Mechanistic Insights into the Autocatalyzed Hydrolysis of I₂O₄: 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.