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

The missing mechanism from iodine sources to particle formation is discovered in the CLOUD chamber and confirmed by the field observations


Iodine emissions have been increasing due to human activities since 1950 and are forecasted to increase in the future. Iodine (I2) and hypoiodous acid (HOI) are primarily emitted from the oceans as a result of ozone reaction with iodide (I-).

Iodine is very efficient in forming atmospheric new particles; however, the gas-phase formation mechanism had been unknown and, thus, not properly accounted for in the atmospheric models.


CERN CLOUD experiments under marine boundary layer conditions: I2 concentrations are measured by cavity-enhanced differential optical absorption spectroscopy (CE-DOAS) and Br--MION-CIMS. Br--MION-CIMS provides precise measurements of I2 at low concentrations.

HOI is measured by Br--MION-CIMS, Iodic acid (HIO3) by NO3--CIMS. Chemical box modelling and quantum chemical calculations were also performed. Chamber experiments were compared to field measurements on Reunion Island (NO3--CIMS) in the Indian Ocean.

Highlighted results:

Laboratory experiments and theoretical calculations showed that the Base case model does not reproduce the observed concentrations of HIO3 and HOI. Therefore, two reactions (R1 and R2, Fig. 1) were added to the Base case model. The extended model and the measurements agree that the concentration of HOI3 and HOI rapidly increase to atmospherically relevant concentrations (107 molecules per cm3) within a few minutes after the initiation of Iphotolysis by green lights (Fig. 1).   

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

Observations of the HIO3 production rate in the CLOUD experiment are consistent with field measurements of HIO3 in the remote free troposphere (Reunion Island, elevation 2200 m), which confirms the atmospheric relevance of the proposed reactions. Moreover, a rapid increase of HIO3 concentrations is observed on Reunion Island already during the twilight phase of sunrise.  

The proposed mechanism can be now used in the atmospheric models to connect iodine sources and particle formation, even at low iodine concentrations.

The concentration of Iodine species measured with NO3- -CIMS and Br--CIMS (Fig. 2) confirms that many of the atmospherically relevant iodine species can be detected when using MION2 with rapidly switching between reagent ions NO3- and Br-.

Figure 2: Concentrations of iodine species measured by the NO3--CIMS and   Br--MION-CIMS, and predicted by base-case and extended models.

Reference: Finkenzeller, H., Iyer, S., He, XC. et al. The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source. Nat. Chem. 15, 129–135 (2023).  Licence information: This article is licensed under a Creative Commons Attribution 4.0 International License

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Iodic acid formation mechanism by Finkenzeller et al.

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