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Abstract. Atmospheric free radicals hydroxyl and hydroperoxyl (OH and HO2, collectively HOx) are the catalysts that cause secondary or photochemical air pollution. Chemical mechanisms for oxidant and acid formation, on which expensive air pollution control strategies are based, must accurately predict these radical concentrations. We have used the fluorescence assay with gas expansion (FAGE) technique to carry out the first simultaneous, in situ measurements of these two radicals in highly polluted air during the Los Angeles Free Radical Experiment. A complete suite of ancillary measurements was also made, including speciated hydrocarbons, carbon monoxide, aldehydes, nitric oxide, nitrogen dioxide, and ozone along with meteorological parameters. Using this suite of measurements, we tested the ability of a lumped chemical mechanism to accurately predict radical concentrations in polluted air. Comparison of model predictions with measured radical concentrations revealed generally good agreement for OH early and late in the day, including the early evening hours, when OH persisted at low concentrations after dark. During midday, however, modeled [OH] was high by about 50%. Agreement for HO 2 was quite good in the early morning hours, but model-calculated HO 2 concentrations were significantly too high during midday. When we used our measured HO 2 concentrations as model input, agreement between calculated and measured OH concentrations was improved. It seems likely that (1) the model's HOx sources are too large, (2) there are unaccounted HO• loss processes in Los Angeles air, and/or (3) the complex parameterization of RO2/HO 2 radical chemistry in the reaction mechanism does not adequately describe the behavior of these radicals in the Los Angeles atmosphere.

Molecular dynamics simulations were used to study the structure and dynamics of the uranyl ion and its aquo, hydroxy, and carbonato complexes in bulk water and near the hydrated quartz (010) surface. All simulations were performed in the constant (NVT) ensemble with three-dimensional periodic boundary conditions, and a slab technique was used to model the quartz-water interface. The uranyl coordination shell exhibits pentagonal bipyramidal symmetry, with carbonate and hydroxide ions readily replacing water molecules in the first shell. Radial distribution functions of the hydroxy and carbonato complexes are characterized by a consistent splitting in the equatorial shell, caused by the close proximity of hydroxide and carbonate oxygen atoms. Average U-O distances are 2.31-2.35 Å for hydroxide ions, 2.35-2.39 Å for carbonate ions, and 2.49-2.55 Å for water molecules. Two protonation states of the quartz surface were considered for adsorption simulations: singly protonated and partially deprotonated. Surface complexes formed only when the initial uranyl position was close to the surface; otherwise, a diffuse species was observed. Outer-sphere surface complexes formed at the singly protonated surface and are characterized by hydrogen bonding between a coordinating water molecule and the surface. Inner-sphere surface complexes formed at the partially deprotonated surface, with water and surface oxygen atoms equidistant to the uranium atom. In both types of surface complex, splitting of the equatorial shell of the uranyl ion was due to the presence of hydroxide or carbonate ions in the first coordination shell.

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The structure of unpromoted precipitated Fe catalysts was determined by Mössbauer emission and X-ray absorption spectroscopies after use in the Fischer-Tropsch synthesis (FTS) reaction in well-mixed autoclave reactors for various periods of time. X-ray absorption near-edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS) analysis, and Mössbauer spectroscopy showed consistent trends in the structural evolution of these catalysts during reaction. The nearly complete formation of Fe carbides during initial activation in CO was followed by their gradual re-oxidation to form Fe 3 O 4 with increasing time-on-stream. Fe 3 O 4 became the only detectable Fe compound after 450 h. The observed correlation between FTS rates and Fe carbide concentration, and the unexpected re-oxidation of the catalysts as CO conversion decreased, suggest that the deactivation of Fe catalysts in FTS reactions parallels the conversion of Fe carbides to Fe 3 O 4 . It appears that the CO activation steps responsible for replenishing carbidic surface species and for removing chemisorbed oxygen are selectively inhibited by deactivation of surface sites, leading to the oxidation of Fe carbide even in the presence of a reducing reactant mixture.

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