Cumulative column density ratios in a plane slab of A_V = 10 mag illuminated by the left side for a grid of UV fields and hydrogen nuclei densities. The ratios were calculated using the Meudon code (Le Petit et al. 2006; Goicoechea et al. 2007). G₀ is given in units of the Habing field. Levels drawn in red correspond to the values measured in Mon R2. Symbols indicate the physical conditions of the protypical PDRs: Mon R2 (square), Orion Bar (triangle), NGC 7023 (circle), and Horsehead (cross). Note that the CO⁺/HCO⁺ and HCO/HCO⁺ ratios are excellent diagnostics of the UV incident field. The IF and MP positions in Mon R2 have the abundance characteristics of dense PDRs with high UV fields, while the chemistry of the MP2 position is more typical of a low UV PDR.