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

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 layer 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 4 iodine atoms are absent. The method is validated by measuring the absorption cross section of IO at 355 nm, σ355 nm, 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: σ355 nm, I2O3 < 5 × 10–19 cm2 molecule−1, σ355 nm, I2O4 = (3.9 ± 1.2) × 10–18 cm2 molecule−1, σ355 nm, I3O6 = (6.1 ± 1.6) × 10–18 cm2 molecule−1, σ355 nm, I3O7 = (5.3 ± 1.4) × 10–18 cm2 molecule−1 and, σ355 nm, I5O12 = (9.8 ± 1.0) × 10–18 cm2 molecule1. Photodepletion at λ = 532 nm was only observed for OIO, which enabled determining 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 at 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.