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

The study addresses the unresolved gas-phase formation mechanism of iodic acid (HIO3), a key driver of iodine-induced new particle formation. Iodine is an efficient nucleator compared to sulfuric acid and can nucleate without bases, contributing to atmospheric particle formation across marine and polar regions and even continental and free-tropospheric environments. Iodine emissions, largely oceanic and enhanced by surface O3 reactions with iodide and sea-ice changes, have increased markedly since 1950. Despite widespread detection of HIO3 and iodine oxides, atmospheric models rarely include iodine-driven particle formation and lack a mechanistic link between iodine sources and HIO3 production. Prior hypotheses for HIO3 precursors (hydrated iodine atoms or IO, OIO radicals, and larger iodine oxides) remained speculative without experimental confirmation, leaving a gap in explaining observed HIO3 and associated particle formation. This work aims to experimentally determine and validate the gas-phase pathway(s) that form HIO3 under atmospherically relevant conditions and to test their relevance with field observations.

Previous suggestions for HIO3 formation invoked hydrated iodine atoms, hydrated IO radicals, OIO, and larger iodine oxides (I2O4, I2O5, I2O6). However, none had been experimentally demonstrated under relevant conditions, and models relying on these pathways failed to reproduce observed HIO3, especially under OH-free or low-radical conditions. Established iodine chemistry included a theoretical OIO + OH pathway to HIO3, which is ineffective in the OH-free conditions typical of selective I2 photolysis experiments and likely in parts of the atmosphere. Field studies report iodine-induced nucleation over remote oceans and IOx in stratospheric aerosols, suggesting a broader role for iodine, but a definitive gas-phase HIO3 source mechanism has been missing. The literature also documents iodine’s catalytic ozone destruction efficiency and multiphase redox cycling of iodate (IO3−), highlighting iodine’s potential for catalytic cycles in atmospheric chemistry though not previously linked to particle formation.

Laboratory experiments were conducted in the 26.1 m3 CERN CLOUD chamber under marine boundary layer conditions at 283 K and 263 K, with typical I2 ≈ 8 pptv (range <0.5–330 pptv), O3 ≈ 40 ppbv (range <1–80 ppbv), and RH ≈ 40% (<3–90%). I2 was photolyzed using green LEDs (centered at 523 nm), generating iodine radicals without UV and minimizing OH production. Key species were monitored with CE-DOAS (I2), Br−-MION-CIMS (I2, HOI, and iodine oxides), and NO3−-CIMS (HIO3). Chamber wall-loss lifetimes were characterized via H2SO4, and experiments with large particle surface area were excluded to avoid additional sinks. HIO3 production rates were derived from measured concentrations and known wall losses assuming steady state. Sensitivity tests varied O3, RH, and fan speeds (modulating wall-loss lifetimes) under otherwise constant conditions. A photochemical box model incorporating state-of-the-art iodine and HOx chemistry was extended with chamber-specific processes (wall and particle losses, dilution, measured actinic fluxes) and new reactions: (R1) IOIO + O3 → IOIO4 and (R2) IOIO4 + H2O → HIO3 + HOI + HO2 (with updated IOIO thermal lifetime). The model was constrained by measured I2, O3, H2O2, photolysis frequencies, temperature, and loss processes. IO, OIO, IOIO, I2O2, I2O3, HI, and HIO3 were assumed to share H2SO4-like wall-loss rates; empirical HOI uptake (25% of H2SO4) closed observed HOI behavior. Quantum chemical calculations (M06-2X/aug-cc-pVTZ-PP geometries and frequencies, CCSD(T)/CBS(T,Q) single-point energies; iodine pseudopotentials from EMSL) mapped reaction coordinates and barriers for IOIO reactions with O3 and H2O to HIO3 + HOI + 1O2, benchmarking against known bond dissociation energies and proton affinities. Master equation simulations (MESMER) estimated effective temperature-dependent rate coefficients, including effects of excess energy. Field relevance was tested by comparing laboratory-derived HIO3 production rates with in situ HIO3 (NO3−-CIMS) and near-site IO (MAX-DOAS) observations at the Maïdo observatory (Réunion Island, 2200 m a.s.l.), representing the remote lower free troposphere with similar IO levels, condensational sink, and temperature as in CLOUD.

- Mechanism identification: IOIO is a quantitative precursor to HIO3 via a two-step sequence: (R1) IOIO + O3 → IOIO4, followed by (R2) IOIO4 + H2O → HIO3 + HOI + O2 (HO2 also considered in chamber model). Experimental time series show rapid increases of HIO3 and HOI to >1×10^7 molec cm−3 within minutes of I2 photolysis, reproduced only when including R1 and R2. - Kinetics and yields: HIO3 production rate (PHIO3) scales first-order with the I atom production rate (pI) across 10^4–10^6 molec cm−3 s−1. The HIO3 yield η = PHIO3/pI is substantial (≈10–20%) and nearly constant (~20%) at higher pI; at low pI, reduced η is explained by wall losses of intermediates (notably IO). Enhanced stirring experiments (reducing HIO3 wall-loss lifetime from ~8 min to ~2 min) caused >10× suppression of HIO3, consistent with an IO wall-loss sensitive multistep mechanism. - Precursor specificity: Larger iodine oxide clusters (IxOy, x≥3) are not required to form HIO3 under CLOUD conditions. If they dominated, a higher-order dependence on pI would be observed; measurements show first-order behavior. IOIO forms sufficiently early; larger clusters form too late to explain observed HIO3 timing. - Rate coefficients (298 K): Experimentally derived k1 (R1) ≥ 1.5×10^−13 molec−1 cm^3 s−1; theory supports k1 within uncertainty. For R2, theory predicts k2 = 5.7×10^−16 molec−1 cm^3 s−1, implying conversion within fractions of a second at typical RH (e.g., τ ≈ 0.023 s at 10% RH). Updated theory indicates IOIO thermal lifetimes are much longer than previously thought, consistent with persistent conversion to HIO3 even at low O3 and 263 K. - Insensitivity to O3, H2O, T: For ranges probed, PHIO3 normalized by PIOIO shows no pronounced dependence on O3, H2O, or temperature (263–283 K). The rate-limiting step is IOIO formation (from IO + IO); once formed, IOIO is fully and rapidly converted to HIO3. - Model–measurement closure: The extended model reproduces time evolution and steady-state levels of IO, OIO, IOIO, I2O4, HOI, and HIO3, as well as superlinear responses to wall-loss changes. Neglecting IO wall loss overestimates yields at low pI, confirming wall interactions. - Field validation: At the Maïdo observatory, observed PHIO3 aligns with CLOUD results when plotted versus IO concentrations, matching the IOIO formation rate line at 283 K. HIO3 increases rapidly during sunrise even under twilight, consistent with efficient activation of iodine reservoirs and the proposed mechanism. - Atmospheric implication: Each HIO3 formed consumes three O3 molecules. Given efficient particulate iodate reduction and iodine recycling, the mechanism completes a catalytic iodine cycle that repeatedly forms HIO3 and promotes particle formation while destroying O3.

The results resolve the long-standing question of how HIO3 forms in the gas phase under atmospherically relevant conditions. Demonstrating that IOIO is efficiently and quantitatively converted to HIO3 via oxidation by O3 followed by reaction with H2O links iodine radical production directly to HIO3 formation and new particle formation. The first-order dependence on iodine radical production explains HIO3 presence even in remote, low-iodine regions. Agreement between CLOUD chamber experiments, the extended chemical model, quantum chemistry, and free-tropospheric field measurements underscores the robustness and atmospheric relevance of the mechanism. This pathway provides the missing connection between oceanic iodine emissions (enhanced by surface O3), iodine radical chemistry, and nucleation/growth of iodine oxoacid particles. Because particulate iodate (IO3−) is efficiently reduced in multiphase reactions, re-emitting iodine back to the gas phase, the mechanism enables a catalytic cycle: a single iodine atom can repeatedly contribute to HIO3 formation and particle nucleation while also contributing to O3 destruction, potentially amplifying iodine’s role in atmospheric composition and aerosol–cloud–climate interactions.

This study experimentally identifies and validates the dominant gas-phase formation mechanism of iodic acid: IOIO oxidation by O3 to IOIO4 followed by rapid reaction with H2O to form HIO3 and HOI. The pathway operates efficiently at low iodine concentrations, exhibits first-order dependence on iodine radical production, and is supported by quantum chemical kinetics. The mechanism reproduces laboratory observations across varied conditions and explains field measurements in the remote lower free troposphere, closing the gap between iodine sources and particle formation in atmospheric models. Implications include a catalytic iodine cycle that promotes particle formation and O3 loss. Future work should incorporate temperature-dependent k1 and k2 into global models, assess the mechanism’s impact on global aerosol budgets under changing O3 and iodine emissions, investigate its role in the stratosphere and other environments, and further constrain uncertainties in larger iodine oxide chemistry and heterogeneous processes.

- Chamber-specific processes: Results rely on chamber wall-loss characterizations and assumptions (e.g., efficient IO wall uptake, H2SO4-like wall-loss rates for several iodine species). While varied and tested, these introduce uncertainties when extrapolating to the open atmosphere. - Theoretical uncertainties: Quantum chemical energies for iodine systems have ≈3 kcal mol−1 uncertainty (except OIO BDE), translating to about an order-of-magnitude uncertainty in calculated rate constants. Effective thermal lifetimes and rates include modeled excess-energy effects. - Larger IxOy chemistry: Some discrepancies remain for I2O3 predictions; larger iodine oxide pathways are uncertain and may vary at extremely high iodine levels not typical of the studied conditions. - Limited environmental range: Experiments covered 263–283 K, specific RH and O3 ranges, and OH-free green-light photolysis. Applicability to conditions with strong UV, different oxidants, or very low/high RH requires further validation. - Heterogeneous processes: The uptake and release of HOI and iodate reduction on particle surfaces were treated empirically or discussed qualitatively; detailed molecular-level mechanisms and rates remain to be fully constrained, especially for stratospheric relevance.