Time evolution of rate R_{H, CO} obtained by summing the rates of the reactions that involve the individual bins with different binding energies, as defined in Eq. (27). We note the decline of the rates with time, given by the increasing T_d that evaporates more and more reactants from the surface of the grains. The models are A (blue), B (orange), and C (green), as indicated in Fig. 2, while solid and dashed lines indicate multiple- and single-bin approaches, respectively.The vertical dotted lines and the circular markers are the same as in Fig. 2, employed as a reference for Fig. 4.