Iodine-mediated nucleation and particle growth: laboratory measurements of IO production and its implications

Reactive iodine chemistry significantly influences atmospheric oxidation and new particle formation. The detailed chemical mechanism linking iodine monoxide (IO) radicals to particle nucleation and growth, however, remains insufficiently constrained. Here, we designed a dual-optical cell Differential Optical Absorption Spectroscopy (DOAS) system to generate and measure IO. Combined with laboratory measurements and an observation-constrained chemical model, we revealed the direct correlation between IO and particle formation under controlled O₃ and I₂ levels, and further provided modeling insights into the potential roles of higher iodine oxides and iodic acid (HIO₃) in aerosol dynamics. Results showed that IO although not a nucleating species, can promote the formation of intermediates, driving nucleation and size-dependent growth. Observation-constrained model simulations revealed that HIO₃ concentrations exhibited a power-law dependence on particle growth rates (GR) across different size bins. Notably, under dry laboratory conditions (RH ∼5.5%), HIO₃ may remain an important contributor to particle growth, while higher iodine oxides like I₃O₇, played minor roles. An increase in O₃ from 10 to 50 ppbv was associated with enhanced HIO₃ production and a more than 20% increase in GR for the 1.8-3.2 nm size range. These results suggest that, under dry iodine-rich conditions, HIO₃ may be relevant to particle growth, although the extent of its atmospheric importance will depend on local precursor levels and environmental conditions. In addition, variations in ambient O₃ may influence HIO₃ formation and, consequently, iodine-mediated particle growth. These findings provide experimental and modeling evidence that nonlinear O₃-iodine interactions may need to be considered for accurately evaluating the impact of iodine chemistry on aerosol loading and regional air quality.