Lewis, Thomas R.; Gómez Martín, Juan Carlos; Blitz, Mark A.; Cuevas, Carlos A.; Plane, John M. C.; Saiz-Lopez, Alfonso

Iodine oxides (inline-formulaIxOy) 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 inline-formulaIxOy, inline-formulax=2, 3, 5, inline-formulay=1–12 at inline-formulaλ=355 nm by combining pulsed laser photolysis of inline-formulaI2∕O3 gas mixtures in air with time-resolved photo-ionization time-of-flight mass spectrometry, using inline-formulaNO2 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, inline-formula M8inlinescrollmathml italic σ normal 355 nobreak0.125em normal nm , nobreak0.125em normal IO = 57pt12ptsvg-formulamathimgbe0b89e71de67d168ed90f8a2b661021 acp-20-10865-2020-ie00001.svg57pt12ptacp-20-10865-2020-ie00001.png (inline-formula1.2±0.1) inline-formula M10inlinescrollmathml × normal 10 - normal 18 37pt14ptsvg-formulamathimgf6c81bdba2af4ac70869ecec47c18b8f acp-20-10865-2020-ie00002.svg37pt14ptacp-20-10865-2020-ie00002.png  cminline-formula2, which is found to be in good agreement with the most recent literature. The results obtained are inline-formula M12inlinescrollmathml italic σ normal 355 0.125emnobreak normal nm , nobreak0.125em normal I normal 2 normal O normal 3 nobreak0.125em italic < nobreak0.125em normal 5 × normal 10 - normal 19 110pt17ptsvg-formulamathimg89df5c5e5946ba7aeb89f39871240bc9 acp-20-10865-2020-ie00003.svg110pt17ptacp-20-10865-2020-ie00003.png  cminline-formula2 molec.inline-formula−1, inline-formula M15inlinescrollmathml italic σ normal 355 nobreak0.125em 0.125emnobreak normal nm , nobreak0.125em normal I normal 2 normal O normal 4 = 67pt12ptsvg-formulamathimg601b6317c4e4a7953820911945bbea5e acp-20-10865-2020-ie00004.svg67pt12ptacp-20-10865-2020-ie00004.png (inline-formula M16inlinescrollmathml normal 3.9 ± normal 1.2 ) × normal 10 - normal 18 85pt15ptsvg-formulamathimg647bec75b1120dbca7f13987dd1093ef acp-20-10865-2020-ie00005.svg85pt15ptacp-20-10865-2020-ie00005.png  cminline-formula2 molec.inline-formula−1, inline-formula M19inlinescrollmathml italic σ normal 355 0.125emnobreak nobreak0.125em normal nm , 0.125emnobreak normal I normal 3 normal O normal 6 = 67pt12ptsvg-formulamathimgd1771de8d0c9f2e5dfec77138fe18f40 acp-20-10865-2020-ie00006.svg67pt12ptacp-20-10865-2020-ie00006.png (inline-formula M20inlinescrollmathml normal 6.1 ± normal 1.6 ) × normal 10 - normal 18 85pt15ptsvg-formulamathimg52c0333e9ad35cfc53e3929dba0ac936 acp-20-10865-2020-ie00007.svg85pt15ptacp-20-10865-2020-ie00007.png  cminline-formula2 molec.inline-formula−1, inline-formula M23inlinescrollmathml italic σ normal 355 0.125emnobreak nobreak0.125em normal nm , 0.125emnobreak normal I normal 3 normal O normal 7 = 67pt12ptsvg-formulamathimg1ee4752f6836cab5b34ac39907257efc acp-20-10865-2020-ie00008.svg67pt12ptacp-20-10865-2020-ie00008.png (inline-formula M24inlinescrollmathml normal 5.3 ± normal 1.4 ) × normal 10 - normal 18 85pt15ptsvg-formulamathimgbd2c1223e60ca150f383536a9b77231f acp-20-10865-2020-ie00009.svg85pt15ptacp-20-10865-2020-ie00009.png  cminline-formula2 molec.inline-formula−1, and inline-formula M27inlinescrollmathml italic σ normal 355 nobreak0.125em nobreak0.125em normal nm , 0.125emnobreak normal I normal 5 normal O normal 12 = 72pt12ptsvg-formulamathimg0066eb2199b028de142478a940638fbf acp-20-10865-2020-ie00010.svg72pt12ptacp-20-10865-2020-ie00010.png (inline-formula M28inlinescrollmathml normal 9.8 ± normal 1.0 ) × normal 10 - normal 18 85pt15ptsvg-formulamathimg5035400845f2b18d7999be66f56115a8 acp-20-10865-2020-ie00011.svg85pt15ptacp-20-10865-2020-ie00011.png  cminline-formula2 molec.inline-formula−1. Photodepletion at inline-formulaλ=532 nm was only observed for OIO, which enabled determination of upper limits for the absorption cross sections of inline-formulaIxOy 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 inline-formulaJ(inline-formulaIxOy). Our results confirm a high inline-formulaJ(inline-formulaIxOy) scenario where inline-formulaIxOy is efficiently removed during daytime, implying enhanced iodine-driven ozone depletion and hindering iodine particle formation. Possible inline-formulaI2O3 and inline-formulaI2O4 photolysis products are discussed, including inline-formulaIO3, which may be a precursor to iodic acid (inline-formulaHIO3) in the presence of inline-formulaHO2.

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Lewis, Thomas R. / Gómez Martín, Juan Carlos / Blitz, Mark A. / et al: Determination of the absorption cross sections of higher-order iodine oxides at 355 and 532 nm. 2020. Copernicus Publications.

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