Iodine in the Atmosphere II: Cluster Formation Potential of Iodine Oxyacids and Iodine Oxides

Iodine-driven nucleation is thought to be a significant source of new particle formation, especially in marine and polar regions. Despite numerous studies, the mechanism is still not fully understood. To shed further light on this, we apply ZORA-DLPNO-CCSD(T_0)/TZVPP//ωB97X-D3BJ/aug-cc-pVTZ-PP to calculate the thermochemistry of iodine-containing clusters up to tetramers, and simulate the cluster formation potential for several nucleation paths using the atmospheric cluster dynamics code (ACDC). We find that iodine oxyacid--amine nucleation can be competitive with sulfuric acid--amine nucleation if iodic acid is present in a 10:1 ratio compared to sulfuric acid. Therefore, the importance of the iodine-driven pathway is regionally dependent. Likewise, we find that increasing the relative humidity from 34% to 73% only changes the cluster formation potential by a factor of 2. Nucleation pathways consisting of only iodic and iodous acid are unable to explain the relative nucleation rates previously observed in experiments. In contrast, the simultaneous nucleation of iodine oxides, assisted by iodine oxyacids, is better able to describe the trend. This indicates that a nucleation pathway starting with iodine oxides is more likely to be able to explain observed particle numbers. However, this current model does not include all the hydrates of the clusters and does not account for the hydrolysis reactions of the iodine oxides. This would need to be incorporated in future studies.