Radial density distribution of the metastable supersaturated vapor via restricted ensemble simulations.

Extensive restricted canonical ensemble Monte Carlo simulations [D. S. Corti and P. Debenedetti, Chem. Eng. Sci. 49, 2717 (1994)] for the supersaturated Lennard--Jones (LJ) vapor were performed. These simulations were conducted at different densities and reduced temperatures from 0.7 to 1.0 and the radial density distribution functions were obtained, most of which are unavailable via integral equation theory due to phase separations. Among different constraints imposed on the system studied, the one with the local minimum of the excess free energy was taken to be the one that approximates the equilibrium state of the metastable LJ vapor. For the slightly saturated state points, where integral equation theory does have a solution, compared with our simulations, differences of the radial density distribution functions were found and they are attributed to ignoring density fluctuations in the integral equation theory.

The free energy of the metastable supersaturated vapor via restricted ensemble simulations. II. Effects of constraints and comparison with molecular dynamics simulations.

Extensive restricted canonical ensemble Monte Carlo simulations [D. S. Corti and P. Debenedetti, Chem. Eng. Sci. 49, 2717 (1994)] were performed. Pressure, excess chemical potential, and excess free energy with respect to ideal gas data were obtained at different densities of the supersaturated Lennard-Jones (LJ) vapor at reduced temperatures from 0.7 to 1.0. Among different constraints imposed on the system studied, the one with the local minimum of the excess free energy was taken to be the approximated equilibrium state of the metastable LJ vapor. Also, a comparison of our results with molecular dynamic simulations [A. Linhart et al., J. Chem. Phys. 122, 144506 (2005)] was made.

Study of thermal properties of the metastable supersaturated vapor with the integral equation method.

Pressure, excess chemical potential, and excess free energy data for different densities of the supersaturated argon vapor at reduced temperatures from 0.7 to 1.2 are obtained by solving the integral equation with perturbation correction to the radial distribution function [F. Lado, Phys. Rev. 135, A1013 (1964)]. For those state points where there is no solution, the integral equation is solved with the interaction between argon atoms modeled by Lennard-Jones potential plus a repulsive potential with one controlling parameter, alpha exp(-rsigma) and in the end, all the thermal properties are mapped back to the alpha=0 case. Our pressure data and the spinodal obtained from the current method are compared with a molecular dynamics simulation study [A. Linhart et al., J. Chem. Phys. 122, 144506 (2005)] of the same system.

The free energy of the metastable supersaturated vapor via restricted ensemble simulations.

Pressure, excess chemical potential, and excess free energy, with respect to ideal gas data at different densities of the supersaturated Lennard-Jones particle vapor at the reduced temperature 0.7 are obtained by the restricted canonical ensemble Monte Carlo simulation method [D. S. Corti and P. Debenedetti, Chem. Eng. Sci. 49, 2717 (1994)]. The excess free energy values depend upon the constraints imposed on the system with local minima exhibited for densities below the spinodal density and monotonic variation for densities larger than the spinodal density. The results are compared with a molecular dynamics simulation study [A. Linharton et al., J. Chem. Phys. 122, 144506 (2005)] on the same system. The current study verifies the conclusion drawn by the simulation work that clustering of Lennard-Jones atoms exists even in the vicinity of spinodal. Our method gives an alternative to molecular dynamic simulations for the determination of equilibrium properties of a metastable fluid, especially close to the spinodal, and does not require a very large system to carry out the simulation.

Equilibrium adsorption on single and aggregated nanospheres.

The mean field density functional theory has been used to explore Ar adsorption onto single and double identical carbon dioxide nanospheres. We have studied adsorption from subsaturation up to saturation at several temperatures. For the single sphere case, the adsorption excesses and density profiles approach those of plane cases as the spherical substrate size increases; at each temperature, the transition from thin-film to thick-film adsorption is strongly dependent on the size of substrate and the adsorption behavior approaches to that of a plane when the substrate size goes to a large value. For the double sphere case, the cluster formed within the region analogous to the pendular region formed by two contacting identical nanospheres of radius of 1.7 nm has also been studied. The two sphere structure favorites the cluster forming in the interstitial region but does not affect the adsorption transition.

Simulations of aerosol aggregation including long-range interactions.

Current understanding of solid aerosol particle aggregation is limited to simulation models based on diffusive and ballistic motion of the colliding particles. The role of the long-range van der Waals forces in aggregation phenomena, although important, has never been examined. In an effort to address this issue, a simulation model, based on molecular dynamics techniques, is developed. Using this model to simulate thermal collisions of single spheres with small aggregates of similar spheres, we examine the effects of retardation of the long-range van der Waals forces, particle transport, ambient temperature, and pressure on the collision rates and mass and structure distributions of the aggregated particles. The model calculations were performed at simulated temperatures of 293 and 1500 K and at simulated pressures of 760 and 3040 torr for glassy carbon primary particles in the free molecular regime with diameters of 6 nm, and in the transition regime with diameters of 30 nm. Inclusion of the long-range van der Waals forces resulted in aggregates with relatively open structures and few branches and collision rate constants that were larger than the corresponding hard sphere rate constants, whereas exclusion of the forces resulted in compact structures with more branches and smaller enhancements in the rate constants. The above effects were found to be more pronounced in the free molecular regime than in the transition regime, which is consistent with the observation that the initial conditions and the interparticle forces play a more significant role in particle transport in the free molecular regime than in the transition regime. The effect of retardation of the forces is an increase in the percentage of open aggregates and the collision rate constants over that of the corresponding nonretarded case. An increase in temperature resulted in a collapse of aggregate structure and a decrease in collision rate constants corresponding to the reduced geometrical cross sections. Again, the effects were found to be more pronounced in the free molecular regime than in the transition regime. No significant difference was observed in the structure of the aggregates or in the collision rate constants with a change in pressure, indicating that the pressure effect, if any, is hidden by the much stronger effect of the long-range van der Waals forces.

Measurement of SO2 effects on the 218Po ion mobility spectrum by alpha-track detection.

A number of investigators have reported the formation of radiolytic ultrafine particles produced by the interaction of ionizing radiation with atmospheric trace gases. Previous studies have suggested that a very high localized concentration of the OH radical produced by the radiolysis of water can react with atmospheric trace gas, like SO2, to produce lower vapor pressure compounds that can then nucleate. To determine trace-gas dependence of the active, positively charged, first decay product of radon, the mobility spectrum of the decay products in the range of 0.06-3.0 cm2 V-1s-1 was measured in a flow-through radon chamber using a specially designed mobility spectrometer. Measurements were taken for different relative humidities and concentrations of SO2 in purified laboratory compressed air. The resultant mobility spectra were compared with radiolytic ultrafine particle activity distribution data. In the case of low humidities, the reduction of available OH concentration and hence neutralization rate led to the increasing intensity of the shoulder around 1.35 cm2 V-1s-1 with increasing SO2, suggesting SO2 clustering around the PoOx+ ion. For high humidity conditions (5 ppm SO2, 30% RH), there was clear droplet formation, H2SO4, clustering around the ion, and also an increase in the background level between the high mobility peak and droplet peak.

Composition measurement of aerosols of submicrometer particles by phase fluctuation absorption spectroscopy.

Phase fluctuation optical heterodyne spectroscopy is employed to obtain the first nonextractive data on size-mass distribution of submicrometer ammonium sulfate aerosols. The required theory of aerosol absorption of electromagnetic energy and subsequent thermalization is presented. Based on this theory, the predicted behavior was confirmed by experiments using IR absorption of ammonium sulfate aerosols. The results show good agreement with measurements using conventional methods including condensation nuclei counter, electron micrography, and filtration. In addition, a conversion constant for the photothermal signal of ammonium sulfate aerosols is calculated to be 2.3 microV/(microg/m3). Furthermore, the signal is shown to be linear in excitation energy and modulation frequency and scalable.

Charged aerosol particles and air pollution.

Possible environmental effects and the importance of charged aerosol particles are examined. Some calculations of the evolution of the charge distribution of the atmospheric aerosol are presented. Several possible environmental effects of particle charge are noted including effects on particle coagulation, dry deposition, and deposition in the lung. Finally some problems in aerosol charging are reviewed as they pertain to various technical operations in air pollution control.