Martian Atmospheric Chemistry of HCl: Implications for the Lifetime of Atmospheric Methane

Kurzfassung

We develop a 1-D atmospheric photochemistry model for Mars to interpret HCl profile measurements collected by the ACS MIR spectrometer aboard the ExoMars Trace Gas Orbiter (TGO) in Mars Year (MY) 34. We include a gas-phase chlorine chemistry scheme and study 1) surface chemistry, 2) hydrolysis, 3) photolysis, and 4) hydration and photolysis of dust grains as possible sources of gas-phase chlorine chemistry. Heterogeneous uptake of chlorine species onto water ice and minerals in Martian dust are loss processes common to all mechanisms. We drive the 1-D model using TGO profile measurements of aerosols and water vapour. Mechanisms 1, 2, 3 three result in significant model biases. Seasonal variations of Martian HCl are best reproduced by mechanism four, yielding low HCl abundances (< 1 ppb) prior to the dust season that rise to 2-6 ppb in southern latitudes during the dust season. We find that the additional Cl atoms released via mechanism four shortens the atmospheric lifetime of methane by a magnitude of 102. This suggests the production of Cl via the UV (or other electromagnetic radiation) induced breakdown of hydrated perchlorate in airborne Martian dust, consistent with observed profiles of HCl, helps reconcile observed variations of methane with photochemical models.