Proton exchange, residence time, and gas uptake measurements are used to explore collisions and reactions of HCl, HBr, and HNO 3 with 70 wt % D 2 SO 4 at 213 K. These studies help to provide a detailed picture of HX (X ) Cl, Br, NO 3 ) energy transfer to sulfuric acid and the fate of the HX molecules immediately after thermalization at the D 2 O/D 2 SO 4 surface. We find that the three molecules readily dissipate their excess kinetic energy and become trapped momentarily in the interfacial region. However, only 11 ( 3% of the thermalized HCl and 22 ( 3% of the thermalized HBr molecules undergo H f D exchange; the HCl and HBr that do not react are found to desorb from the acid within 2 × 10 -6 s. In contrast, more than 95% of the initially trapped HNO 3 molecules are converted to DNO 3 . The HX molecules that undergo exchange dissolve within the deuterated acid for characteristic times of 5 × 10 -5 s (HCl), 3 × 10 -3 s (HBr), and 1 × 10 -1 s (HNO 3 ) before they desorb thermally as DX. The scattering experiments imply that the desorption of thermalized HCl and HBr molecules is, on average, faster than their solvation and reaction in the interfacial and bulk regions of 70 wt % D 2 SO 4 . Although HNO 3 is less acidic than HCl or HBr, it appears to hydrogen bond more strongly to surface D 2 O and D 2 SO 4 , enabling it to be captured by the acid in nearly every collision.

Abstract.Gaseous HC1 and HBr react with sulfuric acid at rates that depend strongly on acid concentration over a range of acidities typical of stratospheric aerosols. We monitor the competition between immediate desorption and H-•D exchange after HC1 and HBr thermalize upon collision with the surface of deuterated sulfuric acid: the exchange probabilities decrease from 0.7 to 0.1 (HC1) and 0.9 to 0.2 (HBr) as the acid concentration is increased from 55 to 70wt % D2SO4 at 213 K. These measurements imply that HC1 and HBr desorb faster than they dissociate at higher acidities, impeding the formation of C1-and Br-in more acidic aerosols. Residence time measurements of HC1 molecules that do enter the acid, however, indicate that HC1 is more soluble than expected at high sulfuric acid concentrations.

The rate and thermodynamics of the adsorption of acetone on ice surfaces have been studied in the temperature range T = 190-220 K using a coated-wall flow tube reactor (CWFT) coupled with QMS detection. Ice films of 75 +/- 25 microm thickness were prepared by coating the reactor using a calibrated flow of water vapor. The rate coefficients for adsorption and desorption as well as adsorption isotherms have been derived from temporal profiles of the gas phase concentration at the exit of the flow reactor together with a kinetic model that has recently been developed in our group to simulate reversible adsorption in CWFTs (Behr, P.; Terziyski, A.; Zellner, R. Z. Phys. Chem. 2004, 218, 1307-1327). It is found that acetone adsorption is entirely reversible; the adsorption capacity, however, depends on temperature and decreases with the age of the ice film. The aging effect is most pronounced at low acetone gas-phase concentrations (< or = 2.0 x 10(11) molecules/cm(3)) and at low temperatures. Under these conditions, acetone is initially adsorbed with a high rate and high surface coverage that, upon aging, both become lower. This effect is explained by the existence of initially two adsorption sites (1) and (2), which differ in nature and number density and for which the relative fractions change with time. Using two-site dynamic modeling, the rate coefficients for adsorption (k(ads)) and desorption (k(des)) as well as the Langmuir constant (K(L)) and the maximum number of adsorption sites (c(s,max)), as obtained for the adsorption of acetone on sites of types (1) and (2) in the respective temperature range, are k(ads)(1) = 3.8 x 10(-14) T(0.5) cm(3) s(-1), k(des)(1) = 4.0 x 10(11) exp(-5773/T) s(-1), K(L) (1) = 6.3 x 10(-25) exp(5893/T) cm(3), c(s,max)(1) < or = 10(14) cm(-2) and k(ads)(2) = 2.9 x 10(-15) T(0.5) cm(3) s(-1), k(des)(2) = 1.5 x 10(7) exp(-3488/T) s(-1), K(L)(2) = 5.0 x 10(-22) exp(3849/T) cm(3), c(s,max)(2) = 6.0 x 10(14) cm(-2), respectively. On the basis of these results, the adsorption of acetone on aged ice occurs exclusively on sites of type (2). Among the possible explanations for the time-dependent two-site adsorption behavior, i.e., crystallographic differences, molecular or engraved microstructures, or a mixture of the two, we tentatively accept the former, i.e., that the two adsorption sites correspond to cubic (1, I(c)) and hexagonal (2, I(h)) sites. The temporal change of I(c) to I(h) and, hence, the time constants of aging are consistent with independent information in the literature on these phase changes.

A kinetic model has been developed to simulate reversible gas adsorption in coated wall flow tube reactors (CWFTs). The motivation of this work is to provide the theoretical framework for modelling studies in support of the results obtained from CWFT studies on the adsorption and desorption behaviour of atmospheric trace gases on ice surfaces at temperatures relevant to the upper troposphere/lower stratosphere (190–230 K). The model consists of an axial sequence of individual flow tube sections of equal volumes in which the gas phase is homogeneously mixed and interactions with the coated wall occur by adsoption and desorption exclusively. The adsorption rate is assumed to be kinetically controlled and not to be transport limited. Moreover, chemical reactions are not considered. Simulations have been performed for the temporal behaviour of the gas phase concentration at the exit of the flow tube as a function of laboratory time for typical operation procedures of CWFTs with moveable injectors i.e. (i) instantaneous injection of the gas, (ii) instantaneous termination of the gas flow and (iii) movements of the injector with constant velocities. It is found that the temporal profiles show complex behaviours due to the overlap of adsorption and desorption upon successive exposure of the gas to different length of the ice surface. The validity of the model is demonstrated for the adsorption of acetone on an ice surface at 200 K as a case study.

A fast-flow apparatus with mass spectrometric detection was used to study the system F + CHFO between 2 and 3.5 mbar total pressure. The rate constant of the primary reaction was evaluated directly to yield at 298 K k(1) = (8.8 2 1.4) * cm3 * molecule-' * s-l.Numerical modelling was used to determine the rate constant at 298 K of the subsequent reaction CFO + CFO -CFzO + CO: kp.1 = (4.9 2 2.0) * lo-" cm3 * molecule-l * s-l. The possible occurrences of secondary reactions, CFO + F + M -CF20 + M, and CFO + FzCFzO + F, can be excluded under the present conditions. 0