  Clouds are ubiquitous in exoplanet atmospheres and represent a challenge for
the model interpretation of their spectra. Complex cloud models are too
numerically costly for generating a large number of spectra, while more
efficient models may be too strongly simplified. We aim to constrain the
atmospheric properties of the directly imaged planet HR 8799e with a free
retrieval approach. We use our radiative transfer code petitRADTRANS for
generating spectra, which we couple to the PyMultiNest tool. We added the
effect of multiple scattering which is important for treating clouds. Two cloud
model parameterizations are tested: the first incorporates the mixing and
settling of condensates, the second simply parameterizes the functional form of
the opacity. In mock retrievals, using an inadequate cloud model may result in
atmospheres that are more isothermal and less cloudy than the input. Applying
our framework on observations of HR 8799e made with the GPI, SPHERE and
GRAVITY, we find a cloudy atmosphere governed by disequilibrium chemistry,
confirming previous analyses. We retrieve that ${\rm
C/O}=0.60_{-0.08}^{+0.07}$. Other models have not yet produced a well
constrained C/O value for this planet. The retrieved C/O values of both cloud
models are consistent, while leading to different atmospheric structures:
cloudy, or more isothermal and less cloudy. Fitting the observations with the
self-consistent Exo-REM model leads to comparable results, while not
constraining C/O. With data from the most sensitive instruments, retrieval
analyses of directly imaged planets are possible. The inferred C/O ratio of HR
8799e is independent of the cloud model and thus appears to be a robust. This
C/O is consistent with stellar, which could indicate that the HR 8799e formed
outside the CO$_2$ or CO iceline. As it is the innermost planet of the system,
this constraint could apply to all HR 8799 planets.

Earth and Planetary Astrophysics