Rate Coefficients for the Reaction IO + CH3O2 by LIF Detection of IO

DILLON, T. J.; MARIA EUGENIA TUCCERI; CROWLEY, J. N.

Orleans

Simposio; 19th International Symposium on Gas Kinetics.; 2006

Laser flash photolysis coupled to LIF detection of IO was used to measure rate coefficients, k1 for the reaction between iodine oxide and methyperoxy radicals (R1). Photolysis of acetone and O2 at l=193 nm, in the presence of CF3I was used to initiate the reaction sequence:               CH3C(O)CH3 + hn (193nm) ®        2CH3+CO             O2 + hn (193nm)                    ®        2O(3P)             CH3 + O2 + M                        ®        CH3O2 + M             O(3P) + CF3I                          ®        IO + CF3             IO + CH3O2                                    ®        (products)                                          (R1)   The temporal profiles of IO were monitored by LIF at around 445.0 nm. [CH3]0, the concentration of CH3 formed in the laser flash was determined in parallel experiments by monitoring the depletion of NO2 in the reaction (R2) by calibrated LIF of NO2 at l=444.8 nm:               NO2 + CH3                                      ®        (products)                                          (R2)   The experiments to determine k1 were conducted under pseudo first-order conditions of [CH3O2]>>[IO], and over the range of bath gas (N2) pressure 30 < P / Torr < 320. Decays of IO  were fit to obtain kini, the initial 1st –order decay rate coefficient. Values of k1 = (5 ± 1)x10-12 cm3 molecule-1 s-1 were obtained from plots of kini vs. [CH3]0. Numerical simulations of an assumed reaction scheme suggests that these determinations of k1 were in fact overestimates, due to the secondary generation of small concentrations of the highly reactive (towards IO) species CH3O and HO2. The true value of k1 (298 K) was closer to 2x10-12 cm3 molecule-1 s-1, considerably smaller than the only previous experimental determination of k1 (298 K) = (6.0 ± 1.3)x10-11 cm3 molecule-1 s-1 {1}, and the estimate (2.3x10-11 cm3 molecule-1 s-1) used in a photochemical model, {2} to calculate the O3 destroying efficiency of (R1) in the marine boundary layer. The atmospheric implications of this result are discussed.   References   {1} Bale, C.S.E., canosa-Mas, C.E., Shallcross, D.E., Wayne, R.P., A discharge-flow study of the kinetics of the reactions of IO with CH3O2 and CF3O2, Phys. Chem. Chem. Phys., 10 (2005), 2164-2172.           {2} Vogt, R., Sander, R., von Glasow, R., and Crutzen, P.J., Iodine chemistry and its role in halogen activation and ozone loss in the marine boundary layer: a model study., Journal of Atmospheric Chemistry, 32, 3, (1999), 375-395.