Franz S. Ehrenhauser

Adsorption of Gas-Phase Phenanthrene on Atmospheric Water and Ice Films

The adsorption of gas-phase phenanthrene on atmospheric water and ice films was investigated in a flow-tube reactor with a view to understanding the processing of semi-volatile organic compounds by fog and snow. Air-water (ice) interface partition constants were obtained by measuring the mass uptake of phenanthrene vapor on thin water and ice films with varying thickness. Adsorption enthalpies and entropies were obtained from the temperature dependence of the interfacial partition constants. The surface adsorption is the predominant mechanism for the uptake of phenanthrene in water and ice films with small film thickness or at low temperature. The adsorption of phenanthrene to ice resembles that to sub-cooled water and theres no significant difference between the adsorption of phenanthrene to water and that to quasi liquid layer (QLL) if we take into account the uncertainties on the thermodynamic quantities measured. Molecular dynamics simulations of phenanthrene at air/water and air/ice interfaces support these experimental observations. The interfacial air-water and air-ice partition constants of phenanthrene increased greatly in the presence of surface-active substances, indicating that surface active materials effectively enhanced the uptake of organic compounds by atmospheric water and ice films.

Design and evaluation of a dopant-delivery system for an orthogonal atmospheric-pressure photoionization source and its performance in the analysis of polycyclic aromatic hydrocarbons.

Atmospheric-pressure photoionization (APPI) mass spectrometry benefits from the addition of an ionization-enhancing dopant such as benzene. A passive dopant-delivery system has therefore been designed for use with the orthogonal APPI source within a commercial liquid chromatographic instrument with mass spectrometric detector. By providing the dopant in the gas phase, the newly designed equipment avoids mixing problems and other difficulties associated with liquid dopant addition. The system is a simple and durable design that can reliably deliver virtually any dopant with sufficient vapor pressure in the temperature range of 20 to 120 degrees C. At the optimum dopant flow rate (10% of the mobile phase flow rate) for high-performance liquid chromatography with narrow-bore (2.1 mm) columns, the system allows for uninterrupted routine analysis for up to two weeks. The performance of the device has been evaluated with benzene as dopant and with a test mixture consisting of four polycyclic aromatic hydrocarbons (PAH): naphthalene, 9H-fluorene, anthracene, and phenanthrene. All four PAH can be detected with an excellent signal-to-noise ratio in the scanning mode and a limit of detection down to 0.42 ng on column (51 pg in single-ion monitoring mode). The concentration calibration curves are linear over a range of three orders of magnitude, with correlation coefficients greater than 0.99. The utilization of benzene as dopant not only increases the sensitivity significantly - 20-fold, compared with dopant-free operation - but the low m/z values of the background ions observed also allow for the effective quantitative and qualitative analysis of PAH.

Molecular Modeling of the Green Leaf Volatile Methyl Salicylate on Atmospheric Air/Water Interfaces

Methyl salicylate (MeSA) is a green leaf volatile (GLV) compound that is emitted in significant amounts by plants, especially when they are under stress conditions. GLVs can then undergo chemical reactions with atmospheric oxidants, yielding compounds that contribute to the formation of secondary organic aerosols (SOAs). We investigated the adsorption of MeSA on atmospheric air/water interfaces at 298 K using thermodynamic integration (TI), potential of mean force (PMF) calculations, and classical molecular dynamics (MD) simulations. Our molecular models can reproduce experimental results of the 1-octanol/water partition coefficient of MeSA. A deep free energy minimum was found for MeSA at the air/water interface, which is mainly driven by energetic interactions between MeSA and water. At the interface, the oxygenated groups in MeSA tend to point toward the water side of the interface, with the aromatic group of MeSA lying farther away from water. Increases in the concentrations of MeSA lead to reductions in the height of the peaks in the MeSA-MeSA g(r) functions, a slowing down of the dynamics of both MeSA and water at the interface, and a reduction in the interfacial surface tension. Our results indicate that MeSA has a strong thermodynamic preference to remain at the air/water interface, and thus chemical reactions with atmospheric oxidants are more likely to take place at this interface, rather than in the water phase of atmospheric water droplets or in the gas phase.

PAH and IUPAC Nomenclature

The nomenclature of polycyclic aromatic hydrocarbons (PAH) and their derivatives has undergone substantial changes since the beginning of the 20th century. The International Union of Applied and Pure Chemistry (IUPAC) has issued rules and recommendations on chemical nomenclature including organic compounds like PAH since 1957. This article presents an overview of the latest version of IUPAC nomenclature for PAH and their derivatives, detailing current changes. In addition, an overview of older nomenclature systems and commonly used, PAH specific terms aiding nomenclature is given.

Mass Transport and Chemistry at the Air–Water Interface of Atmospheric Dispersoids

Atmospheric aerosols contain a significant quantity of water in the form of both bulk water and thin water films. These provide very high surface areas and adsorptive surfaces for organic compounds of low solubility and low vapor pressures. They also provide highly active sites for oxidation reactions with hydroxyl, singlet oxygen, and ozone species in the gaseous atmosphere. Thus, organic compounds are transformed readily by the reaction at the air–water interface of fog/cloud droplets and in the thin water film in atmospheric aerosols. Mathematical models suggest that the reactions in fog and cloud droplets are not limited by gas phase diffusion or mass accommodation at the surface; rather, they primarily depend on the partition constant and reaction rates at the surface. Laboratory data on both the latter parameters are available in the literature for a variety of organic compounds and they are also supported by field data. Considerable data already exist on the aqueous processing of organic species that are highly water soluble in atmospheric aerosols. However, our works suggest that aqueous processing of organic chemicals (possessing low solubility and low vapor pressure) at the interface should also be considered in fate models for secondary organic aerosols in the atmosphere.

Clar Reaction of 7H-benz[de]anthracen-7-one: Isolation and Identification of Tetrabenzo[a,cd,lm,o]perylene

The Clar reaction of 7H-benz[de]anthracen-7-one (20) was reinvestigated as a source for C34H18 polycyclic aromatic hydrocarbons (PAH). The utilization of a ternary sodium chloride/potassium chloride/zinc chloride melt with zinc dust as reducing agent yielded tetrabenzo[a,cd,j,lm]perylene (11) (10.7% yield), dibenzo[a,cd]naphtho[1,8-jk]perylene (16) (10.2% yield), and dibenzo[a,cd]naphtho[3,2,1-lm]perylene (14) (7.4% yield) as main products. Tetrabenzo[a,cd,lm,o]perylene (26) as a minor product of the Clar reaction was purified via semi-preparative high performance liquid chromatography, and its unequivocal identification was demonstrated through NMR data and a microscale-cyclization reaction.