Relative sulfur abundance as function of A_V for a spherical cloud. The model parameters are mass: M = 1   M_⊙, surface density n = 10⁶ cm^-3, and FUV intensity χ = 10⁶ in units of Draine. Three different chemical networks were applied: a) the original UDFA rates; b) the original UDFA rates with all reactions possessing γ < 0 set to zero; and c) all reactions with γ < 0 replaced by the new fits from Table 1.