Time evolution of HCl detections and the meridional column dust opacities for Mars years 34 (blue) and 35 (orange). Panel a: dust optical depth at 9.3 μm derived fromMars Climate Sounder (Montabone et al. 2020). The mean column dust opacities are first zonally averaged, then a meridional average is shown. The meridional averages are taken over bands covering the northern hemisphere (40–70°N) and southern hemisphere (40–70°S). Panel b: maxima of ACS MIR HCl detections below 30 km (circles). For observations in which no HCl was detected at a 5σ level, a 3σ criterion is first used (open circles), and if this is not met, detection limits are given (crosses).