Output of the Horsehead Nebula photon dominated region model III with a low sulfur abundance in the initial conditions (S / H = 5.8 × 10^-8) for the CCH hydrocarbon abundance as a function of visual extinction in the cloud illuminated by the σOri star (see Appendix A for details). The abundance of CCH is shown in the top panel at different times after injection of the a–C:H grain photolysis in the model. The corresponding abundance ratio of CCH with the addition of photolytic reactions divided by the abundance without the addition of the photolytic reactions is shown in the bottom panel for the same times.