Important effects of relative humidity on the formation processes of iodine oxide particles from CH3I photo-oxidation

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 (I x O y ), 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 I x O y and HIO 3 are involved in atmospheric new particle formation. I 2 O y 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 (I 2 O 5 ) produced by photolysis of higher-order I x O y is hydrolyzed, likely by the water dimer, to yield HIO 3 , 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 2 O y . Under such conditions, iodine-containing clusters and particles are formed by aggregation of I 2 O y and HIO 3 , while under dry laboratory conditions, particle formation is driven exclusively by I 2 O y . 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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“…A recent experimental study indicates that the transition between the dry and humid IOP formation mechanisms occurs at around 20% RH. 53 …”

Section: Discussionmentioning

confidence: 99%

“…Note that, in addition, water-limited regions will generally be associated with lower pre-existing aerosol loadings, thereby increasing the survival chance of any newly formed iodine particle. A recent experimental study indicates that the transition between the dry and humid IOP formation mechanisms occurs at around 20% RH …”

Section: Discussionmentioning

confidence: 99%

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

et al. 2022

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“…Under the assumption that the monomers are spherical, we can calculate this number with eqn (6) : where M is the molecular mass and ρ is the density of HIO 3 monomer. The density was determined by R'Mili et al (2022) 59 to be 2.83 g cm −3 at 22% relative humidity. N A is the Avogadro constant and we add 0.3 nm for the mobility correction (difference between mass and mobility diameter).…”

Section: Methodsmentioning

confidence: 99%