Same as Fig. 13, but for two individual regions: W51 Main a), the region 49.4°<ℓ< 49.6°, − 0.5°<b< − 0.3°, and W51 B b) with 48.8°ℓ< 49.4° and − 0.5°<b< − 0.1°. Note that the y axes have different ranges. The area covered by the W51 B cutout is 6 × larger than the W51 A cutout, so its effect on Fig. 13 is greater. The total area covered in Fig. 13 includes some regions with no detected H₂CO, which is why the peak fraction in that figure is lower.