Sheldon K. Friedlander

The effect of particle coalescence on the surface area of a coagulating aerosol

Abstract The initial stage of particle formation in high temperature processes is characterized by a high density of very small particles undergoing rapid coagulation. When these particles are solid this leads to agglomerates with a high specific surface area. However, at high gas temperatures particle coalescence which is very sensitive to the temperature may reduce the surface area and increase the size of the primary particles. In this paper we generalize the Smoluchowski equation to incorporate the coalescence rate into the aerosol dynamics. Individual agglomerates are characterized by their volume, v , and surface area, a . A Liouville term is added to the coagulation equation determining the movement of the distribution function through a -space due to coalescence. For the rate of coalescence a simple two sphere model has been used. Results for the surface area and the average diameter of the individual primary particles are presented for the case of a collision kernel which is independent of the particle structure. As an example, the theory is applied to fine particle formation in combustion processes under nonisothermal conditions.

Dynamics of aerosol agglomerate formation

Abstract The dynamics of the formation of metal oxide agglomerates was studied by introducing metal containing salts of magnesium and zinc into a flat flame. Distributions of the mobility equivalent diameter of the agglomerates formed downstream from the flat flame were self-preserving and in good agreement with the theory for the agglomerating of power law (fractal-like) agglomerates in the free molecule regime. Theory indicates that after sufficiently long times, the average size of the agglomerates varies inversely with the size of the primary particles composing the agglomerates, for a given volumetric concentration of aerosol material. The effect of primary particle size is significant, and supported by experimental evidence.

Origins of aerosol sulfur size distributions in the Los Angeles basin

Low pressure impactor measurements show two distinct types of fine particle sulfur size distributions in Los Angeles, California. These two types of aerosol sulfur have mass median diameters of 0.54 ± 0.07μm and 0.20 ± 0.02 μm, respectively. Factors which may account for the two distribution types, including effects of relative humidity, coagulation, fogs and formation mechanisms, are discussed. Calculations show the 0.5 μm sulfur is consistent with chemical reactions in the aerosol phase, whereas the 0.2 μm sulfur results from homogeneous gas phase SO2 oxidation. While the growth of the total aerosol volume distribution is not too sensitive to the mechanism, the chemical species distribution strongly depends on the growth law, and can be used to establish its form.

Nanoparticle Formation by Laser Ablation

The properties of nanoparticle aerosols of size ranging from 4.9 nm to 13 nm, generated by laser ablation of solid surfaces are described. The experimental system consisted of a pulsed excimer laser, which irradiated a rotating target mounted in a cylindrical chamber 4 cm in diameter and 18-cm long. Aerosols of oxides of aluminum, titanium, iron, niobium, tungsten and silicon were generated in an oxygen carrier gas as a result of a reactive laser ablation process. Gold and carbon aerosols were generated in nitrogen by non-reactive laser ablation. The aerosols were produced in the form of aggregates of primary particles in the nanometer size range. The aggregates were characterized using a differential mobility analyzer and electron microscopy. Aggregate mass and number concentration, electrical mobility size distribution, primary particle size distribution and fractal dimension were measured. System operating parameters including laser power (100 mJ/pulse) and frequency (2 Hz), and carrier gas flow rate (1 l/min) were held constant.A striking result was the similarity in the properties of the aerosols. Primary particle size ranged between 4.9 and 13 nm for the eight substances studied. The previous studies with flame reactors produced a wider spread in primary particle size, but the order of increasing primary particle size follows the same trend. While the solid-state diffusion coefficient probably influences the size of the aerosol in flame reactors, its effect is reduced for aerosols generated by laser ablation. It is hypothesized that the reduced effect can be explained by the collision-coalescence mechanism and the very fast quenching of the laser generated aerosol.

Versatile aerosol concentration enrichment system (VACES) for simultaneous in vivo and in vitro evaluation of toxic effects of ultrafine, fine and coarse ambient particles Part II: Field evaluation

Abstract This study presents results from a field evaluation of a mobile versatile aerosol concentration enrichment system (VACES), designed to enhance the ambient concentrations of ultrafine (less than 0.18 μm ), fine (0– 2.5 μm ), and coarse particles (2.5– 10 μm ) for in vivo and in vitro toxicity studies. The VACES may be coupled to an exposure chamber system to assess exposure-dose effects of any one, or all, of ambient aerosol on either human subjects and/or animals. Alternatively, concentrated ultrafine, fine and coarse particles can be directly collected by impaction onto a medium suitable for application to cell cultures for in vitro evaluation of their toxic effects. The enrichment and preservation of ambient ultrafine, fine and coarse particles by size and chemical composition was determined by comparisons made between the VACES and a co-located multistage MOUDI impactor, used as a reference sampler. Furthermore, preservation of the ultrafine fraction is measured by the enrichment based on ultrafine particle numbers, morphological characteristics as well as their elemental carbon (EC) content. The results suggest that the concentration enrichment process of the VACES does not differentially affect the particle size or chemical composition of ambient PM. The following fractions: (1) mass (coarse and fine PM); (2) number (ultrafine PM); (3) sulfate (fine PM); (4) nitrate (fine PM, after correcting for nitrate losses within the MOUDI); (5) EC (ultrafine PM); and (6) selected trace elements and metals (coarse and fine PM), are concentrated very close to the “ideal” enrichment value of 22—thereby indicating a near 100% concentration efficiency for the VACES. Furthermore, ultrafine particles are concentrated without substantial changes in their compactness or denseness, as measured by the fractal dimension analysis.

In situ determination of the activation energy for restructuring of nanometer aerosol agglomerates

Abstract A new approach to the kinetics of the rearrangement of aerosol agglomerates is presented. This analysis is based on the change in free energy per primary particle during restructuring. The excess free energy compared with the final state drives the agglomerate to become more compact which results in an increase in the coordination number, as estimated from the changing fractal dimension of the aerosol agglomerates The analysis was tested on agglomerates of nanometer silver and copper particles produced by an evaporation-condensation technique and by laser ablation. The assumption that the rearrangement is an activated process was supported by the results. It was concluded that the interaction between the primary particles occurs over a short range. The activation energies were one order of magnitude lower than the bond energies expected from the bulk Hamaker constants for silver and copper. Within the measured size range no significant difference in the activation energies of silver and copper agglomerates was found. A maximum in the activation energy was found for particles about 18 nm in diameter. Larger particles showed decreasing activation energy with increasing diameter. The addition of oxygen (50%) before the agglomeration of the primary particles increased the activation energy by a factor of two and shifted the maximum in the activation energy to 15 nm. The prefactor in the rate coefficient for restructuring increased by four orders of magnitude when oxygen was added

One-step aerosol synthesis of nanoparticle agglomerate films: simulation of film porosity and thickness

A method is described for designing nanoparticle agglomerate films with desired film porosity and film thickness. Nanoparticle agglomerates generated in aerosol reactors can be directly deposited on substrates to form uniform porous films in one step, a significant advance over existing technologies. The effect of agglomerate morphology and deposition mechanism on film porosity and thickness are discussed. Film porosity was calculated for a given number and size of primary particles that compose the agglomerates, and fractal dimension. Agglomerate transport was described by the Langevin equation of motion. Deposition enhancing forces such as thermophoresis are incorporated in the model. The method was validated for single spherical particles using previous theoretical studies. An S-shape film porosity dependence on the particle Peclet number typical for spherical particles was also observed for agglomerates, but films formed from agglomerates had much higher porosities than films from spherical particles. Predicted film porosities compared well with measurements reported in the literature. Film porosities increased with the number of primary particles that compose an agglomerate and higher fractal dimension agglomerates resulted in denser films. Film thickness as a function of agglomerate deposition time was calculated from the agglomerate deposition flux in the presence of thermophoresis. The calculated film thickness was in good agreement with measured literature values. Thermophoresis can be used to reduce deposition time without affecting the film porosity.

Enhanced power law agglomerate growth in the free molecule regime

Abstract Aerosols generated at high temperatures tend to form agglomerates which can be characterized by a power law exponent, similar to a fractal dimension. The coagulation dynamics of these particles can be described by a modified collision kernel for the free molecule regime. The collision kernel for power law (fractal-like) particles is a homogeneous function, and the equation is solved using self-preserving size distribution theory for fractal dimensions between 2 and 3. The effects of fractal dimension and primary particle size on agglomerate growth and the size distribution are very strong. Agglomerate growth is rapid at low fractal dimension and fine primary particle size, because the collision cross-section is much larger for the same agglomerate mass. The effect of primary particle size on the rate of particle growth becomes more significant with decreasing fractal dimension, and the particle size distribution becomes much broader at low fractal dimensions.

Particle analysis by mass spectrometry

Abstract Mass spectra have been measured for single aerosol particles in the micron size range on a continuous, real-time basis. Particle beams of dioctyl phthalate, glutaric acid, adipic acid, ammonium sulfate, and some of the amino acids were generated by expansion of these aerosols through a capillary nozzle (throat diameter = 0.1 mm and length = 5 mm) and a skimmer (diameter = 0.331 mm). The transmission efficiency of the beam generator has been measured for different particle sizes. Individual particles were volatilized by impaction on a hot rhenium V-type filament (300 to 1400°C) and the resulting vapor plume ionized by electron bombardment in the ionizer of a quadrupole mass spectrometer. Ion currents for different masses from individual particles have been measured. The intensity of the characteristic mass peaks of different size aerosol particles increased linearly with their volume. Ammonium sulfate aerosols produce signals at SO+, SO2+, SO3+ mass fragments whereas no SO3+ mass peak could be detected from sodium sulfite particles. The absence of SO3+ from sulfite can be used to differentiate between the sulfate/sulfite contents of aerosol particles.

Regional haze case studies in the southwestern U.S—I. Aerosol chemical composition

Abstract Aerosol chemical composition as a function of particle size was determined in the southwestern U.S.A. during four weeks of sampling in June, July and December, 1979 as a part of project VISITA. Samples were collected at two ground stations about 80 km apart near Page (AZ) and in two aircraft flying throughout the region. Several different size separating aerosol samplers and chemical analysis procedures were intercompared and were used in determining the size distribution and elemental composition of the aerosol. Sulfur was shown to be in the form of water soluable sulfate, highly correlated with ammonium ion, and with an average [NH + 4 ]/[SO 2− 4 ] molar ratio of 1.65. During the summer sampling period, three distinct regimes were observed, each with a different aerosol composition. The first, 24 h sampling ending 30 June, was characterized by a higher than average value of light scattering due to particles (b sp ) of 24 × 10 −6 m −1 and a fine particulate mass ( M f ) of 8.5 μg m −1 . The fine particle aerosol was dominated by sulfate and carbon. Aircraft measurements showed the aerosol was homogeneous throughout the region at that time. The second regime, 5 July, had the highest average b sp of 51 × 10 −6 m −1 during the sampling period with M f of 3.2 μgm −3 . The fine particle aerosol had nearly equal concentrations of carbon and ammonium sulfate. For all three regimes, enrichment factor analysis indicated fine and coarse particle Cu, Zn, Cl, Br, and Pb and fine particle K were enriched above crustal concentrations relative to Fe, indicating that these elements were present in the aerosol from sources other than wind blown dust. Particle extinction budgets calculated for the three regimes indicated that fine particles contributed most significantly, with carbon and (NH 4 ) 2 SO 4 making the largest contributions. Fine particle crustal elements including Si did not contribute significantly to the extinction budget during this study. The December sampling was characterized by very light fine particle loading with two regimes identified. One regime had higher fine mass and sulfate concentrations while the other had low values for all species measured.