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

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(σ(IONO2)/cm2 molecule−1) = 170.4 − 3.773 λ + 2.965 × 10−2λ2 − 1.139 × 10−4λ3 + 2.144 × 10−7λ4 − 1.587 × 10−10λ5, where λ is in nm; the cross section, with 2σ 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 ϕ(IO) ≤ 0.02 and ϕ(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 ϕ(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.