  The Atmospheric Chemistry Suite (ACS) instrument onboard the ExoMars Trace
Gas Orbiter (TGO) ESA-Roscosmos mission began science operations in March 2018.
ACS Mid InfraRed (MIR) channel notably provides solar occultation observations
of the martian atmosphere in the 2.3 - 4.2 $\mu$m spectral range. Here we use
these observations to characterize water ice clouds before and during the MY 34
Global Dust Storm (GDS). We developed a method to detect water ice clouds with
mean particle size $\leq$ 2 $\mu$m, and applied it to observations gathered
between $L_s=165^\circ$ and $L_s=243^\circ$. We observe a shift in water ice
clouds maximum altitudes from about 60 km before the GDS to above 90 km during
the storm. These very high altitude, small-sized ($r_\mathrm{eff} \leq 0.3$
$\mu$m) water ice clouds are more frequent during MY34 compared to non-GDS
years at the same season. Particle size frequently decreases with altitude,
both locally within a given profile and globally in the whole dataset. We
observe that the maximum altitude at which a given size is observed can
increase during the GDS by several tens of km for certain sizes. We notably
notice some large water ice particles ($r_\mathrm{eff}\geq1.5$ $\mu$m) at
surprisingly high altitudes during the GDS (50 - 70 km). These results suggest
that GDS can significantly impact the formation and properties of high altitude
water ice clouds as compared to the usual perihelion dust activity.
