Spiral arm–interarm CO line ratio variation consistency. For the arm–interarm variation in Fig. 12, we include all sight lines with S/N > 5. Here, we increase the S/N threshold from 5 to 20 in steps of 5 to investigate whether the higher line ratios are still preserved in the interarm region. Furthermore, we add a radial constraint, only including sight lines within a given galactocentric radius of 4, 5, and 6 kpc. We normalise the line ratios by the global CO (2–1) brightness weighted median. We offset different line ratios in steps of 0.5 (in the positive direction for increasing S/N and in negative steps for decreasing radial constraint). We see that the trend of higher line ratios in the interarm region is persistent for all computations shown in this figure.