Flow tube laboratory experiments that show both IxOy and HIO3 are involved in atmospheric new particle formation, thereby solving a long-standing question about the mechanism of iodine gas-to-particle conversion.

Iodine chemistry is an important driver of new particle formation in the marine and polar boundary layers. There are, however, conflicting views about how iodine gas-to-particle conversion proceeds. Laboratory studies indicate that the photooxidation of iodine produces iodine oxides (IxOy), which are well-known particle precursors. By contrast, nitrate anion chemical ionization mass spectrometry (CIMS) observations in field and environmental chamber studies have been interpreted as evidence of a dominant role of iodic acid (HIO3) in iodine-driven particle formation.   These experimental results, aided by theoretical calculations, solve these discrepancies by showing that both IxOy and HIO3 are involved in atmospheric new particle formation. I2Oy molecules (y = 2, 3, and 4) react with nitrate core ions to generate mass spectra similar to those obtained by CIMS, including the iodate anion. Iodine pentoxide (I2O5) produced by photolysis of higher-order IxOy is hydrolyzed, likely by the water dimer, to yield HIO3, which also contributes to the iodate anion signal. We estimate that ∼50% of the iodate anion signals observed by nitrate CIMS under atmospheric water vapor concentrations originate from I2Oy. Under such conditions, iodine-containing clusters and particles are formed by aggregation of I2Oy and HIO3, while under dry laboratory conditions, particle formation is driven exclusively by I2Oy.   Finally, a full mechanism for iodine gas-to-particle conversion is provided.   Juan Carlos Gómez Martín, Thomas R. Lewis, Alexander D. James, Alfonso Saiz-Lopez, and John M. C. Plane. Insights into the chemistry of iodine new particle formation: The role of iodine oxides and the source of iodic acid. JACS, DOI: https://doi.org/10.1021/jacs.1c12957