Variation of the gas-phase abundances of fundamental species in carbon chemistry, normalised by their respective peak abundances as a function of visual extinction, A_v. The cloud parameters for the different models are as follows: n_H = 10² cm⁻³, G₀ = 1 (top-left), n_H = 10² cm⁻³, G₀ = 1 × 10⁴, n_H = 10³ cm⁻³, G₀ = 1, and n_H= 10³ cm⁻³, G₀ = 1 × 10⁴ (bottom-right). All the models presented here use a constant cosmic-ray ionisation rate of ζ_H = 2.2 × 10⁻¹⁶ s⁻¹. The pink and grey shaded regions in each panel highlight the transition layer between H–H₂ and C⁺ –C, respectively.