Top panel: formation of CO ice mantles as a function of the cloud lifetime for a cloud density of 1 × 10⁴ cm^-3 and T_gas = T_dust = 7 K, with no photodesorption (dashed gray trace) and if photodesorption balances the accretion (solid black trace). Bottom panel: R_acc(CO) if there is no photodesorption (dashed gray trace), R_acc(CO) if photodesorption balances the accretion (solid black trace), and R_ph − des(CO) represented as a dotted trace for the same initial parameters as a function of the cloud lifetime.