Gary N. Robinson

Heterogeneous uptake of ClONO2 and N2O5 by sulfuric acid solutions

A droplet train apparatus has been used to measure the heterogeneous reactive uptake of gaseous N2O5 and ClONO2 by concentrated sulfuric acid solutions. H2SO4 concentrations in the range of 39 to 69 wt% were investigated between 229 and 260 K. Uptake rates normalized to the gas-liquid collision frequency, γ0, for N2O5 ranged from 0.086 to 0.16, decreasing moderately with increasing temperature and decreasing H2SO4 concentration. Uptake rates for ClONO2, measured over a slightly narrower concentration range of 39–59 wt% H2SO4, ranged between 0.0037 and 0.056, decreasing moderately with H2SO4 temperature but significantly with increasing concentration. Results are compared with measurements from other laboratories using different experimental techniques. In general, the data from the different groups agree well. A phenomenological model is presented which addresses the solubility, diffusion, and chemical reactivity of XNO3 (X=Cl, NO2) in sulfuric acid solutions and accounts for the dependence of the observed uptake rates on H2SO4 concentration and temperature. Two XNO3 hydrolysis pathways are proposed, one involving direct reaction with H2O and the other involving participation of H+ ions to promote bond dissociation. Differences between the concentration dependencies of γ0 for ClONO2 and N2O5 can be ascribed largely to different rates of acid-catalyzed hydrolysis. The implications of these results for the effects of lower stratospheric sulfuric acid aerosols on ozone depletion chemistry are discussed.

Heterogeneous uptake of HCl by sulfuric acid solutions

The uptake of HCI molecules by aqueous sulfuric acid droplets was measured in the temperature range 230-264 K at 39, 49, 54, 59, and 69 wt % acid and as a function of time (2-15 ms). These experiments utilized a droplet train apparatus in which a stream of monodisperse droplets (120-250 μm in diameter) is passed through a low-pressure flow containing HCl(g). The droplet area is changed in a step-wise fashion, while the HCl(g) density is continuously monitored by infrared absorption. The uptake coefficient is obtained from the measured change in the HCl density. The product of H * D l 1/2 (H * , solubility; D l , liquid phase diffusion coefficient) and the mass accommodation coefficient α of the species as a function of temperature and sulfuric acid concentration were obtained from the uptake coefficient. The good agreement of measured and modeled H * D l 1/2 values validates current formulations of HCl reactivity in stratospheric aerosols. While the solubility of HCl decreases steeply with sulfuric acid concentration because increasing acidity reduces the dissociation of HCl into H + and Cl - in solution, the mass accommodation coefficient is independent of acid concentration in the region studied. As with previously studied species, a is inversely proportional to temperature increasing from ∼0.06 at 294 K to near unity at ∼230 K. The mass accommodation coefficient is well expressed in terms of an observed Gibbs free energy as α/(1 - α) = exp (- ΔG obs /RT), suggesting that the clustering model for the accommodation process is applicable in this case as well. The mass accommodation measurements are well fitted by the parameters ΔH obs = -13.8 ± 0.9 kcal mol -1 and ΔS obs = -52.2 ± 0.3 cal mol -1 K -1 . Under stratospheric conditions α for HCI is unity. Implications of the HCl uptake studies for atmospheric chemistry are examined.

Decomposition of halomethanes on α‐alumina at stratospheric temperatures

We present data that suggest the possibility of a previously unsuspected heterogeneous process involving the decomposition of halocarhon source gases on alumina solid-propellant rocket motor exhaust particles deposited in the stratosphere. These results, obtained under ultrahigh vacuum conditions, provide evidence that halomethane compounds undergo dissociative chemisorption on α-alumina surfaces at stratospheric temperatures, leading to the release of more reactive halogen containing species. Simple calculations indicate that halocarbon concentrations will be severely depleted in the immediate wake of the rocket plume; any serious global impact at current projected launch rates requires the existence of one or more processes that regenerate active sites on the surface subsequent to the dissociative chemisorption of halocarbons.

Sulfuric acid-induced corrosion of aluminum surfaces

The sulfuric acid-induced corrosion of smooth (2 nm average roughness) aluminum surfaces has been studied in real times using an in situ Fourier transform infrared reflection absorption spectrometer and a quartz crystal microbalance. Submicron thick, 35 to 55 weight percent (5 to 12 molal), sulfuric acid films were formed on room temperature metal surfaces by the reaction of gas-phase SO{sub 3} and H{sub 2}O vapor in a flowing gas system at a total pressure of {approximately}200 Torr. The deposition of the acid films and subsequent changes in their chemical composition resulting from corrosion of the aluminum substrate could be monitored using characteristic infrared absorption features. The corrosion process always significantly perturbed the spectral signature of the films from that which was observed on inert gold surfaces. Using changes in spectral features that are linked to the production of Al{sup 3+} as indicators of corrosion, the authors conclude the rate of corrosion of the metal is strongly enhanced by both higher relative humidities and increased rates of sulfuric acid deposition.

Temperature-dependent surface potentials of fluorinated alkanethiolate self-assembled monolayers

Self-assembled monolayers (SAMs) consisting of twenty-two carbon, methyl-terminated alkanethiolates adsorbed on vapor-deposited gold have been fluorinated in vacuum using an effusive F atom source. The reactive uptake of fluorine as a function of F atom exposure was calibrated using X-ray photoelectron spectroscopy. The surface potentials (Vs) of SAMs that were fluorinated to different degrees were measured as a function of temperature using a high-sensitivity vibrating probe electrostatic voltmeter. The surface potential grew increasingly negative with increasing fluorine uptake, reflecting the charge asymmetry that is induced in the alkanethiolate chains as a result of the substitution of fluorine for hydrogen. The Vs of the most highly fluorinated SAMs displayed a negative temperature dependence. This observation may be indicative of a pyroelectric effect in these monolayers although a definitive conclusion awaits further measurements.