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 IONO(2) was measured by the pulsed photolysis at 193 nm of a NO(2)/CF(3)I 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 log(10)(sigma(IONO(2))/cm(2) molecule(-1)) = 170.4 - 3.773 lambda + 2.965 x 10(-2)lambda(2)- 1.139 x 10(-4)lambda(3) + 2.144 x 10(-7)lambda(4)- 1.587 x 10(-10)lambda(5), where lambda is in nm; the cross section, with 2sigma uncertainty, ranges from (6.5 +/- 1.9) x 10(-18) cm(2) at 240 nm to (5 +/- 3) x 10(-19) cm(2) 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 NO(3) production at 248 nm were measured using laser induced fluorescence of IO at 445 nm, and cavity ring-down spectroscopy of NO(3) at 662 nm, yielding phi(IO) </= 0.02 and phi(NO(3)) = 0.21 +/- 0.09. It is likely that photolysis to I + NO(3) is the only significant channel, as shown by accompanying quantum chemistry calculations. The low phi(NO(3)) is explained by the production of hot NO(3), most of which dissociates to NO(2) + O. In terms of atmospheric relevance, the noon photolysis frequency of J(IONO(2)) = (3.0 +/- 2.1) x 10(-3) s(-1) (40 degrees N, July) is fast enough to limit the effectiveness of IONO(2) as a daytime reservoir of iodine oxides, but the formation and subsequent photolysis of IONO(2) is very inefficient as an ozone-depleting cycle.