Gómez Martín JC, Lewis TR, James AD, Saiz-Lopez A, Plane JMC. Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid.
J Am Chem Soc 2022;
144:9240-9253. [PMID:
35604404 PMCID:
PMC9164234 DOI:
10.1021/jacs.1c12957]
[Citation(s) in RCA: 11] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
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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. 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. 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. 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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