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

Abstract

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.