The absorption cross sections of the gas-phase IO radical and the kinetics and products of the IO self-reaction have been measured using the technique of laser photolysis with time-resolved UV-vis absorption spectroscopy. The IO absorption cross section at the peak of the (4,0) vibronic band of the (A 2 Π r X 2 Π) transition at 427.2 nm, determined using the reaction O( 3 P) + CF 3 I to form IO and calibrated relative to the O 3 cross section was found to be (1.9 ( 0.17) × 10 -17 cm 2 molecule -1 at 295 K and 1.13 nm fwhm spectral resolution. The IO cross sections were found to exhibit a negative temperature dependence. The kinetics of the IO selfreaction were measured using the reaction O( 3 P) + I 2 to form IO, and the self-reaction rate coefficient k 1 , determined from the loss of IO radicals in the absence of ozone, was found to be (8.2 ( 1.3) × 10 -11 molecules -1 cm 3 s -1 at 295 K and 760 Torr. The self-reaction rate coefficient was found to be independent of pressure between 100 and 760 Torr, and to display a negative temperature dependence between 222 and 325 K, described by k 1 ) (4.1 ( 3.4) × 10 -11 exp{(220 ( 230)/T} molecules -1 cm 3 s -1 . All errors are 2σ. Four potential product channels exist for the IO self-reaction: IONo direct measurement of I atom production was performed. I 2 formation was observed, but attributed to IO-catalyzed I atom recombination (I + IO + M f I 2 O + M; I + I 2 O f I 2 + IO). OIO formation was observed and shown to result from the IO selfreaction. Formation of a broadband absorbing product underlying the IO absorption at low (λ < 400 nm) wavelengths was observed, and tentatively attributed to I 2 O 2 . The OIO cross sections and yield from the IO + IO reaction were determined via measurement of OIO production from the IO + BrO reaction which allowed limits to be placed on the branching ratio for OIO formation in the IO self-reaction at 295 K and 760 Torr. Branching ratios for all reaction channels were found to lie in the ranges 0.07 e k 1a /k 1 e 0.15, k 1b /k 1 e 0.05, 0.30 e k 1c /k 1 e 0.46, and 0.42 e k 1d /k 1 e 0.55 at 295 K and 760 Torr. The results are compared with previous studies of the IO self-reaction, and their implications for atmospheric iodine chemistry are considered.

The rates of the dimerization of butadiene t o form vinylcyclohexene and of the reaction of butadiene with ethylene t o form cyclohexene have been measured in the temperature range of 400°-6000C, and a t low converions.The reactions were found t o be homogeneous bimolecular associations, leading essentially to the products indicated. Combination of the rate data for the dimerization reaction with data obtained by other workers at lower temperatures shows that the activation energy is temperature dependent. This fact is correlated with statistical rate calculations. Such calculations are also carried out for the cyclohexene reaction, the reverse reaction of decomposition of cyclohexene into butadiene and ethylene and the resulting equilibrium. I t is shown that a cyclic tran-ition complex accounts best for all available rate data, provided adjustments are made for the vibrational frequencies assocated mith bonds t o be formed or t o be broken. It is assumed that these frequencies are lowered because of the extension of these bonds in the transition configurations.

The kinetics of the gas phase HO2 self-reaction have been studied using flash photolysis of Cl2/CH3OH/O2/N2 mixtures coupled with time-resolved broadband UV absorption spectroscopy. The HO2 self-reaction rate coefficient (HO2 + HO2 --> H2O2 + O2 (R1)) has been determined as a function of temperature (236 < T < 309 K, at 760 Torr) and pressure (100 < p < 760 Torr, at 296 K). In addition, the effects of water vapour ((0-6.0) x 10(17) molecules cm(-3), 254 < T < 309 K at 760 Torr, 400 < p < 760 Torr at 296 K) and methanol vapour ((0.06-4.7) x 10(17) molecules cm(-3), 254 < T < 309 K, at 760 Torr) on the rate coefficient have been characterised. The observed rate coefficient, k1, was found to exhibit a negative temperature dependence with both pressure dependent and pressure independent components, in agreement with previous studies. Furthermore, the rate coefficient k1 was found to be enhanced in the presence of elevated H2O or CH3OH concentrations, as reported previously. This study reports the most extensive characterisation of the rate coefficient k1 as a function of T, p, [H2O] and [CH3OH]. The present results indicate that k1 is higher at low temperatures, and that enhancement of k1 by H2O is greater, than has been reported previously. The pressure dependence of k1 at ambient temperature is in good agreement with previous studies. The rate enhancement by CH3OH reported here is in good agreement with previous studies at ambient temperatures but is smaller at low temperatures than the most recent previous investigation suggests. The rate coefficient k1 is adequately parameterised by: k1(760 Torr) = {(1.8 +/- 0.8) x 10(-14) exp((1500 +/- 120)/T/K)} x {1 + (2.0 +/- 4.9) x 10(-25) [H2O] exp((4670 +/- 690)/T/K)} x (1 + (0.56 +/- 1.00) x 10(-21) [CH3OH] exp((2550 +/- 500)/T/K)} cm(-3) molecule(-1) s(-1), where [H2O] and [CH3OH] are in molecules cm(-3). Errors are 1 sigma, and statistical only. The atmospheric implications of these results are briefly discussed.

Abstract:IO and BrO radicals are intermediates in the atmospheric photo-oxidation of iodo-and bromocarbons and can act as catalysts for ozone loss. We have studied the kinetics and mechanisms of the reactions of IO with itself and with BrO to establish their role in the atmospheric chemistry of iodine. We have found that iodine dioxide, OIO, is produced in these reactions. The results of these and other experimental observations together with a recent computational study suggest an unexpectedly high photochemical stability for OIO. It is shown that OIO formation and its attachment to particles could account for the high enrichment of iodine in the small size fraction of marine aerosol, which is important for the transport of iodine from the sea to the continents. OIO may be a route to the formation of iodate, which is present in atmospheric precipitation. OIO formation also implies a reduced efficiency for iodine catalysed ozone loss.

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