Mechanistic Insights into I₂O₅ Heterogeneous Hydrolysis and Its Role in Iodine Aerosol Growth in Pristine and Polluted Atmospheres

Abstract. Higher iodine oxides are intricately linked to marine aerosol formation; however, the underlying physicochemical mechanisms remain poorly constrained, particularly for I₂O₅, which is stable yet conspicuously absent in the atmosphere. While reactivity with water has been implicated, the direct hydrolysis of I₂O₅ (I₂O₅ + H₂O → 2HIO₃) fails to account for this discrepancy due to its high activation barrier (21.8 kcal mol^-1). Herein, we have probed heterogeneous hydrolysis of I₂O₅ mediated by prevalent chemicals over oceans through Born-Oppenheimer molecular dynamics and well-tempered metadynamics simulations. Our results demonstrate that self-catalyzed pathways involving I₂O₅ and its hydrolysis product HIO₃ substantially reduce the reaction barrier, thereby accelerating the conversion of I₂O₅ to HIO₃ in pristine marine environments. In polluted regions, interfacial hydrolysis of I₂O₅ mediated by acidic or basic pollutants (e.g., H₂SO₄ or amines) proceeds with even greater efficiency, characterized by remarkably low barriers (≤1.3 kcal mol^-1). Collectively, these proposed heterogeneous reactions of I₂O₅ are highly effective, acting as a hitherto unrecognized sink for I₂O₅ and a source of HIO₃—processes that facilitate marine aerosol growth and rationalize the high iodate abundances detected in aerosols. These findings provide mechanistic insight into the elusive I₂O₅-to-HIO₃ conversion, offering a critical step toward improving the representation of iodine chemistry and marine aerosol formation in atmospheric models, with implications for climate prediction and environmental impact assessment.