  One of the ultimate goals of the ESA Ariel space mission is to shed light on
the formation pathways and evolution of planetary systems in the Solar
neighbourhood. Such an endeavour is only possible by performing a large
chemical survey of not only the planets, but also their host stars, inasmuch as
stellar elemental abundances are the cipher key to decode the planetary
compositional signatures. This work aims at providing homogeneous abundances of
C, N, and O of a sample of 181 stars belonging to the Tier 1 of the Ariel
Mission Candidate Sample. We applied the spectral synthesis and the equivalent
width methods to a variety of atomic and molecular indicators (C I lines at
5052 and 5380.3 A, [O I] forbidden line at 6300.3 A, C_2 bands at 5128 and 5165
A, and CN band at 4215 A) using high-resolution and high S/N spectra collected
with several spectrographs. We provide carbon abundances for 180 stars,
nitrogen abundances for 105 stars, and oxygen abundances for 89 stars. We
analyse the results in the light of the Galactic chemical evolution, and in
terms of the planetary companions properties. Our sample basically follows the
typical trends with metallicity expected for the [C/Fe], [N/Fe], and [O/Fe]
abundance ratios. The fraction between C and O abundances, both yields of
primary production, is consistent with a constant ratio as [O/H] increases,
whereas the abundance of N tends to increase with the increasing of the O
abundance, supporting the theoretical assumption of a secondary production of
nitrogen. The [C/N], [C/O], and [N/O] ratios are also correlated with [Fe/H],
which might introduce biases in the interpretation of the planetary
compositions and formation histories if host stars of different metallicity are
compared. We provide relations that can be used to qualitatively estimate
whether the atmospheric composition of planets is enriched or not with respect
to the host stars.
