In this study, transmission FT-IR spectroscopy is used to investigate the adsorption of water on aluminum oxide surfaces, including both single-crystal and particle surfaces. The FT-IR spectra of the (0001) surface of R-Al2O3 single crystals at 296 K in the presence of 0.2 to 20 Torr H2O vapor pressure corresponding to 1 to 95% relative humidity (RH) have been measured. The FT-IR spectra are shown to change as a function of RH. At low relative humidity, below approximately 10%, water adsorbs on the surface in an ordered fashion with the formation of a stable hydroxide layer. At intermediate RH, between 10 and 70% RH, water adsorbs molecularly in a structured overlayer. At high RH, greater than 70%, water adsorption is more disordered and consistent with the formation of a liquid-like water layer. The adsorption of nitric acid on R-Al2O3(0001) prior to water adsorption is shown to only slightly modify the water infrared spectra. A discussion of the quantification of the number of water layers on the R-Al2O3(0001) surface as a function of RH as well as a comparison of water adsorption on single-crystal R-Al2O3(0001) to that on R-and γ-Al2O3 powders are presented here.

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Transition metals such as iron are reactive components of environmentally relevant surfaces. Here, dark reaction of Fe(III) with catechol and guaiacol was investigated in an aqueous solution at pH 3 under experimental conditions that mimic reactions in the adsorbed phase of water. Using UV-vis spectroscopy, liquid chromatography, mass spectrometry, elemental analysis, dynamic light scattering, and electron microscopy techniques, we characterized the reactants, intermediates, and products as a function of reaction time. The reactions of Fe(III) with catechol and guaiacol produced significant changes in the optical spectra of the solutions due to the formation of light absorbing secondary organics and colloidal organic particles. The primary steps in the reaction mechanism were shown to include oxidation of catechol and guaiacol to hydroxy- and methoxy-quinones. The particles formed within a few minutes of reaction and grew to micron-size aggregates after half an hour reaction. The mass-normalized absorption coefficients of the particles were comparable to those of strongly absorbing brown carbon compounds produced by biomass burning. These results could account for new pathways that lead to atmospheric secondary organic aerosol formation and abiotic polymer formation on environmental surfaces mediated by transition metals.

Second-harmonic studies were carried out to determine the interfacial acidity, the potential, and the interfacial energy density of an acid-functionalized silica/water interface between pH 2 and 12. The interfacial potential changes over 3 orders of magnitude, from 10-2 mV to several tens of millivolts, and the interfacial energy density changes by 7 orders of magnitude, from less than 10-7 mJ/m2 to several millijoules per square meter. The methodology presented in this study provides quantitative thermodynamic information necessary for understanding and predicting how solvated species interact with functionalized organic adlayers at liquid/solid interfaces over a wide pH range.

The heterogeneous reactivity of gaseous nitric acid on R-Al 2 O 3 , R-Fe 2 O 3 , SiO 2 , MgO, and CaO, as well as authentic samples of Gobi dust and Saharan sand, was investigated at 295 K using a Knudsen cell reactor. Through total uptake calculations and mass dependent studies, nitric-acid diffusion into the underlying layers is shown to be an important process for all of the systems studied. As such, models that account for the increased surface area and the concomitant increase in "internal collisions" were used to calculate uptake coefficients. The initial uptake coefficients for R-Al 2 O 3 , R-Fe 2 O 3 , SiO 2 , MgO, and CaO all lie in the 10 -5 to 10 -3 region at gas concentrations between 10 11 and 10 12 molecules/cm 3 . As expected, on the basis of their high SiO 2 composition, the measured initial uptake coefficients for the authentic dust samples are close to the value found for SiO 2 and are between 2 and 6 × 10 -5 . Uptake of nitric acid on most of the oxide particles and the authentic dust samples results in irreversible adsorption. The effect of surface adsorbed water on both CaO and Gobi dust was investigated and is found to significantly enhance the initial uptake coefficient. Atmospheric implications of these results in terms of the importance of heterogeneous reactions of nitric acid on mineral aerosol are discussed.