The heterogeneous reactions of HNO3(g) + NaCl(s) -HCl(g) + NaNOs(s) (eq 1) and N205(g) + NaCl(s) -ClN02(g) + NaNO3(s) (eq 2) were investigated over the temperature range 223-296 K in a flow-tube reactor coupled to a quadrupole mass spectrometer. Either a chemical ionization mass spectrometer (CIMS) or an electron-impact ionization mass spectrometer (EIMS) was used to provide suitable detection sensitivity and selectivity. In order to mimic atmospheric conditions, partial pressures of HN03 and N2O5 in the range 6 x Torr were used. Granule sizes and surface roughness of the solid NaCl substrates were determined by using a scanning electron microscope. For dry NaCl substrates, decay rates of HN03 were used to obtain y(1) = 0.013 f 0.004 (lo) at 296 K and '0.008 at 223 K, respectively. The error quoted is the statistical error. After all corrections were made, h e overall error, including systematic error, was estimated to be about a factor of 2. HC1 was found to be the sole gas-phase product of reaction 1. The mechanism changed from heterogeneous reaction to predominantly physical adsorption when the reactor was cooled from 296 to 223 K. For reaction 2 usingdry salts, y(2) was found to be less than 1.0 x at both 223 and 296 K. The gas-phase reaction product was identified as ClN02 in previous studies using an infrared spectrometer. An enhancement in reaction probability was observed if water was not completely removed from salt surfaces, probably due to the reaction of N2O5(g) + H2O(s) -2HN03(g). Our results are compared with previous literature values obtained using different experimental techniques and conditions. The implications of the present results for the enhancement of the hydrogen chloride column density in the lower stratosphere after the El Chichon volcanic eruption and for the chemistry of HCl and HN03 in the marine troposphere are discussed.

The kinetics of the reactions of CH2Br and CH2I radicals with O2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O2 reaction in the range 0.2-15 x 10(17) cm(-3) of He at 298 K. In this same density range the CH2Br + O2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O2 reaction is found to depend on temperature as k(CH2I + O2)=(1.39 +/- 0.01)x 10(-12)(T/300 K)(-1.55 +/- 0.06) cm3 s(-1)(220-450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O2 reaction in the third-body region is k(CH2Br + O2+ He)=(1.2 +/- 0.2)x 10(-30)(T/300 K)(-4.8 +/- 0.3) cm6 s(-1)(241-363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the F(cent) value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/-25%.

Directly measured low-pressure thermal HCO dissociation rate constants and isotope effects are presented for the first time. The temperature range of the measurements is 637-832 K. A theoretical model developed in the preceding paper is found to be highly consistent with these results and with all available H + CO thermal addition rate constant measurements. The calculations are used to extend the measured dissociation rate constant to combustion temperatures. The calculated low-pressure dissociation rate constant k¡ in various buffer gases is accurately represented from 300 to 3000 K by At,(Ar) = 3.09 X 10-7 7•°/* , A:,(He) = 3.80 X 10"7r1e"171/-Rr, A:,(N2) = 3.07 X lO^rV170/*7; and A:,(H2) = 5.79 X lO^r'e"17,0/*7", where kx is in cm3 4/(molecule s) and R is in kcal/(mol deg). The calculations suggest that {AE)lot, the average energy transfer between metastable HCO* and the buffer gas, varies between -40 and -50 cm"1 for buffer gases N2, H2, He, and Ar.

The gas-phase kinetics of the reactions of HCO with four molecules (02, N02, Cl2, and Br2) have been studied as a function of temperature in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants for each reaction were determined at a minimum of five temperatures to obtain Arrhenius parameters (k = A exp(-EJRT)). The results obtained are as follows (the numbers in the brackets are log A/(cm3 molecule-1 s-1), £a/(kJ mol-1), and the temperature ranges covered): HCO + 02 {-10.9 (±0.3), 1.7 (±1.5), 295-713 K); HCO + N02 {-10.6 (±0.3), -1.8 (±2.0), 294-713 K); HCO + Cl2 {-11.2 (±0.3), 0.3 (±2.0), 296-582 K¡; HCO + Br2 {-10.8 (±0.3), -3.7 (±2.0), 296-669 K). The reactivity of HCO was found to be between that of CH3 and C2H5 in the reactions of these radicals with Cl2 and Br2, which is consistent with proposed correlations of reactivity in exothermic reactions based on free-radical ionization potentials.

The heterogeneous reaction of ClONOz + NaCl -Cl2 + NaN03 (eq 1) was investigated over a temperature range 220-300 K in a flow-tube reactor interfaced with a differentially pumped quadrupole mass spectrometer. Partial pressures of ClONOz in the range 10-8-10-5 Torr were used. Granule sizes and surface roughness of the NaCl substrates were determined by using a scanning electron microscope, and in separate experiments, surface areas of the substrates were measured by using BET analysis of gas-adsorption isotherms. For dry NaCl substrates, both the decay rates of ClONO2 and the growth rates of Cl2 were used to obtain reaction probabilities, y1 = (4.6 f 3.0) x at 296 K and (6.7 f 3.2) x at 225 K, after considering the internal surface area. The error bars represent 1 standard deviation. The C12 yield based on the ClONO2 reacted was measured to be 1.0 f 0.2. In order to mimic the conditions encountered in the lower stratosphere, the effect of water vapor pressures between 5 x and 3 x Torr on reaction 1 was also studied.With added H20, reaction probabilities, y l = (4.1 f 2.1) x at 225 K, were obtained. A trace of HOC1, the reaction product from the ClON02 4-H20 -HOC1 i-HNO3 reaction, was observed in addition to the Cl2 product from reaction 1. The implications of this result for the enhancement at 296 K and (4.7 f 2.9) x of hydrogen chloride in the stratosphere after the El Chichon volcanic eruption and for the marine troposphere are discussed.