The heterogeneous uptake of gaseous N 2 O 5 by ammonium sulfate [(NH 4 ) 2 SO 4 ], ammonium bisulfate [NH 4 HSO 4 ], and sulfuric acid [H 2 SO 4 ] aerosols as a function of relative humidity has been investigated at room temperature and atmospheric pressure. Ammonium-containing aerosols were generated by a constantoutput atomizer and conditioned by passing through a diffusion dryer. Sulfuric acid aerosols were produced by the homogeneous reaction of SO 3 and H 2 O in a borosilicate vessel. Addition of a dry or wet N 2 flow controlled the relative humidity (RH) of these aerosol flows. Using a chemical ionization mass spectrometer (CIMS) for N 2 O 5 concentration monitoring and a scanning mobility particle spectrometer (SMPS) for aerosol characterization, reaction probabilities (γ) in the range of 0.001 to 0.1 for the uptake of N 2 O 5 were determined as a function of RH. The results are expressed as follows: γ[(NH 4 ) 2 SO 4 ] ) 2.79 × 10 -4 + 1.30 × 10 -4 × (RH) -3.43 × 10 -6 × (RH) 2 + 7.52 × 10 -8 × (RH) 3 , γ[NH 4 HSO 4 ] ) 2.07 × 10 -3 -1.48 × 10 -4 × (RH) + 8.26 × 10 -6 × (RH) 2 , and γ[H 2 SO 4 ] ) 0.052 -2.79 × 10 -4 × (RH). We suggest that the water content and phase in the ammonium-containing aerosols control the reactivity of N 2 O 5 while liquid-phase ionic reactions primarily dominate the uptake in sulfuric acid aerosols.

The oxidation of propylene preadsorbed on the Pt(111) surface has been characterized in oxygen pressures up to 0.02 Torr using fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) above the carbon K edge. During oxidation of adsorbed propylene, a stable intermediate was observed and characterized using these soft X-ray methods. A general in situ method for determining the stoichiometry of carbon-containing reaction intermediate species has been developed and demonstrated for the first time. Total carbon concentration measured during temperature-programmed reaction studies clearly indicates a reaction intermediate is formed in the 300 K temperature range with a surface concentration of 0.55 × 1015 carbon atoms/cm2. By comparing the intensity of the C−H σ* resonance at the magic angle with the intensity in the carbon continuum, the stoichiometry of this intermediate can be determined unambiguously. Based on calibration with molecular propylene (C3H6) and propylidyne (C3H5), the intermediate has a C3Η5 stoichiometry for oxygen pressures up to 0.02 Torr. A set of normal and glancing angle FYNES spectra above the carbon K edge was used to characterize the bonding and structure of this intermediate. Spectra of known coverages of adsorbed propylene and propylidyne served as standards. The spectra of di-σ propylene, propylidyne, and the intermediate were curve fit as a group with consistent energies and widths of all primary features. Based on this procedure, the intermediate is 1,1,2-tri-σ 1-methylvinyl. The stoichiometry and temperature stability range of the 1-methylvinyl intermediate formed in oxygen pressures up to 0.02 Torr is identical with the stoichiometry and stability of the same intermediate formed during oxidation of preadsorbed propylene by excess coadsorbed atomic oxygen.

The uptake of gas-phase methanol by liquid sulfuric acid has been investigated over the composition range of 40-85 wt % H 2 SO 4 and the temperature range of 210-235 K. Laboratory studies were performed with a flow-tube reactor coupled to an electron-impact ionization mass spectrometer to detect trace gases. While reversible uptake was the primary mechanism at low acid concentrations, an irreversible reaction between methanol and sulfuric acid at low temperatures, forming methyl hydrogen sulfate and dimethyl sulfate, was observed at all concentrations. At compositions >65 wt % H 2 SO 4 , more than 90% of uptake was found to be reactive. On the basis of the uptake data and the calculated liquid-phase diffusion coefficients, the product of the effective Henry's law constant (H*) and the square root of the overall liquid-phase reaction rate (k l ) was calculated as a function of acid concentration and temperature. The results suggest that the reaction with sulfuric acid forming methyl hydrogen sulfate and dimethyl sulfate is the dominant loss mechanism of methanol and that the oxidation of methanol is only a minor source of hydroxyl radicals in the upper troposphere. † Part of the special issue "Harold Johnston Festschrift". Dedicate to Professor Harold Johnston for his contributions to atmospheric chemistry.

The uptake of acetone vapor by liquid sulfuric acid has been investigated over the range of 40−87 wt % H2SO4 and between the temperatures 198 and 300 K. Studies were performed with a flow-tube reactor, using a quadrupole mass spectrometer for detection. At most concentrations studied (40−75 wt %), acetone was physically absorbed by sulfuric acid without undergoing irreversible reaction. However, at acid concentrations at or above 80 wt %, reactive uptake of acetone was observed, leading to products such as mesityl oxide and/or mesitylene. From time-dependent uptake data and liquid-phase diffusion coefficients calculated from molecular viscosity, the effective Henry's law solubility constant (H*) was determined. The solubility of acetone in liquid sulfuric acid was found to increase with increasing acid concentration and decreasing temperature. In the 75 wt % and 230 K range, the value for H* was found to be ∼2 × 106 M/atm. This value suggests that acetone primarily remains in the gas phase rather than absorbing into sulfate aerosols under atmospheric conditions.