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

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 (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.

Critical Role of Iodous Acid in Neutral Iodine Oxoacid Nucleation

“…The good agreement on [(HIO 3 ) 1 (HIO 2 ) 1 ] also suggests that (HIO 3 ) 1 (HIO 2 ) 1 is well captured by the nitrate-CIMS and it is unlikely a surrogate for other iodine species as suggested by a recent study. 37 Although the agreement between our calculations and the CLOUD data is good at −10 °C, the agreement is poorer at +10 °C, but nevertheless acceptable within theoretical and experimental uncertainties (see below). The median J ratio (ACDC/CLOUD) is 56 (Figure S6) but reaches as high as 789.…”

“… 32 − 35 In contrast to ambient observations, recent laboratory studies with high iodine concentrations shed doubts on this mechanism and proposed that I 2 O y could have been interpreted as gaseous HIO 3 . 36 , 37 …”

“…32−35 In contrast to ambient observations, recent laboratory studies with high iodine concentrations shed doubts on this mechanism and proposed that I 2 O y could have been interpreted as gaseous HIO 3 . 36,37 Sophisticated experiments were carried out in the Cosmic Leaving Outdoor Droplets (CLOUD) chamber at CERN to study iodine particle formation at atmospherically relevant conditions to resolve the puzzles. With a finely tuned nitrate-CIMS, gaseous HIO 3 was unambiguously measured.…”

The Competition between Hydrogen, Halogen, and Covalent Bonding in Atmospherically Relevant Ammonium Iodate Clusters

“…33 However, the role of gas phase HIO 3 is unclear as some fraction of the detected iodic acid may result from the chemical ionization process. 49 Since acid−base chemistry plays such an important role in these early stage NPF clusters, the role of ammonia has also been explored in iodine containing clusters as it is important in stabilizing sulfuric acid and nitric acid containing clusters. 50,51 Quantum chemical calculations found that ammonia can aid in the hydrolysis of I 2 O 5 and it can also interact with HIO 3 , increasing nucleation rates of these clusters through the lowering of transition state energies via increased hydrogen bond donors and acid−base interactions.…”

The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source

“…This variability is iodine atoms 10,34 , hydrated IO radicals 34 , iodine dioxide (OIO) radicals 35 and larger iodine oxides (I 2 O 3 , I 2 O 4 and I 2 O 5 ; refs. 34,[36][37][38] ). However, these mechanisms remain speculative and have not been demonstrated experimentally, leaving atmospheric HIO 3 observations unexplained.…”