Ratios of emission from , HCO⁺, HNC, and CS(2−1) relative to CO, and C¹⁸O relative to . The regression lines were fit to the data at R ≤ 6.6 kpc (R ≤ 6.3 kpc for HCO⁺) to avoid contamination from the W31 complex at V ≤ 10 km s^-1 that appears as a false excess of emission at larger galactocentric radii for species with higher dipole moments. The mean ratios ± 1σ and slopes of the fitted regression lines are shown in the individual panels.