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The kinetics for the uptake of N 2 O 5 with NaCl [N 2 O 5 (g) + NaCl(s) f ClNO 2 (g) + NaNO 3 (s) (reaction 1)] and KBr[N 2 O 5 (g) + KBr(s) f products (reaction 6)] have been studied in a Teflon-coated Knudsen reactor. The product of reaction 1 is found to be ClNO 2 , in agreement with previous studies. The only brominecontaining gaseous product observed for reaction 6 is Br 2 ; we propose a redox reaction in which Br -is oxidized to molecular bromine with the concurrent formation of nitrite. The hypothesis is supported by the observation of nitrous acid in the product spectrum of reaction 6. The observed uptake coefficients are found to depend strongly on the total external surface area of the salt substrates, prepared by a number of methods. With samples of well-defined total external surface, we are able to determine the following values for the uptake coefficient: γ 1 ) (5.0 ( 2.0) × 10 -4 ; γ 6 ) (4.0 ( 2.0) × 10 -3 . The values are smaller than previously reported. The unexpected dependence of the uptake kinetics on the surface presentation led us to conduct further experiments on the HNO 3 reaction with salt: HNO 3 (g) + NaCl(s) f HCl(g) + NaNO 3 (s) (reaction 2). From our new experiments conducted on various substrates, we find γ 2 ) (2.0 ( 1.0) × 10 -2 , in good agreement with our previous measurements on salt powder. The validity of calculating correction factors for the uptake coefficients, as recently proposed by Keyser and co-workers, is discussed in detail, and specific experimental tests of the theory are described. The role of internal surfaces in the overall observed uptake kinetics for the reactions of N 2 O 5 is found to be remarkably well described by the treatment given by Keyser and co-workers. Additional experiments on the uptake kinetics of nitric acid, however, cast doubt that this treatment can be generally extended to all species of atmospheric interest. The atmospheric implications of these findings are discussed briefly.

A new low-pressure flow reactor operated as a Knudsen cell and intended for chemical kinetic studies is described. The reactor is specifically designed to study the kinetics of heterogeneous reactions. Gas-phase species are detected either by mass-spectrometric sampling or by in situ optical techniques, e.g., laser-induced fluorescence, resonantly enhanced multiphoton ionization. A feature of the reactor is its modular design, allowing full interchangeability of several sample holders at minimal effort, allowing the measurement of uptake coefficients ranging from 10−7 to 1.0. Sample supports operating at low and high temperatures have been developed which cover the stated temperature range. Several experimental examples of the utility of the reactor are detailed. The reliability and error bars of the kinetic results due to the errors and uncertainties associated with the experimental procedures are discussed, in particular for fast heterogeneous processes. It is found that even in the molecular flow regime, for fast reaction, the effects of diffusion limitations within the cell must be taken into account. This fact has been shown here from an experimental point of view. In a companion article the phenomena are studied using Monte Carlo simulation of the gas dynamics under molecular flow conditions.

The ultraviolet spectrum of the ethylperoxy radical (C2H502) and the reactions C2H5O2 + C2H5O2 -products (1) and C2H5O2 + H02 -C2H502H + 0 2 ( 5 ) have been studied using the flash photolysis/UV absorption technique. The spectrum was taken between the wavelengths of 210 and 290 nm and at the temperatures of 298 and 600 K. The room temperature spectrum is found to be in good agreement with previous determinations, with a maximum cross section umax = (4.89 f 0.60) X lo-'* cm2 molecule-' at 240 nm. The temperature dependence of the broadness of the spectrum as well as the value of umax is analyzed by fitting the data to a Gaussian function that predicts the temperature behavior of broad, structureless UV absorptions. Our results on the C2H5O2 self-reaction are also in good agreement with previous studies, with kl/cm3 molecule-' s-I = (6.7 f 0.6) X exp((60 f 40)/TJ for the temperature range 248-460 K. At higher temperatures, we observe non-second-order kinetic behavior which can be attributed to the thermal decomposition of the ethoxy radical, a product of reaction 1. Our results for the reaction C2H5O2 + HO2 are significantly different from the only previous determination of its temperature dependence, especially at and below room temperature, with ks/cm3 molecule-' s-I = (1.6 f 0.4) X lO-I3 expi( 1260 f 130)/ 7') over the temperature range of 248-480 K; our room temperature rate constant is about a factor of 2 greater than the currently accepted value of k5, with k5(298)/cm3 molecule-' s-I = (1.10 f 0.21) X lo-". This result holds implications for the understanding of the reactivity of R02 species with H02, which is important for the chemical modeling of the troposphere.

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