A wetted-wall cylindrical flow reactor was used to measure uptake coefficients, g, of O 3 on aqueous surfaces at 293 K. The loss of O 3 from the gas-phase following contact with varying areas of aqueous surface was determined by UV absorption. The use of Na 2 S 2 O 3 as an aqueous-phase scavenger for O 3 ensured that uptake coefficients were in a reaction-controlled rather than mass accommodation-controlled regime. Observed uptake coefficients were corrected for radial gas-diffusion to yield values of g corr. From extrapolation of a plot of 1/g corr against the inverse square-root of the Na 2 S 2 O 3 activity, a value of a ¼ 4 Â 10 À2 was derived for the true mass accommodation coefficient of O 3. Evaluation of uncertainties indicate a conservative lower limit of 10 À2 for a. The data do not rule out that the upper limit approaches unity. However, it is shown that the measured value of a is sufficiently high that mass accommodation does not limit heterogeneous processing of O 3 in the atmosphere for droplets of diameter >10 mm. A value of 3.7 þ0:7 À0:6 Â 10 8 L mol À1 s À1 is derived for the aqueous-phase reaction rate coefficient between O 3 and Na 2 S 2 O 3 at 293 K.

The potential energy curves of the two lowest 2Σ+ (X,B) and the two lowest 2Π (A,C) electronic states of the AlF+ and AlCl+ molecular ions have been calculated using highly correlated multireference configuration interaction (MR-CI) wave functions. It is found that both 2Σ+ states and the C 2Πr state are bound, whereas the A 2Πi state is repulsive. Electronic transition moment functions for all six pairs of states of both ions have also been calculated and used for evaluation of the radiative transition probabilities between bound vibrational states. The calculated charge distributions show that the X and C states are dominated by Coulombic attraction between a doubly charged positive aluminum and the singly charged negative halogen ion. The results provide a new assignment of the photoelectron spectra of the neutral AlF. They are also in good agreement with the recently observed optical B–X emission spectra of both molecular ions and C–X bands of AlCl+. The absence of C–X emission of AlF+ is most likely due to predissociation of the C 2Πr state by the repulsive A 2Πi state.

The reactive uptake of phenol, 2-nitrophenol, and 3-methylphenol (m-cresol) was measured in a vertical wettedwall flow reactor over the temperature range 278-298 K using bromine as an aqueous phase scavenger. First-order decays in gas-phase concentration as a function of increased gas-liquid contact time in the reactor were monitored by UV absorption downstream of the contact zone. Mass accommodation coefficients, R, were derived from measured uptake coefficients by correcting for limitations to mass transfer from radial gas-phase diffusion. Temperature-dependent expressions fitted to the data yielded values of R that decrease from 3.7 × 10 -2 to 6.6 × 10 -3 for phenol, 1.5 × 10 -2 to 1.1 × 10 -3 for 2-nitrophenol, and 1.0 × 10 -2 to 5.1 × 10 -3 for m-cresol over the range 278 K to 298 K. (Estimated overall uncertainty in R values of ∼(30%). These are the first published accommodation data for the latter two aromatic species. The thermodynamic data derived from the values of R were interpreted in terms of the critical cluster model for mass accommodation, yielding average critical cluster sizes of 3.2 ( 0.6, 4.1 ( 1.0, and 2 ( 0.5, for phenol, 2-nitrophenol, and m-cresol, respectively. The larger critical cluster size for 2-nitrophenol is likely attributable to its strong intramolecular hydrogen bonding which significantly reduces the hydrogen bonding strength of this species relative to the other two phenols. It is also demonstrated that the magnitude of the observed enthalpy of mass accommodation for these aromatic compounds correlates well with their excess energy of dissolution. The studied aromatic compounds are important intermediates in the tropospheric oxidation of monoaromatics and react readily in the aqueous phase. Thus knowledge of the mass accommodation coefficient is required to accurately quantify their rate of aqueous phase oxidation.

Optical spectra of ten AX+ ions (A=B, Al, Ga, In; X=F, Cl, Br) have been observed in the visible and near UV; a total of 18 band systems were newly discovered. The emission was produced by chemiluminescent reactions A++X2 at low (2–10 eVCM) kinetic energy in a beam-gas arrangement. A position-sensitive photon counting detector with large surface area and very low dark count rate was employed, the resolution was mostly 5–50 Å FWHM. Three types of band systems were observed: (1) For all AX+ combinations except BCl+ and BBr+, a very broad quasicontinuum with undulatory structure appears. On the basis of electronic state correlation arguments, photoelectron data, some ab initio calculations and, in one case, a known emission spectrum (InCl+) these band systems were identified as B 2Σ+–X 2Σ+ transitions. It is concluded that the excited state potentials are considerably displaced against the ground state, and their energetics are given. (2) For six species AX+, narrow band systems were observed in the 2500 Å region. They could be clearly identified as being due to C 2Π–X 2Σ+ transitions by means of comparison with the systematics of the analogous A 2Π–X 2Σ+ transitions of the isoelectronic alkaline earth halides, by the resolved fine structure, and, in the case of AlF+, by an ab initio calculation. (3) In the GaCl+, GaBr+, and InBr+ spectra, narrow features accompany the C–X transitions. They are attributed to D 2Σ+–X 2Σ+ transitions, analogous to the alkaline earth halide B 2Σ+–X 2Σ+ band systems. Qualitative electronic state correlations are discussed, and the expected dominant configurations in different regions of the AX+ ground and excited states are given. These are in accord with recent ab initio results on AlF+.

Although 2-nitrophenol has been identified as an important environmental chemical there is scarcity in the literature regarding the temperature dependence of its Henry's law coefficient, H. Here a bubble purge method was used to measure H for 2-nitrophenol over the temperature range 278-303 K. A novel approach in the data treatment allowed correction of the data for non-equilibrium partitioning in the apparatus to obtain the true equilibrium H value. The experimentally derived temperature-dependent expression for H of 2-nitrophenol is lnH (M atm(-1)) = (6290/T (K)) - 16.6. The standard enthalpy and entropy of gas-to-liquid transfer for 2-nitrophenol in aqueous solution are -52.3 +/- 8.1 kJ mol(-1) and -138 +/- 28 J mol(-1) K(-1), respectively. (Errors are 95% confidence intervals.)

Light emission from the impact of Al+(1S,3P) ions on H2 molecules at 5.2 to 13.8 eVCM was spectrally analyzed. It was found to consist of two band systems. Of the known A 2Π–X 2Σ+ transition, numerous new bands were observed, allowing a first experimental determination of vibrational constants. In addition, a predicted B 2Σ+–X 2Σ+ system was observed for the first time. Relative emission cross sections for the two systems and for the 1S and 3P reactant species were measured. They parallel closely the trends in the isovalent system B++H2. One important difference is, however, the existence of a large (∼3.5 eV) activation barrier in the dominant Al++H2 reaction channel. This is explained in terms of adiabatic potential hypersurfaces.

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