Time evolution of vertical profiles of the transmission level of solar occultation absorption spectra recorded with ACS MIR. Panels a–d: five time periods in the northern hemisphere that coincide with observed changes in dust activity and the detection of HCl. Panels e–h: same for the southern hemisphere. Lines are coloured by the dust optical depth at 9.3 μm derived from Mars Climate Sounder observations (Montabone et al. 2020).