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 (I 2 O 4 ), which is the top candidate among iodine oxides (I x O y ), is proposed to fill this gap. I 2 O 4 drives nucleation under dry conditions but is not correlated with the NPF rate in humid marine air. Although gas-phase hydrolysis to HIO 3 and HIO 2 has been widely proposed as the primary loss pathway of I 2 O 4, direct hydrolysis is kinetically hindered (the activation barrier is ∼25.8 kcal mol –1 ). Here, we identify the product (HIO 3 )-autocatalyzed hydrolysis of I 2 O 4 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. I 2 O 4 first seeds initial clusters but is rapidly converted to HIO 3 and HIO 2 via this pathway, which dominates iodine nucleation under nearly all marine conditions. The residual gas-phase I 2 O 4 concentration decreases to