Time evolution of CO depletion factor, f_D, evaluated in astrochemical model Grid 1 (left column) and Grid 2 (right column) as a function of the environmental variables of: (a) top row: number density of H nuclei, n_H; (b) second row: temperature, T; (c) third row: cosmic ray ionisation rate, ζ; (d) fourth row: visual extinction, A_v. In each panel the three remaining variables are held fixed at their fiducial level (i.e. n_H = 10⁵ cm⁻³, T = 15 K, ζ = 10⁻¹⁷ s⁻¹, A_n = 100 mag). Each time-evolving model is shown by a horizontal line with fD indicated by the colour scale.