  We have conducted laboratory experiments to study the chemistry in hot
Jupiter atmospheres with C/O ratio of 0.35. We have compared our results with
the ones obtained previously for atmospheres with a C/O ratio of 1 to
investigate the influence of the C/O ratio on the chemistry and formation of
photochemical organic aerosol. We found that the C/O ratio and the gas mixture
compositions strongly influence the pathways responsible for the formation of
CO2. Thermochemical reactions are primarily responsible for the formation of
CO2 in low C/O ratio atmospheres, while photochemistry is the dominant process
in high C/O ratio atmospheres even if the final CO2 concentration is the same
in both cases. Our results show that low C/O atmospheres at the thermochemical
equilibrium contain a higher water abundance, while high C/O atmospheres are
significantly depleted in water. However, in low C/O atmospheres, the water
abundance is not affected by UV photolysis, while our previous work
demonstrated that significant amount of water can be produced in high C/O ratio
atmospheres. This contrast in water production suggests that photochemistry
should be considered when interpreting exoplanet transit spectra. Finally, we
did not observe the formation of a detectable amount of non-volatile
photochemical aerosols in low C/O atmospheres, in contrast to our previous
study. We infer that for C/O ratio < 1, water likely inhibits organic growth
and aerosol formation, suggesting that photochemical organic aerosols are
likely to be observed in planets presenting a carbon enrichment compared to
their host stars.
