Masks used in this work. Top: mask used to compute power spectrum after excluding the brightest CO emission regions in the Galactic ridge. Bottom: mask used for fitting global CO mixing coefficients at low Galactic latitude the Galactic plane, obtained from the sky fraction observed in the Dame et al. (2001) survey, excluding regions with CO emission larger than 3 K_RJ km s⁻¹. This results in the brightest 0.5% of pixels (in the Galactic ridge) being masked.