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.
Light scattering and extinction measurements combined with laser-induced incandescence for the real-time determination of soot mass absorption cross-section.
