  State of the art spectral retrieval models of exoplanet atmospheres assume
constant chemical profiles with altitude. This assumption is justified by the
information content of current datasets which do not allow, in most cases, for
the molecular abundances as a function of pressure to be constrained. In the
context of the next generation of telescopes, a more accurate description of
chemical profiles may become crucial to interpret observations and gain new
insights into atmospheric physics. We explore here the possibility of
retrieving pressure-dependent chemical profiles from transit spectra, without
injecting any priors from theoretical chemical models in our retrievals. The
"2-layer" parameterisation presented here allows for the independent extraction
of molecular abundances above and below a certain atmospheric pressure. By
simulating various cases, we demonstrate that this evolution from constant
chemical abundances is justified by the information content of spectra provided
by future space instruments. Comparisons with traditional retrieval models show
that assumptions made on chemical profiles may significantly impact retrieved
parameters, such as the atmospheric temperature, and justify the attention we
give here to this issue. We find that the 2-layer retrieval accurately captures
discontinuities in the vertical chemical profiles, which could be caused by
disequilibrium processes -- such as photo-chemistry -- or the presence of
clouds/hazes. The 2-layer retrieval could also help to constrain the
composition of clouds and hazes by exploring the correlation between the
chemical changes in the gaseous phase and the pressure at which the condensed
phase occurs. The 2-layer retrieval presented here therefore represents an
important step forward in our ability to constrain theoretical chemical models
and cloud/haze composition from the analysis of future observations.
