Laboratory Measurements of Ferric Chloride (FeCl) under Venusian Conditions.

Ferric chloride (FeCl) in sulfuric acid cloud droplets has been proposed to explain the inhomogeneous near-ultraviolet (UV) absorption visible at the Venusian cloud tops. However, the absorption spectrum of FeCl in concentrated sulfuric acid does not appear to have been measured previously; here we report measurements under appropriate conditions of temperature and HSO/HO solution strengths. The choice of solvent has a significant effect on the measured spectrum. The reaction of FeCl in aqueous HSO to form ferric sulfate (Fe(SO)) was shown to be suppressed by adding HCl to the solution (as would occur in the Venusian atmosphere). The FeCl spectrum in sulfuric acid is shown to be in good agreement with observations of the unknown absorber in Venus' atmosphere. The presence of Fe(SO), which absorbs strongly below 320 nm, should be considered when reconstructing Venusian spectra to avoid misattribution of absorption in this spectral region to SO, potentially leading to an overestimation of the SO cloud top concentrations.