Mechanistic Insights into the Autocatalyzed Hydrolysis of I ₂ O ₄ : A Paradigm for Reactive Nucleation

Marine new particle formation (NPF) sustains the abundance of global cloud condensation nuclei (CCN); however, intense iodine-driven NPF bursts cannot be explained by iodine oxoacids alone. Iodine tetroxide (I₂O₄), which is the top candidate among iodine oxides (IxOy), is proposed to fill this gap. I₂O₄ drives nucleation under dry conditions but is not correlated with the NPF rate in humid marine air. Although gas-phase hydrolysis to HIO₃ and HIO₂ has been widely proposed as the primary loss pathway of I₂O₄, direct hydrolysis is kinetically hindered (the activation barrier is ∼25.8 kcal mol⁻¹). Here, we identify the product (HIO₃)-autocatalyzed hydrolysis of I₂O₄ as the novel and dominant reactive pathway that governs marine iodine nucleation. Facilitated by strong halogen and hydrogen bonds, this autocatalyzed pathway lowers the activation barrier to 1.7 kcal mol⁻¹, yielding an effective reaction rate that is competitive with cluster collision rates. I₂O₄ first seeds initial clusters but is rapidly converted to HIO₃ and HIO₂ via this pathway, which dominates iodine nucleation under nearly all marine conditions. The residual gas-phase I₂O₄ concentration decreases to <1% of its initial abundance in humid air, quantitatively explaining its persistent ambient scarcity in the marine atmosphere. Collectively, these results not only resolve the longstanding I₂O₄ paradox but also establish a paradigm for reactive nucleation in marine iodine chemistry, while further advancing our understanding of marine iodine cycling.