Evolution with H atom fluence of the C–H (a), C=O (b), N–H (c), and N=C=O (HOCN) (d) normalized column density (filled circles). In the first three cases, the normalization factor is the corresponding initial column densities, while for HNCO, the data are normalized to the initial value of the column density of NH₂CHO. The best fit to the data (solid lines), following first order kinetic relations (1 − a₁(1 − e^−σ_dF_H) and a₂(1 − e^−σ_fF_H)) are also shown. The first equation refers to the destruction with the boundary condition that considers a residual reactant for F_H = ∞ (i.e., H atoms cannot react with the deeper ice layers). The second equation refers to the formation limited by the availability of the reactants. The estimated destruction (σ_d) and formation (σ_f) cross-sections and the asymptotic destruction of NH₂CHO (a₁) and asymptotic formation of HNCO (a₂) are also reported.