Influence of vertical mixing on millimetre CO observations. The top row shows the CO concentration є(CO) = n_CO/n_〈H〉 in the disc on linear scales for radius r and vertical height z. The two dashed contour lines enclose the region of maximum CO concentration, where χ is the UV strength and n = n_〈H〉 is the total hydrogen nuclei particle density. Below dust temperature T_dust ≈ 23 K, CO is mostly frozen. The second row shows the ¹³CO J= 2 → 1 line velocity profile observed from a distance of 140 pc at a disc inclination of 45°. The third row shows the integrated line map after continuum subtraction and the bottom row shows a channel map at radial velocity v + 0.94 km s⁻¹. To plot the intensities in Kelvin, we consider I_v c²/(2v²)/k, where I_v [erg cm⁻² s⁻¹ Hz⁻¹ sr⁻¹] is the spectral intensity, c the speed of light, v the frequency, and k the Boltzmann constant.