Variations of the HDO/CO₂ line depth ratio as a function of the HDO mixing ratio for two extreme profiles (black lines: Model 2; red lines: Model 5), for two different values of the airmass (am = 1.0 and 5.0). It can be seen that, for low values of the line depth ratio, the behavior is similar to the H₂O₂ case (Fig. A.4). The curves converge in a single point where this ratio is close to 0.7−0.9, and diverge again for higher values of the line depth ratio.