Mechanistic insights into I ₂ O ₅ heterogeneous hydrolysis and its role in iodine aerosol growth in pristine and polluted atmospheres

Abstract. Higher-order 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 relatively 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 an unheeded step toward improving the representation of iodine chemistry and marine aerosol formation in atmospheric models, with implications for climate prediction and environmental impact assessment.