| Summary: | HCl was discovered in the atmosphere of Mars for the first time during the global dust storm in Mars year (MY) 34 (July 2018)
using the Atmospheric Chemistry Suite mid-infrared channel (ACS MIR) on the ExoMars Trace Gas Orbiter. The simultaneity of
variations in dust and HCl, and a correlation between water vapour and HCl, led to the proposal of a novel surface-atmosphere
coupling analogous to terrestrial HCl production in the troposphere from salt aerosols. After seasonal dust activity restarted in MY 35
(August 2020), we have been monitoring HCl activity to determine whether such a coupling was validated. Here we present a new
technique for analyzing the absorption features of trace gases close to the ACS MIR noise level and report that HCl mixing ratios
are observed to rapidly increase in both hemispheres coincidentally with the onset of the MY 35 perihelion dust season. We present
the temporal evolution of the vertical distribution of HCl (0.1–6 ppbv) and of dust activity in both hemispheres. We also report two
observations of > 2 ppbv HCl below 10 km in the northern hemisphere during the aphelion period.
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