Time evolution of [CO] (gas [dotted] and ice [solid] phases) (left), [HCO⁺] (middle) and [N₂H⁺] (right) for the overall best fitting model (black lines, ζ = 2.2 × 10⁻¹⁸ s⁻¹) and best ‘late-time’ model (red lines, ζ = 2.2 × 10⁻¹⁶ s⁻¹) for the XR search of the P2 clump, i.e. with n_H = 1.0 × 10⁵ cm⁻³, A_v = 20 mag and T = 15 K. The time of the best models along each of these tracks is marked with a ‘+’ symbol. Horizontal dotted lines show the observed values of abundances, with the uncertainties shown by the shaded bands.