Top: line ratios of HNC(1−0)/HCN(1−0) as a function of the mean and the dispersion of the PDF. Bottom: line ratios of HCO⁺ (1−0)/HCN(1−0) as a function of the mean and the dispersion of the PDF. The dark brown stripe represents observational data. The overlapping region of these two line ratios is colored in black in the bottom panel. For all the grid points a typical A_V = 10 mag and Γ_mech  = 10^-22 cm^-3 is used. In both panels, Mach numbers are reported on the right axes and G is 100.