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Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid

Juan Carlos Gómez Martín, Thomas R. Lewis, Alexander D. James, Alfonso Saiz-Lopez and John M. C. Plane

J. Am. Chem. Soc., https://doi.org/10.1021/jacs.1c12957

Abstract:

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 (HIO 3) in iodine-driven particle formation. Here, we report flow tube laboratory experiments that solve these discrepancies by showing that both IxOy and HIO3 are involved in atmospheric new particle formation. I 2Oy 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 (I2 O5 ) 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 I 2Oy. Under such conditions, iodine-containing clusters and particles are formed by aggregation of I 2 Oy and HIO3 , while under dry laboratory conditions, particle formation is driven exclusively by I 2Oy. An updated mechanism for iodine gas-to-particle conversion is provided. Furthermore, we propose that a key iodine reservoir species such as iodine nitrate, which we observe as a product of the reaction between iodine oxides and the nitrate anion, can also be detected by CIMS in the atmosphere.

 

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