Title: Seasonal evolution of $\mathrm{C_2N_2}$, $\mathrm{C_3H_4}$, and $\mathrm{C_4H_2}$ abundances in Titan's lower stratosphere,
Abstract: We study the seasonal evolution of Titan's lower stratosphere (around
15~mbar) in order to better understand the atmospheric dynamics and chemistry
in this part of the atmosphere. We analysed Cassini/CIRS far-IR observations
from 2006 to 2016 in order to measure the seasonal variations of three
photochemical by-products: $\mathrm{C_4H_2}$, $\mathrm{C_3H_4}$, and
$\mathrm{C_2N_2}$. We show that the abundances of these three gases have
evolved significantly at northern and southern high latitudes since 2006. We
measure a sudden and steep increase of the volume mixing ratios of
$\mathrm{C_4H_2}$, $\mathrm{C_3H_4}$, and $\mathrm{C_2N_2}$ at the south pole
from 2012 to 2013, whereas the abundances of these gases remained approximately
constant at the north pole over the same period. At northern mid-latitudes,
$\mathrm{C_2N_2}$ and $\mathrm{C_4H_2}$ abundances decrease after 2012 while
$\mathrm{C_3H_4}$ abundances stay constant. The comparison of these volume
mixing ratio variations with the predictions of photochemical and dynamical
models provides constraints on the seasonal evolution of atmospheric
circulation and chemical processes at play.