Determination of the absorption cross sections of higher-order iodine oxides at 355 and 532 nm

Abstract

Abstract. Iodine oxides (IxOy) play an important role in the atmospheric chemistry of iodine. They are initiators of new particle formation events in the coastal and polar boundary layers and act as iodine reservoirs in tropospheric ozone-depleting chemical cycles. Despite the importance of the aforementioned processes, the photochemistry of these molecules has not been studied in detail previously. Here, we report the first determination of the absorption cross sections of IxOy, x=2, 3, 5, y=1–12 at λ=355 nm by combining pulsed laser photolysis of I2∕O3 gas mixtures in air with time-resolved photo-ionization time-of-flight mass spectrometry, using NO2 actinometry for signal calibration. The oxides selected for absorption cross-section determinations are those presenting the strongest signals in the mass spectra, where signals containing four iodine atoms are absent. The method is validated by measuring the absorption cross section of IO at 355 nm, σ355nm,IO= (1.2±0.1) ×10-18 cm2, which is found to be in good agreement with the most recent literature. The results obtained are σ355nm,I2O3<5×10-19 cm2 molec.−1, σ355nm,I2O4= (3.9±1.2)×10-18 cm2 molec.−1, σ355nm,I3O6= (6.1±1.6)×10-18 cm2 molec.−1, σ355nm,I3O7= (5.3±1.4)×10-18 cm2 molec.−1, and σ355nm,I5O12= (9.8±1.0)×10-18 cm2 molec.−1. Photodepletion at λ=532 nm was only observed for OIO, which enabled determination of upper limits for the absorption cross sections of IxOy at 532 nm using OIO as an actinometer. These measurements are supplemented with ab initio calculations of electronic spectra in order to estimate atmospheric photolysis rates J(IxOy). Our results confirm a high J(IxOy) scenario where IxOy is efficiently removed during daytime, implying enhanced iodine-driven ozone depletion and hindering iodine particle formation. Possible I2O3 and I2O4 photolysis products are discussed, including IO3, which may be a precursor to iodic acid (HIO3) in the presence of HO2.

Modelling the impacts of iodine chemistry on the northern Indian Ocean marine boundary layer

“…Over Europe, combined halogen chemistry, which includes I, Br, and Cl, significantly reduces the concentrations of O 3 by as much as 10 ppbv. The contribution because of only iodine is also larger than in the current domain, which is expected because of the higher IO concentrations simulated in Europe (Li et al, 2019) and the iodine source parameterization being reduced in this study.…”

“…This change in NO x is smaller than simulated in Europe, with NO 2 predicted to increase across most of Europe and most regions showing an increase between 50-200 pptv. However, this was the increase reported due to the inclusion of all the halogens, and the impact of only iodine would be lower, even though higher levels were simulated across Europe (Li et al, 2019).…”

“…Li et al (2020) predicted higher levels in the south China Sea, with IO values ranging between 1-3 pptv. By comparison, results from the Community Multiscale Air Quality Modelling System (CMAQ) predicted peaks of 4-7 pptv in the coastal regions around Europe, while the open-ocean concentrations were below 1 pptv (Li et al, 2019). Thus, in comparison, the values in the Indian Ocean are lower, especially in July, than other regions studied hitherto using regional models, implying a reduced impact of iodine chemistry on the atmosphere in the northern Indian Ocean environment.…”

“…Recent modelling studies have made an attempt at quantifying the impact of iodine on a global scale (Saiz-Lopez et al, 2012bSherwen et al, 2016;Stone et al, 2018) and at regional scales (Li et al, 2019(Li et al, , 2020Muñiz-Unamunzaga et al, 2018;Sarwar et al, 2015). Although both approaches have shown significant effects of iodine on the atmosphere, a strong difference is observed in different regions due to the existing chemical regimes.…”

“…In China, the contribution of iodine to the halogen-mediated effect on atmosphere oxidizing capacity has been calculated to be up to 29 % (Li et al, 2020). Using the Community Multiscale Air Quality Model (CMAQ), Li et al (2019) showed that combined halogen chemistry (chlorine, bromine, and iodine) induces variable effects on OH (ranging from −0.023 to 0.030 pptv) and HO 2 (in the range of −3.7 to 0.73 pptv), reduces nitrate radical (NO 3 ) concentrations (∼ 20 pptv) and O 3 (by as much as 10 ppbv -equivalent to nmol mol −1 ), decreases NO 2 in highly polluted regions (by up to 1.7 ppbv), and increases NO 2 (up to 0.20 ppbv) in other areas. Another study using the same model suggested that in the Northern Hemisphere, halogen chemistry, without higher iodine oxide photochemical breakdown, leads to a reduction in surface ozone by ∼ 15 %, whereas a simulation including their breakdown leads to reductions of ∼ 48 % (Sarwar et al, 2015).…”