  The aim of this study is to measure the vertical distribution of HCN on
Titan's stratosphere using ground-based submm observations acquired
quasi-simultaneously with the Herschel ones. This allows us to perform a
consistency check between space and ground-based observations and to build a
reference mean HCN vertical profile in Titan's stratosphere. Using APEX and
IRAM 30-m, we obtained the spectral emission of HCN (4-3) and (3-2) lines. We
applied a line-by-line radiative transfer code to calculate the synthetic
spectra of HCN, and a retrieval algorithm based on optimal estimation to
retrieve the temperature and HCN distributions. Our derived HCN abundance
profiles 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. We
also demonstrate that the retrieved HCN distribution is sensitive to the data
information. Comparisons between our results and the values from Herschel show
similar abundance distributions, with maximum differences of 2.5 ppm ranging
between 100 and 300 km. These comparisons also allow us to inter-validate both
data sets and indicate reliable and consistent measurements. The inferred
abundances are also consistent with the distribution in previous observational
studies, with the profiles from ALMA, Cassini/CIRS, and SMA (the latest ones
below ~230 km). Our HCN profile is also comparable to photochemical models by
Krasnopolsky (2014) and Vuitton et al. (2019) below 230 km and consistent with
that of Loison et al. (2015) above 250 km. However, it appears to show large
differences 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.
