Photochemical lifetimes of HCl on Mars calculated by the 1-D photochemistry model. (a) The photochemical partial column lifetime (within 5–35 km) of atmospheric HCl without (solid) and without (dashed) the heterogeneous uptake of HCl onto CaCO₃ in airborne dust and water ice crystals, photochemical loss rates (s⁻¹ ) and subsequent photochemical lifetime (τ) of HCl at 45° N at (b) 71° L_s and (c) 251° L_s.