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

Abstract

Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O₃ surface concentrations. Although iodic acid (HIO₃) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO₃ via reactions (R1) IOIO + O₃ → IOIO₄ and (R2) IOIO₄ + H₂O → HIO₃ + HOI + ⁽¹⁾O₂. The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO₃ in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation.

Fig. 1. Coincident formation of HIO₃ and HOI in the early stages of iodine oxidation.

a–g, Time-resolved measurements of key iodine species (a,b,d show precursors to HIO₃ (f) and HOI (g), and c and e show higher-oxide routes) are compared with model predictions after the start of I₂ photolysis at green wavelengths within the CERN CLOUD chamber. Measured concentrations (grey lines) of HIO₃ and HOI exceed 10⁷ molecules per cm³ (molec cm⁻³) within minutes. Established gas-phase iodine chemistry (model base case, dashed blue lines) forms neither HIO₃ nor HOI, contrary to the observations, and overestimates the concentrations of IOIO and I₂O₄. The extended model (solid red line), including reactions (R1) and (R2) and considering a longer thermal lifetime of IOIO, achieves good mass and temporal closure for HIO₃, HOI, IOIO and I₂O₄. Source data

Fig. 2. HIO₃ yield η and rate order.

The HIO₃ production rate pHIO₃ scales in first order with the I atom production rate pI (median (solid line) and 25–75% and 5–95% inter-percentile ranges (dark and light grey shading)). The yield η is substantial (~20%) and near constant for pI larger than 10⁵ molec cm⁻³ s⁻¹. At smaller pI, losses of intermediates to chamber walls reduce η. This effect is captured by the model (red line (median)) and is explained by IO radical wall losses (compare blue dashed and red dotted lines (medians)). If larger I_xO_y clusters were the HIO₃ precursor, a higher-order yield would be expected—this is not consistent with the observations. Source data

Fig. 3. Quantum chemical calculations support HIO₃ and HOI as co-products of hypoiodide IOIO oxidation.

Reaction coordinate for the gas-phase reactions (R1) and (R2) as free energy ΔG(T = 298 K). The energies are calculated using theory at the CCSD(T)/CBS(T,Q)//M062X/aug-cc-pVTZ-PP level of theory. ΔG(TS3) (not rate-limiting) is calculated at the CCSD(T)/aug-cc-pVTZ-PP//M062X/aug-cc-pVTZ-PP level, due to memory limitations. The reaction coordinate supports that atmospheric concentrations of O₃ and H₂O lead to a quantitative conversion of IOIO into HIO₃, HOI and singlet O₂.

Fig. 4. Comparison with field measurements.

Good consistency is observed between HIO₃ production rates measured in the CLOUD laboratory (red) and at the Maïdo field site (blue). IO radical concentrations at CLOUD overlap with those found in the remote lower free troposphere. The solid black line is the IOIO formation rate from IO radicals (at 283 K), and corresponds to the rate-limiting step of HIO₃ formation under both field and laboratory conditions. Source data

Fig. 5. Simplified gas-phase iodine chemistry in the remote atmosphere.

After activation of iodine reservoirs (step 1), HIO₃ is efficiently formed (step 2) and subsequently nucleates and grows particles extremely efficiently (step 3). Iodate (IO₃⁻) can be reduced and re-emitted to the gas phase (step 4), closing an iodine-catalysed reaction cycle forming particles and destroying O₃. HIO₃ formation from IOIO links iodine sources and new particle formation even at lower IO concentrations. This mechanism is currently missing from atmospheric models.

Extended Data Fig. 1. Response in the HIO₃ concentration to varying the mixing fan speed.

A strong sensitivity of the HIO₃ concentration to changes in the wall loss lifetime t_wall (dashed black line) is observed. While other parameters are held constant, stirring of the CLOUD atmospheric simulation chamber is reduced at 19:57 UTC, increasing the wall loss lifetime by a factor of four. HIO₃ concentrations recover by a factor 12. The superlinear response is evidence for a reasonably long-lived precursor (that is, IO) that gets lost to the chamber walls. At 20:25 UTC, light is turned off, HIO₃ production stops, and the HIO₃ concentration is efficiently lost to the chamber walls. The model reproduces the observed behaviour if IO is considered to efficiently get lost to the chamber wall. Source data

Extended Data Fig. 2. Time and mass closure of hypothetical HIO₃ formation mechanisms.

Sensitivity studies assume hypothetical mechanisms that form HIO₃ from different precursors in the model. After the start of I₂ photolysis (Δt = 0), ΔHIO₃ is defined as HIO₃(Δt) - HIO₃(Δt = 0). HIO₃ measurements (thick black line, grey shading indicates 50 % uncertainty) and simulated time profiles assuming different hypothesised mechanisms in the model (coloured thin lines). The four panels a-d show the closure at different temperatures, and HIO₃ concentrations. The formation of HIO₃ via reactions R1 and R2 is the only mechanism compatible with observations regarding temporal and mass closure. Source data

Extended Data Fig. 3. Detection of iodine oxide radicals and I_xO_y species, including the key species IOIO, IOIO₄, HOI, and HIO₃.

Concentrations of iodine species as measured by the ${\mathrm{NO}}_3^{-}$-CIMS and the Br⁻-MION-CIMS, and as modelled by the base-case and extended model. The bottom panel shows the loss rate of sticky molecules to the chamber walls, to particle surfaces, and to dilution. The grey shaded period shows an experiment with extremely high IO_x concentrations, where IOIO₄ is clearly detected, but extreme particle concentrations and chamber inhomogeneities lead to higher model-measurement differences. The base-case model does not form any HOI or HIO₃ in UV-dark conditions. The extended model reproduces both and improves the closure also for other molecules. Calibration factors are given in Supplementary Table 3. T = 263 K. Source data

Extended Data Fig. 4. Sensitivity studies of the HIO₃ production towards changes in O₃, H₂O, and temperature.

For the ranges probed there is no pronounced sensitivity of HIO₃ production (normalised by IOIO production) observed. The linear rate order lines (long dashes) assume either O₃ or H₂O were controlling the rate limiting step towards HIO₃ formation. No such dependence is observed. The robustness in HIO₃ formation is evidence that neither O₃ nor H₂O (nor temperature) control the rate limiting step under the conditions probed. Measurements and predictions of the extended model agree within uncertainties. Measurements: 5-95 % whiskers, 25-75 % boxes, median. Model: median only. The grey shading indicates the combined measurement model uncertainty (65 %, 2 − σ standard deviation). Source data