogeneous chemical reactions using simplified column models of the martian atmosphere. Whilst these models have advantages, they cannot evaluate the crucial impact of atmospheric dynamics and transport on the chlorine cycle. In this work we present the results of a full 4-dimensional modelling study of the transport and chemical interactions of chlorine and water species in the martian atmosphere, using the UK version of the Laboratoire de Météorologie Dynamique (LMD) Martian Global Climate Model. The model uses a modified version of the LMD photochemistry scheme (Lefevre et al. 2004) that has been augmented with a chlorine sub-model with 11 new gas species, 47 gas-phase reactions and 5 photolysis reactions in order to simulate the major chemical pathways of atmospheric chlorine (Duffy et al. 2014, Duffy 2015). By varying the locations, times and rates at which HCl is introduced and removed from the model, 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 constrain the rates of creation and destruction of HCl. Aoki, S. et al.: Annual appearance of hydrogen chloride on Mars and a striking similarity with the water vapor vertical distribution observed by TGO/NOMAD. Geophys. Res. Lett., 48(11):e2021GL092506, 2021. Duffy, M.K.D. et al.: Numerical simulations of the possible atmospheric origin of martian perchlorate. In F. Forget et al., editors, Fifth International Workshop on the Mars Atmosphere: Modelling and Observations, 2014. Duffy, M.K.D.: Studies of ozone and chlorine chemistry in the Martian atmosphere: implications for recent and future missions. PhD thesis, The Open University, 2015. Korablev, O. et al.: Transient HCl in the atmosphere of Mars. Sci. Adv., 7(7):eabe4386, 2021. Lefèvre, F. et al.: Three-dimensional modeling of ozone on Mars. J. Geophys. Res.-Planet, 109(E7), 2004. Olsen, K.S. et al.: Seasonal reappearance of HCl in the atmosphere of Mars during the Mars year 35 dusty season. Astron. Astrophys., 647:A161, 2021