  In 2020, hydrogen chloride (HCl) in the gas phase was discovered in the
atmosphere of Mars with the Atmospheric Chemistry Suite (ACS) onboard the Trace
Gas Orbiter (TGO) mission (Korablev et al., 2021). Its volume mixing ratio
(VMR) shows a seasonal increase of up to 5 ppbv during the perihelion season,
followed by a sudden drop to undetectable levels, contradicting previous
estimations of the HCl lifetime of several months. In the Earth's stratosphere,
heterogeneous uptake of HCl onto water ice is known to be a major sink for this
species. Modelling of associated chemistry involving heterogeneous reactions
indicates that H2O ice becomes the most effective sink for HCl above 20 km with
the characteristic time shorter than 12 hours. In this work, we use
simultaneous measurements of water ice particles and HCl abundance obtained by
the ACS instrument and show particular structures in the vertical profiles,
forming detached layers of gas at the ice free altitudes ('ice-holes'). We
demonstrate that the heterogeneous uptake of HCl onto water ice operates on
Mars and is potentially a major mechanism regulating the HCl abundance in the
atmosphere of Mars.
