On the photochemistry of IONO2 : absorption cross section (240–370 nm) and photolysis product yields at 248 nm

D. M. Joseph, S. H. Ashworth, J. M. C. Plane

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The absolute absorption cross section of IONO2 was measured by the pulsed photolysis at 193 nm of a NO2/CF3I mixture, followed by time-resolved Fourier transform spectroscopy in the near-UV. The resulting cross section at a temperature of 296 K over the wavelength range from 240 to 370 nm is given by log10(s(IONO2)/cm2 molecule-1) = 170.4 - 3.773 ? + 2.965 × 10-2?2 - 1.139 × 10-4?3 + 2.144 × 10-7?4 - 1.587 × 10-10?5, where ? is in nm; the cross section, with 2s uncertainty, ranges from (6.5 ± 1.9) × 10-18 cm2 at 240 nm to (5 ± 3) × 10-19 cm2 at 350 nm, and is significantly lower than a previous measurement [J. C. Mössinger, D. M. Rowley and R. A. Cox, Atmos. Chem. Phys., 2002, 2, 227]. The photolysis quantum yields for IO and NO3 production at 248 nm were measured using laser induced fluorescence of IO at 445 nm, and cavity ring-down spectroscopy of NO3 at 662 nm, yielding [curly or open phi](IO) = 0.02 and [curly or open phi](NO3) = 0.21 ± 0.09. It is likely that photolysis to I + NO3 is the only significant channel, as shown by accompanying quantum chemistry calculations. The low [curly or open phi](NO3) is explained by the production of hot NO3, most of which dissociates to NO2 + O. In terms of atmospheric relevance, the noon photolysis frequency of J(IONO2) = (3.0 ± 2.1) × 10-3 s-1 (40° N, July) is fast enough to limit the effectiveness of IONO2 as a daytime reservoir of iodine oxides, but the formation and subsequent photolysis of IONO2 is very inefficient as an ozone-depleting cycle.
Original languageEnglish
Pages (from-to)5599-5607
Number of pages9
JournalPhysical Chemistry Chemical Physics
Issue number41
Early online date30 Aug 2007
Publication statusPublished - 2007

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