Iodine is a trace atmospheric constituent crucial in atmospheric processes. Unlike sulfuric acid, methanesulfonic acid, and nitric acid, which require additional vapors (ammonia or dimethylamine) for particle formation, iodine, along with highly oxygenated organic molecules (HOMs), can nucleate particles independently. Iodine's nucleation rates surpass those of sulfuric acid at comparable iodic acid (HIO3) concentrations. HIO3 also enhances nanoparticle growth rates. Current atmospheric models largely neglect iodine particle formation, focusing primarily on its ozone-depleting properties. While sulfur emissions are projected to decrease, iodine emissions have tripled since 1950 due to human activities, mainly from oceans through ozone reactions with dissolved iodide. This marine source is amplified by ozone pollution, sea ice thinning, and now accounts for ~3 Tg yr−1 of iodine emissions. Over the past 70 years, iodine concentrations have tripled in various environmental records. Iodine's high reactivity participates in catalytic cycles impacting ozone loss and potentially particle formation. Its ozone destruction potential significantly exceeds that of bromine and chlorine. Despite its atmospheric ubiquity and HIO3 detection in various locations, the gas-phase formation mechanism of HIO3 remained unclear, hindering the connection between iodine sources and particle formation in atmospheric models. Various precursors have been proposed, but these mechanisms lacked experimental verification.

The literature highlights iodine's significant role in atmospheric chemistry, particularly its impact on ozone depletion and particle formation. Studies have shown that iodine is highly reactive and participates in catalytic reaction cycles. While its catalytic role in ozone destruction is well-established, its role in particle formation has not been fully understood. Several studies reported the detection of iodic acid (HIO3) in various atmospheric environments, but the gas-phase formation mechanism remained elusive. Previous research suggested several potential precursors for HIO3, including hydrated iodine atoms, hydrated IO radicals, iodine dioxide (OIO) radicals, and larger iodine oxides (I2O4, I2O5, and I2O6). However, these mechanisms were largely speculative and lacked experimental validation. The lack of a robust HIO3 formation mechanism hampered the integration of iodine's role in particle formation into atmospheric models. Recent field observations of iodine-induced nucleation over remote oceans and IOx in stratospheric aerosols highlighted the widespread role of iodine in particle formation, but the missing HIO3 source mechanism prevented a complete understanding of the connection between iodine sources and particle formation.

This study employed laboratory experiments at the CERN CLOUD chamber, a large-volume (26.1 m3) atmospheric simulation chamber. Experiments were conducted under marine boundary layer conditions at 283 K and 263 K, with typical I2 concentrations of 8 pptv, 40% relative humidity, and 40 ppbv O3. The experiments utilized photolysis of I2, measured by cavity-enhanced differential optical absorption spectroscopy (CE-DOAS) and bromide chemical ionization mass spectrometry (Br−-MION-CIMS). HIO3 and HOI were measured quantitatively by NO3−-CIMS and Br−-MION-CIMS, respectively. The experiments were complemented by a chemical box model, constrained by measurements of I2 concentrations, actinic fluxes, temperature, humidity, wall losses, and chamber dilution. The model initially included established iodine chemistry, but was extended to incorporate two key reactions involving IOIO and ozone and water, based on experimental observations and theoretical calculations. Quantum chemical calculations (density functional theory (DFT) methods M062X/aug-cc-pVTZ-PP, followed by coupled-cluster single-point energy corrections) were used to explore the reactivity of IOIO with O3, H2O, and other reactants to understand HIO3 and HOI formation. The model was evaluated against experimental data, particularly the temporal evolution and concentrations of various iodine species and the HIO3 production rates under different conditions, and by comparing with field measurements from the Maïdo observatory in Réunion Island. The model simulations used established iodine chemistry reaction rates, and then incorporated additional reactions, accounting for the wall losses of various species and other atmospheric processes.

The CLOUD chamber experiments and the extended chemical box model revealed that iodooxy hypoiodite (IOIO) efficiently converts to HIO3 through a two-step process: (R1) IOIO + O3 → IOIO3, and (R2) IOIO3 + H2O → HIO3 + HOI + O2. The laboratory-derived reaction rate coefficients were corroborated by quantum chemical calculations. The model accurately predicted HIO3 and HOI concentrations, showing excellent agreement with measurements. HIO3 formation was found to be first-order with respect to I atom production rate (pI), explaining the presence of HIO3 even in remote oceans where pI is low. This first-order dependence also rules out larger iodine oxides (IxOy, x≥3) as precursors for HIO3 under typical atmospheric conditions. The extended model accurately predicted HIO3 production rates over a wide range of experimental conditions, showing that HIO3 formation is robust against variations in ozone, water vapor, and temperature. The rate-limiting step is the formation of IOIO, which is quantitatively converted into HIO3. The near-linear relationship between HIO3 production rate and I atom production rate highlights the significance of this pathway even at low iodine concentrations. Quantum chemical calculations support the proposed mechanism, showing exothermic reactions with low energy barriers. The calculated reaction rate coefficients for reactions (R1) and (R2) were consistent with the experimental findings. A comparison with field measurements from the Maïdo observatory in Réunion Island demonstrated the atmospheric relevance of the proposed HIO3 formation mechanism. The observatory data show excellent consistency between laboratory and field measurements of HIO3 production rates, further supporting the proposed mechanism's significance. The iodine recycling in the particle phase, resulting from the reduction of particulate iodate, closes a catalytic iodine cycle that enhances particle formation and destroys ozone. This multiphase cycle involves the formation and reduction of iodate (IO3-), leading to the release of volatile iodine species back into the gas phase. This cycle explains the correlation of particulate 131I with aerosol surface area distribution.

The findings address the long-standing question of the gas-phase formation mechanism of HIO3, a crucial species in atmospheric aerosol formation. The study provides a clear pathway linking iodine sources to new particle formation, explaining HIO3 observations in diverse atmospheric environments. The first-order dependence of HIO3 formation on I atom production rate highlights the mechanism's importance even at low iodine concentrations, extending its relevance beyond hotspots. The quantitative agreement between laboratory experiments and field observations validates the mechanism's atmospheric relevance. The catalytic cycle involving HIO3 formation and iodate reduction suggests a significant contribution of iodine to aerosol formation and ozone depletion. This catalytic cycle effectively amplifies the impact of iodine on atmospheric chemistry. The results highlight the need for incorporating this new HIO3 formation mechanism into atmospheric models to improve their accuracy in predicting aerosol formation and ozone levels, particularly in remote regions. This improved understanding of iodine chemistry has implications for climate modeling and air quality predictions.

This study presents a novel gas-phase formation mechanism for atmospheric iodic acid (HIO3) through the efficient conversion of iodooxy hypoiodite (IOIO). The findings, supported by laboratory experiments, theoretical calculations, and field observations, establish a critical link between iodine sources and particle formation. This mechanism, previously missing from atmospheric models, explains the widespread presence of HIO3 and underscores iodine's catalytic role in aerosol formation and ozone depletion. Future research should focus on incorporating this mechanism into atmospheric models to improve predictions of aerosol formation and ozone levels, as well as investigating the impact of this catalytic cycle on other atmospheric processes and exploring the HIO3 formation mechanism in other atmospheric environments, such as the stratosphere.

The study's limitations include the potential for uncertainties in the quantitative measurements of some iodine species due to the complexities of the chemical ionization mass spectrometry techniques. Additionally, the model's accuracy depends on the accuracy of the assumed reaction rates and loss processes, especially those for the wall losses in the CLOUD chamber. Future studies could address this by investigating the specific reaction rates and the accommodation coefficients to the chamber walls for specific iodine species. The focus of the study was on the gas-phase formation of HIO3; further investigations are needed to fully understand the multiphase chemistry involving aerosols and other heterogeneous processes involving iodine.