Select transition lines of the carbon chain molecules, CH₃CCH and c-C₃H₂, the complex organic molecules CH₃OH, and the precursor complex organic molecule H₂CO, at the central pixel of the AFGL 2591 GBT and IRAM maps. The chosen lines represent a range of different upper energy levels, which are indicated in black on the plot. The dashed grey line represents the observed transition frequency at this location. The observed data is shown in black. The model spectra, to be discussed in Sect. 4.2 are shown in colour. The red solid line represents the total model, and the sole component if only one component is fit. For the multi-component fit in CH₃OH the separate components are shown in blue, yellow, and brown dashed lines.