Distributions for three selected CO line ratio in different galactic environments. The locations of the various galactic regions are shown in Fig. 10. For each region, the CO (2–1) brightness temperature weighted median (see Eq. (8)) is indicated by the circle and the 16th and 84th percentile range by the bars. The numerical values are given in Table 4. For the ¹²CO (2–1)/(1–0) line ratio, we find evidence of larger values in the interarm than in the spiral arm. For ¹²CO (3–2)/(2–1) we find the opposite trend. ¹²CO (3–2)/(2–1) and ¹³CO/¹²CO (1–0) show a clear enhancement of the line ratio in the galaxy centre.