Time evolutions of the abundances of CO (left column), HCO⁺ (middle column) and N₂H⁺ (right column) evaluated in astrochemical model Grid 1 (dotted lines) and Grid 2 (solid lines), exploring the effects 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) bottom row: visual extinction, A_v. In each row, 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_v = 100 mag).