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The aim of this study is to measure the vertical distribution of HCN onTitan's stratosphere using ground-based submm observations acquiredquasi-simultaneously with the Herschel ones. This allows us to perform aconsistency check between space and ground-based observations and to build areference mean HCN vertical profile in Titan's stratosphere. Using APEX andIRAM 30-m, we obtained the spectral emission of HCN (4-3) and (3-2) lines. Weapplied a line-by-line radiative transfer code to calculate the syntheticspectra of HCN, and a retrieval algorithm based on optimal estimation toretrieve the temperature and HCN distributions. Our derived HCN abundanceprofiles are consistent with an increase from 40 ppb at ~100 km to 4 ppm at~200 km, which is an altitude region where the HCN signatures are sensitive. Wealso demonstrate that the retrieved HCN distribution is sensitive to the datainformation. Comparisons between our results and the values from Herschel showsimilar abundance distributions, with maximum differences of 2.5 ppm rangingbetween 100 and 300 km. These comparisons also allow us to inter-validate bothdata sets and indicate reliable and consistent measurements. The inferredabundances are also consistent with the distribution in previous observationalstudies, with the profiles from ALMA, Cassini/CIRS, and SMA (the latest onesbelow ~230 km). Our HCN profile is also comparable to photochemical models byKrasnopolsky (2014) and Vuitton et al. (2019) below 230 km and consistent withthat of Loison et al. (2015) above 250 km. However, it appears to show largedifferences with respect to the estimates by Loison et al. (2015), Dobrijevic &Loison (2018), and Lora et al. (2018) below 170 km, and by Dobrijevic & Loison(2018) and Lora et al. (2018) above 400 km, although they are similar in shape.We conclude that these particular photochemical models need improvement.

Ground-based HCN submillimetre measurements in Titan's atmosphere: an intercomparison with Herschel observations