PDR model line ratios for subsolar (solid lines) and solar metallicity (dashed lines): HCO⁺/HCN (top) and HCN/CO (bottom). Different colors indicate different FUV field strengths G₀ = 10 (red), 50 (green), and 100 (blue). Every panel of a subfigure shows the results for one density; from left to right and top to bottom n_H = 0.1, 0.5, 1, 5, 10, 50, and 10² × 10⁴ cm^-3. Gray areas mark the range of observed ratios in M 33. The dashed horizontal lines show the values from the stacked spectra.