We study the seasonal evolution of Titans 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.
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