Relationships between HCl, H2O, aerosols, and temperature in the Martian atmosphere

Detecting trace gases such as hydrogen chloride (HCl) in the atmosphere of Mars is among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Terrestrially, HCl is closely associated with active volcanic activity, so its detection on Mars was expected to point to some form of active magmatism or outgassing. However, after its discovery using the mid-infrared channel of the TGO Atmospheric Chemistry Suite (ACS MIR), a clear seasonality was observed, beginning with a sudden increase in HCl abundance from below detection limits to 1-3 ppbv in both hemispheres coincident with the start of dust activity, followed by very sudden and rapid loss at the southern autumnal equinox. In this study, we have investigated the relationship between HCl and atmospheric dust by making comparisons in the vertical distribution of gases measured with ACS and aerosols measured co-located with the Mars Climate Sounder (MCS). This study includes HCl, water vapour, and ozone measured using ACS MIR, water vapour and temperature measured with the near infrared channel of ACS (ACS NIR), and temperature, dust opacity, and water ice opacity measured with MCS. In Part I, we present the methods, observations of HCl, and describe the seasonal evolution of the vertical structure of each of these above quantities. The studied time period encompasses solar longitude 180°-360° in Mars years 34-36, covering the dusty period around perihelion. In Part II, we investigate the quantitative correlations between each quantity and discuss the possible source and sinks of HCl, their likelihood given the correlations, and any issues arising from them.