Ferric chloride photochemistry in sulfuric acid droplets: Constraints on SO2 oxidation by O2

Venusian sulfuric acid clouds play a central role in the planet’s sulfur cycle, radiative balance, and atmospheric chemistry. One long-standing problem in Venus chemistry is the significant depletion of sulfur dioxide (SO₂) across the cloud layers. Observations show that the concentration of SO₂ decreases by three orders of magnitude from the bottom to the top of the cloud layers. Gas-phase chemistry alone cannot fully account for this depletion, suggesting that heterogeneous uptake and oxidation of SO₂ in cloud droplets may serve as an additional sink. In terrestrial atmospheric chemistry, dissolved SO₂ can be oxidized by oxygen (O₂) through Fe(III)-catalyzed reactions. The reaction could be a candidate pathway for reactive uptake of SO₂ into Venusian sulfuric acid droplets.

This chemistry may also be relevant to another unresolved issue in Venus’ atmosphere: the photochemical stability of the CO₂-dominated atmosphere. Photolysis of CO₂ produces CO and O₂, yet O₂ remains extremely scarce in the gas phase. Photochemistry in sulfuric acid droplets containing SO₂ and iron has been proposed as a possible O₂ sink, in which sulfite-derived radicals rapidly react with O₂ and form peroxymonosulfate (HSO₅^–) ¹. Because HSO₅^– can potentially oxidize CO to CO₂, this chemistry may also contribute to CO₂ regeneration in the cloud layer. Thus, ferric chloride photochemistry in sulfuric acid droplets could potentially link SO₂ depletion, O₂ removal, and . In addition, ferric chloride has recently been discussed as a possible contributor to the unknown UV absorption in Venus’ clouds, motivating further investigation of iron chloride chemistry in sulfuric acid droplets.

Here, we present preliminary laboratory experiments designed to test whether iron-containing sulfuric acid droplets can promote SO₂ oxidation by O₂ under Venus-relevant acidic conditions. Single droplets were levitated using an electrodynamic balance and analyzed by Mie resonance and Raman scattering. Ferric chloride was introduced as an iron source, and both acid composition and iron abundance were selected within ranges relevant to Venus cloud conditions. The droplets were exposed to gas mixtures containing SO₂ and O₂ , which was used to test whether photochemical Fe redox cycling and radical formation can promote SO₂ oxidation by O₂. To interpret the reactivity, we also calculated Fe speciation in sulfuric acid solutions, indicating that dissolved rather than as hydroxo or sulfito complexes, commonly invoked in dilute aqueous Fe-catalyzed SO₂ oxidation. This contrasts with our previous NO₂ oxidation experiments using the same setup, where clear droplet growth was observed when both SO₂ and NO₂ were supplied at 100 ppm². This comparison suggests the low efficiency of Fe-mediated SO₂ oxidation by O₂ under low-O₂ and highly acidic conditions. Possible limiting factors include the low O₂ abundance, suppressed formation of reactive S(IV) species in strongly acidic droplets, and sulfate-dominated Fe speciation.

These preliminary results provide an experimental constraint on ferric chloride photochemistry and Fe-mediated O₂ oxidation in Venus-analog sulfuric acid droplets. Future work will quantify the detection limit, and vary O₂ abundance, UV flux, Fe concentration, and sulfuric acid concentration. These experiments will help assess whether iron-containing sulfuric acid cloud droplets can act as sites for SO₂ depletion and O₂ sink chemistry in Venus’ atmosphere.

 

(1)   Rowland, G.; Eldik, R.; Phillips, L. F. Photochemistry of Concentrated Sulfuric Acid in the Presence of SO2 and Fe(II), and Implications for the Cloud Chemistry of Venus. Journal of Photochemistry and Photobiology A-chemistry 2002, 153, 1–10.

(2)   Ubukata, S.; Karyu, H.; Nakagawa, H.; Koyama, S.; Minamikawa, R.; Kuroda, T.; Terada, N.; Gen, M. Uptake of SO₂ into Sulfuric Acid Droplets through the Oxidation by NO₂ under Venus-Analogous Conditions. ACS Earth Space Chem. 2025, 9 (6), 1525–1533.