Photochemistry of HCl in the martian atmosphere

The recent discovery of HCl in the martian atmosphere using the ExoMars Trace Gas Orbiter (Korablev et al., 2021, Sci. Adv. 7, eabe4386) during a global dust storm indicates a correlation between the HCl appearance and dust. Here this idea is studied using one-dimensional photochemical modeling on two summer midlatitude sites in the aphelion and perihelion seasons (L-S = 70 degrees at 40 degrees N and L-S = 250 degrees at 40 degrees S). Dissociation energies of chlorides lead to the conclusion that FeCl3 is the most active chloride that can form gaseous chlorine species in reactions with H and O, while NaCl is probably the most abundant chloride and may contribute to the production of HCl as well. Heterogeneous loss of the atmospheric chlorine is expected by Cl on FeCl2 and by HCl and HOCl on water ice. We have included five heterogeneous reactions that control HCl abundances, and probabilities of four of those are unknown and chosen to fit the observed HCl. The models involve 35 reactions of the standard CO2-H2O-NO chemistry and 38 reactions of chlorine chemistry that determine vertical profiles of nine chlorine species. Along with the five heterogeneous chlorine reactions, three reactions of heterogeneous loss of OH, HO2, and H2O2 on water ice are included in the models. Alternative mechanisms of the production of HCl in the martian atmosphere are related to electric discharges and not considered here because of the lack of observational evidences for those discharges. The background atmospheres at the chosen sites, including vertical profiles of temperature, density, H2O, and dust and water ice aerosol, are calculated using the Thermal Emission Spectrometer database. The calculated abundances of O-3, H2O2, and the O-2 dayglow at 1.27 mu m agree with observations and other models. The predicted HCl at perihelion varies from 2 ppb at the surface to 10 ppb above 55 km, in agreement with the observed 0.5-5 ppb at 5-35 km. The model cannot reproduce the observed cutoff in HCl above 35 km. The lifetime of HCl is 70 days at perihelion, close to some estimates based on the observations. The abundance of HCl in aphelion is approximate to 1.5 ppt at equilibrium and 60 ppt if the HCl lifetime is constant. The effects of HCl chemistry on major photochemical products are low on Mars. Photolysis of the high-altitude water vapor near perihelion facilitates production of H and hence HCl, and this explains the observed correlation between H2O and HCl.