CO depletion factor, f_D, evaluated in astrochemical model Grid 1 (dotted lines) and Grid 2 (solid lines), at fixed times of 10⁵ (blue), 10⁶ (green), 10⁷ (red) and 10⁸ yr (black) as a function of the environmental variables of: (a) top left: number density of H nuclei, n_H; (b) top right: temperature, T; (c) bottom left: cosmic ray ionisation rate, ζ; (d) bottom right: visual extinction, A_v. In each panel, the three remaining variables are held fixed at their fiducial level, that is n_H = 10⁵ cm⁻³, T = 15 K, ζ = 10⁻¹⁷ s⁻¹, A_v = 100 mag.