Top: distance distributions used for the mass estimates. Solid line corresponds to the NH3 observations by Wienen et al. (2012). Dashed lines show the three models discussed in Sect. 4.3. Bottom: molecular gas mass estimate for the inner Galaxy (| ℓ | < 60° and | b | < 1.4°) from the ATLASGAL APEX/LABOCA and Planck/HFI combined maps of dust emission at 870 μm. Dots show the observed total mass as a function of a single adopted distance. Dashed line shows the mass estimation estimated with a Monte Carlo simulation. Red line shows the total estimated molecular gas mass based on CO measurements (Solomon et al. 1987).