Laboratory measurements of ferric chloride (FeCl₃) under Venusian conditions

Ferric chloride (FeCl₃) 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 FeCl₃ in concentrated sulfuric acid does not appear to have been measured previously; here we report measurements under appropriate conditions of temperature and H₂SO₄/H₂O solution strengths. The choice of solvent has a significant effect on the measured spectrum. The reaction of FeCl₃ in aqueous H₂SO₄ to form ferric sulfate (Fe₂(SO₄)₃) was shown to be suppressed by adding HCl to the solution (as would occur in the Venusian atmosphere). The FeCl₃ spectrum in sulfuric acid is shown to be in good agreement with observations of the unknown absorber in Venus’ atmosphere. The presence of Fe₂(SO₄)₃, which absorbs strongly below 320 nm, should be considered when reconstructing Venusian spectra to avoid misattribution of absorption in this spectral region to SO₂, potentially leading to an overestimation of the SO₂ cloud top concentrations.