A Study of the Recombination of IO with NO₂ and the Stability of INO₃:  Implications for the Atmospheric Chemistry of Iodine

Abstract

The recombination reaction between IO and NO2 was studied over a large range of temperature (216−474 K) and pressure (20−760 Torr). IO, produced by the pulsed photolysis at 193 nm of NO2 to yield O in the presence of CF3I, was probed at 445.0 nm [IO(A2Π3/2 − X2Π3/2), (2,0)] and monitored by nonresonant LIF at 458.6 nm [IO(A2Π3/2 − X2Π3/2), (2,1)]. The resulting rate coefficients were then fitted by RRKM theory, using molecular parameters and a heat of formation for INO3 (Δ = 70 ± 16 kJ mol-1, best fit value 80 kJ mol-1) derived from quantum calculations at the B3LYP/6-311+g(2d,p) level. This yielded k rec,0 = (1.3 ± 0.2) × 10-30 (T/300 K)-(4.5±0.6) cm6 molecule-2 s-1, k rec, ∞ = (6.5 ± 1.0) × 10-12 (T/300 K)-(1.3±0.8) cm3 molecule-1 s-1, and F c = 0.57, over the experimental range of temperature and pressure. The RRKM calculations, in conjunction with measurements made at 474 K, were used to determine the rate of thermal dissociation of INO3 for the conditions of the lower atmosphere: k diss(240−305 K, 760 Torr) = 1.1 × 1015 exp(−12060/T) s-1, with an upper limit about a factor of 2 higher. The atmospheric significance of these results is discussed briefly.

The UV-visible absorption cross-sections of IONO<sub>2</sub>

“…Thermal decomposition of IONO 2 may also contribute to the night time loss of IONO 2 . However, recent observations have shown that the lifetime of IONO 2 with respect to thermal decomposition at 298 K is approximately 6 min with a lower limit of 2.4 min (Allan and Plane, 2002).…”

“…However, recent observations imply that reaction (R3) is a slow process at ambient temperature (Dillon, 2001;Allan and Plane, 2002). A recent laboratory study has provided the first experimental evidence that IONO 2 can be lost from the gas phase via a heterogeneous process (Holmes et al, 2001).…”

Measurements and modelling of I<sub>2</sub>, IO, OIO, BrO and NO<sub>3</sub> in the mid-latitude marine boundary layer

“…Ab initio quantum calculations, using a level of theory described previously (Allan and Plane, 2002b), show that the binding energy of the OIO dimer is only 72 kJ mol −1 , while that of the IO dimer (in the form IOIO) is about 86 kJ mol −1 . Using these binding energies, vibrational frequencies and rotational constants as input for a Rice-Ramsberger-Kassel-Markus (RRKM) calculation, we predict that the rates of dissociation of these dimer molecules will be fast at a typical mid-latitude MBL nighttime temperature of 285 K and a pressure of 1 atmosphere.…”

“…Assuming an accommodation coefficient, γ INO 3 , of 0.05 and a clean MBL aerosol volumetric surface area of 10 −7 cm 2 cm −3 , the loss rate due to uptake on aerosols is ∼2.3×10 −5 s −1 . An upper limit to the rate of thermal dissociation of INO 3 at 285 K has been determined to be 4.5×10 −4 s −1 (Allan and Plane, 2002b). Hence, at night in the MBL there is probably a competition between these processes.…”

Iodine chemistry in the troposphere and its effect on ozone

“…[ . It also makes a negative contribution to the radiative flux in the tropical troposphere (Saiz-Lopez et al, 2012b), and produces higher-order iodine oxides (I x O y ) which have been proposed to participate in the formation of ultrafine aerosol particles in coastal environments (Hoffmann et al, 2001;O'Dowd et al, 2002;Jimenez et al, 2003;McFiggans et al, 2004Pechtl et (Atkinson et al, 2007); 2 Dillon et al, 2006b;3 JPL-2010(Sander et al, 20114 Gómez Martín et al, 2007;5 Kaltsoyannis 6 Gálvez et al, 2013;7 Bösch et al, 2003;8 Gómez Martín and Plane, 2009;9 Chambers et al, 1992;10 Chameides and Davis, 1980;11 Dillon et al, 2006a;12 McFiggans et al, 2000;13 Jenkin et al, 1985;14 Plane et al, 2006;15 Allan and Plane, 2002;16 Lancar et al, 1991;17 Laszlo et al, 1997;18 Bedjanian et al, 1997;19 Gilles et al, 1997;20 Dillon et al, 2008. a The empirical expressions of the form w 1 · exp (w 2 · T ) were obtained by non-linear least squares fitting of Rice-Ramsperger-Kassel-Marcus (RRKM) theoretical results for the indicated reaction rate constants and thermal dissociation rates in the (27-1013) hPa pressure range. RRKM calculations were carried out using the MESMER algorithm (Glowacki et al, 2012) as indicated in the corresponding references (e.g.…”