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.
Highlights
Photolabile iodine-containing molecules are emitted into the lower atmosphere from the sea surface and from marine biota
As described in the experimental section, kinetic profiles of the growth and removal of the target iodine oxide species shown in Fig. 3 were carried out in order to define the time periods with the most suitable kinetic profiles for photolysis measurements (Fig. 5)
Fragmentation of iodine oxides was a significant problem in these experiments, as predicted from ionization energy calculations of larger iodine oxides to possible photofragments at 10.5 eV (Gómez Martín et al, 2020)
Summary
Photolabile iodine-containing molecules are emitted into the lower atmosphere from the sea surface and from marine biota. Lewis et al.: Determination of the absorption cross sections iodine atoms is reaction with ozone to form iodine monoxide (IO). This radical photolyses readily (Gómez Martín et al, 2005), creating an ozone-neutral cycle and establishing a steady-state concentration of I and IO, which are termed collectively active iodine or IOx. Any other chemical cycle involving IO which recycles atomic iodine without concomitant generation of atomic oxygen leads to ozone depletion, e.g. Any other chemical cycle involving IO which recycles atomic iodine without concomitant generation of atomic oxygen leads to ozone depletion, e.g. (Gómez Martín et al, 2009):
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