A newly designed and constructed Knudsen cell has been tested by measuring the reaction probability for gaseous N 2 O 5 on 65% H 2 SO 4 /H 2 O at 210-230 K to be 0.075 ( 0.047 (2σ), which is in excellent agreement with the literature value. This cell has been applied to the study of gaseous HNO 3 reactions with NaCl small crystals and ground powders at 298 K. A rapid initial uptake of HNO 3 and production of gaseous HCl are observed when the crystals and powders are pumped but not heated prior to reaction. After this rapid initial reaction, a constant uptake of HNO 3 and formation of HCl is observed from which a reaction probability of (1.4 ( 0.6) × 10 -2 (2 σ) is calculated. When possible systematic errors (including uncertainties in the effective surface area available for reaction) are taken into account, the overall uncertainty is estimated to be about a factor of 2. The measured reaction probability is independent of the size or preparation of the salt crystals as well as the number of layers of salt in the sample holder. This reaction probability is in excellent agreement with results from the previous work of Rossi and co-workers 21,23 and Leu and co-workers 15 using powders but significantly larger than that measured by Laux et al. 12 using single crystals and an ultrahigh vacuum system. Prior heating of the salts while pumping decreased the extent of the initial rapid reaction but did not affect the subsequent constant reaction uptake probability. Experiments on the reaction of HNO 3 with NaCl crystals that had been previously exposed to D 2 O to replace any surface-adsorbed H 2 O and on the reactions of DNO 3 with NaCl show that under all experimental conditions studied here, some water remains on the surface and plays a key role in the uptake of HNO 3 . We propose a new model for the reaction of HNO 3 with NaCl powders in which HNO 3 is taken up into strongly adsorbed water (SAW) on the salt. This SAW, for which there is prior evidence in the literature, 34 appears likely to be held at defect sites on the powders. Acidification of this SAW leads to degassing of HCl due to dissolution of NaCl into the SAW from the underlying salt. As gaseous HNO 3 continues to be taken up, HCl degasses and nitrate precipitates out as NaNO 3 . This model represents a fundamental change in the description of the heterogeneous reactions of salt powders. The lower reaction probability for single crystals observed by Laux et al. 12 is consistent with the lack of surface-adsorbed water on relatively defect-free single crystals. No uptake of the gases NO 2 , NO, HCl, ClNO, ClNO 2 , or H 2 O was observed on the finely ground NaCl powder from which an upper limit to the reaction probabilities for these gases with NaCl of ∼10 -5 was derived. The atmospheric implications of this model are discussed.

The formation of binary complexes of sulfuric acid with hydrogen chloride, water, nitric acid, chlorine nitrate, and hydrogen peroxide has been investigated by means of ab initio calculations at the Hartree−Fock and MP2 levels of theory. The hydrogen bonds for the complexes with water, hydrogen peroxide, and nitric acid are found to be strong, whereas hydrogen chloride and chlorine nitrate form only weak complexes. For the first time, structural data for binary complexes of sulfuric acid with hydrogen chloride, nitric acid, chlorine nitrate, and hydrogen peroxide have been obtained. The geometry of the calculated sulfuric acid/water complex with the highest stability is in good agreement with the available data in the literature. The stability of the sulfuric acid complexes with hydrogen peroxide and nitric acid are indications that these gases may also take part in the nucleation process of stratospheric sulfuric acid aerosol.

In this study, both relative rate and absolute kinetic measurements were made at 298 K for the reaction of chlorine atoms with the C 1 -C4 alkanes. The relative rate studies were carried out using two different reaction chambers and GC to follow the loss of the organics upon reaction with atomic chlorine generated from the photolysis of Clz or ClNO. Relative rate studies were also carried out for the chloro-substituted methanes. Absolute rate constant studies employed a fast flow discharge system with resonance fluorescence detection of atomic chlorine. Our relative rate constants for the ethane/propane and ethaneln-butane pairs are approximately 15 and 20% lower respectively than recent measurements carried out on a similar series of alkanes. The absolute rate constant for propane is 20% lower than previous absolute rate measurements and the currently recommended value. However, the ratios of the absolute rate constants measured in this study agree to within 10% with our independently measured relative rate constants. Thus, these studies provide for the first time a consistent set of reaction rate constants for the reaction of C1 with the simple alkanes. While the rate constants derived for C H X l and CH2C12 are in reasonably good agreement with the literature, that for CHC1, is approximately 50% higher (but within the wide error bars) of current recommendations. The trends in the rate constants are discussed in the context of recently derived structure-activity relationships.

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