Nanoparticles play a major role in industrial processes and natural phenomena in a variety of fields including chemical engineering, chemistry, physics, public health and biology. Nanoparticles are suspended in fluids during production, handling, processing, and by unintentional and/or undesirable release to the environment. In many cases the suspending fluid is a gas, as for example in large scale production, air pollution, clean room applications and many more. The small size of nanoparticles makes it possible to compare their transport properties to the fluid itself. Brownian particle diffusion is one of the most important mechanisms leading to significant transport rates. Diffusion is well known in mass transfer, however, differences arise because of the wide spectrum of nanoparticle sizes and their morphology (aggregate structure) - a concept understood in particle science and technology but needed in many diverse fields of nanoparticle applications. Nanoparticle transport can be controlled by external force fields because these forces may act exclusively on the nanoparticles and have negligible effects on fluid molecules. These include thermophoresis and forces in electrical fields.

Source resolution of the organic component of the fine fraction of the ambient aerosol (dp < 3.5 fim) has been carried out by combining source information from the inorganic component with thermal analysis and local emission inventories. The primary and secondary carbon containing components were identified using a source fingerprint thermogram. The primary carbon emitted from automobiles was calculated using the ambient lead concentration and a recent estimate of the lead to carbon ratio for this source. The remaining primary carbon was apportioned to the other sources according to the local emission inventory. Source apportionment of the secondary component was performed by scaling the secondary carbon to a recent emission inventory for reactive organic gases (ROG) neglecting biogenic contributions and assuming equal potential for aerosol formation for the various ROG. The method was applied to data collected at Lennox and Duarte on 10/23/80. The major source at both sites was automobiles which were responsible for 68% and 36% of the carbon containing component of the fine aerosol at Lennox and Duarte respectively. Industrial sources made a relatively higher contribution at Duarte, 26%, than at Lennox 12%. Good correlation was found between ozone and secondary carbon. However, a weaker correlation was found between sulfates and secondary carbon. Contributions to the visibility budget by various chemical species were calculated at Lennox and Duarte on 10/23/80 using an appropriate regression model. It was found that the carbon containing component was responsible for 27% and 44% of the incident light extinction at Lennox and Duarte, respectively.

Binary SiO 2 /TiO 2 and SiO 2 /Fe 2 O 3 nanoparticle (diameter < 100 nm) aerosols of varying mole ratios of Ti or Fe to Si were generated in a premixed Bunsen-type aerosol flame reactor. The distribution of species within the particles was investigated using transmission electron microscopy, electron energy loss spectrometry, x-ray diffraction, and Fourier transform infrared spectroscopy. Phase segregation was observed to varying degrees in qualitative agreement with segregation expected from binary phase diagrams for the bulk systems. Differences between the SiO 2 /TiO 2 and SiO 2 /Fe 2 O 3 systems can be explained by considering the variation in the thermodynamically stable liquid-phase solubility and differences in the ability of iron and titanium ions to substitute for silicon ions in the network structure.

The nanoparticle aggregate volumes are determined from the mobility diameter using the idealized aggregate (IA) theory proposed by Lall and Friedlander [(2006). On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: I. Theoretical analysis. Journal of Aerosol Science, 27, 260]. The use of IA theory makes it possible to account for aggregate number and size of primary particles and aggregate orientation in the electric field. The theory is tested using an aerosol particle mass analyzer (APM) which determines particle mass based on particle motion in a centrifugal and electrical force field. Unlike electrical mobility analysis, the APM mass measurements are independent of particle morphology because the centrifugal force is directly proportional to the mass. The aggregate volumes based on IA theory are compared with the aggregate volumes measured by the APM. The comparison is made for iron oxide (density=5.7g/cc) and carbon (density=2g/cc) aggregates, both generated by laser ablation. A differential mobility analyzer (DMA) was used to classify the aggregates corresponding to mobility diameters of 80, 100, and 120nm. For each mobility diameter, the aggregate volume was calculated from IA theory; the primary particle diameter was measured by electron microscopy. The aggregate mass for each mobility diameter was measured directly by the APM without the use of IA theory. The aggregate volume was determined from the mass measured by the APM and the primary particle density. The agreement between the DMA and APM aggregate volume measurements was good for both materials studied. The results support the application of IA theory. In a further application of IA theory, literature data for DMA–APM measurements of the ultrafine atmospheric aerosol were used to calculate the fraction of aggregates.

A method is proposed for using size distribution data obtained with a differential mobility analyzer (DMA) to calculate the total surface area and volume of electrically conducting nanoparticle chain aggregates deposited in the human respiratory tract. The method consists of two steps: (1) the electrical mobility diameter of conducting aggregates determined with a DMA is used to calculate the diffusion-equivalent diameter and (2) the theory of idealized aggregates developed by Lall and Friedlander [(2006). On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: I. Theoretical analysis. Journal of Aerosol Science 37, 260] is applied in calculations for the aggregate surface area and volume. The results of calculations using the volume distribution of a sample of diesel exhaust particles indicate that deposited aggregates have a larger total surface area than do electrical-mobility-equivalent spheres, even though the deposited aggregates have a much smaller total volume. The proposed method for calculating diffusion-equivalent diameter from electrical mobility diameter is also employed to show that the deposition data reported in literature for diesel exhaust particles and petrol particles agree well when the deposition fractions are plotted as functions of diffusion-equivalent diameter.

Dry aerosol synthesis applying the flame spray pyrolysis was used to manufacture and directly (in situ) deposit tin dioxide nanoparticles on sensor substrates. For the first time this technique was used to synthesize a combination of two porous layers for gas-sensor fabrication. Two different sensing layers were deposited on ceramic substrates, i.e., pure tin dioxide and palladium-doped tin dioxide. The top layer was a palladium-doped alumina as a filter. The fabricated sensors were tested with methane, CO, and ethanol. In the case of CH_4, the pure tin dioxide sensor with the Pd/Al_2O_3 filter showed higher sensor signals and improved selectivity with respect to water vapor compared to single tin dioxide films. At temperatures up to 250 °C the Pd doping of the tin dioxide strongly increased the sensitivity to all gases. At higher temperatures the sensor signal significantly decreased for the Pd/SnO_2 sensor with a Pd/Al_2O_3 filter, indicating high catalytic activity.

In the present work a previously developed model of fractal aggregates evolution from an initial morphology (as described by their fractal dimension) towards to that defined by the prevailing coagulation mechanism is extended in two directions. Firstly a new constitutive law for the fractal dimension of the aggregate resulting from a coagulation event is generalized and secondly a restructuring mechanism is added to the population balance model. Several techniques from detailed Monte Carlo simulations to simple monodisperse (in both volume and fractal dimension) approximations are employed for the solution of the corresponding bivariate coagulation equation. The parametric evolution of the fractal dimension of aggregates for the case of Brownian coagulation in the continuum regime is studied and the results indicate that the existence of restructuring makes the evolution dynamics of the fractal dimension distribution of the aggregate population much richer than in the case of simple coagulation examined previously. As an application of the present approach, the morphological data of Xiong and Friedlander [(2001) Morphological properties of atmospheric aerosol aggregates. Proceedings of the National Academy of Sciences of USA, 98, 11851–11856] on atmospheric aggregates are examined and are shown to be consistent with a combined coagulation–restructuring process.

Differential mobility analyzers (DMAs) are sometimes used to characterize aerosols that contain aggregates of low fractal dimension. However, these instruments are normally calibrated for spherical particles and the calibrations are not directly applicable to aggregates. A method proposed by Lall and Friedlander [(2006). On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis, I: Theoretical analysis. Journal of Aerosol Science, in press] for characterizing ultrafine aggregate number, surface area and volume distributions by electrical mobility measurements was tested experimentally. The method is best applied to idealized aggregates composed of uniform primary particles smaller than the mean free path of the gas. It relates the number and size of the primary particles that compose the aggregate to the mobility diameter of a spherical particle. Aggregate number distributions were obtained by calculations based on aggregate drag and aggregate charging efficiency; surface area and volume were obtained by summing over the primary particles that compose the aggregate.The theory was tested experimentally using silver aggregates generated by an evaporation-condensation method. Primary particle diameter was 18.5+/-3.5 nm. To obtain distributions with respect to particle volume, aggregates were sintered to form spheres. It was assumed that the aggregate volume does not change upon sintering and coagulation was neglected. Thus the number of aggregates in a given volume range (number distribution, dN/dlog v vs. v) should not change after sintering. Agreement between aggregate number distribution based on idealized aggregates and the values measured for spheres of sintered aggregates was good. The agreement also indicates that the aggregate volumes based on idealized aggregates were accurate. The aggregate number distribution and volume based on the conventional calibration for spheres were significantly overpredicted. A separate experimental test of the theory was made using literature data for diesel aggregates. Primary particle diameter was 31.9 +/- 7.2 nm. Aggregate volumes calculated from theory agreed well with aggregate volumes measured by transmission electron microscope analysis. (C) 2006 Elsevier Ltd. All rights reserved.

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.

Electrical mobility analyzers are usually calibrated for spherical particles, and provide number, area and volume distributions for spherical particles. However, these instruments cannot be directly used to obtain the surface area and volume distributions for aggregates. Aggregates are important in technological applications, such as the manufacture of fine powdered materials, and in air pollution and atmospheric sciences. Thus, nanoparticle chain aggregates of low fractal dimension are another important limiting case, in addition to spheres; a method is described which makes it possible to relate aggregate surface area and volume distributions to the electrical mobility diameter. This is accomplished by equating the migration velocity of an aggregate to that of a sphere. Particles of equal migration velocities will trace similar paths in the mobility analyzer and have the same mobility diameter (neglecting the Brownian diffusive spread). By equating the migration velocities of a sphere and aggregate, the number and size of the primary particles composing the aggregate can be related to the diameter of a sphere with the same migration velocity.

Nanoparticle chain aggregates (NCAs) are often sized and collected using instruments that rely on inertial transport mechanisms. The instruments size segregate aggregates according to the diameter of a sphere with the same aerodynamic behavior in a mechanical force field. A new method of interpreting the aerodynamic diameter of NCAs is described. The method can be used to calculate aggregate surface area or volume. This is useful since inertial instruments are normally calibrated for spheres, and the calibrations cannot be directly used to calculate aggregate properties. A linear relationship between aggregate aerodynamic diameter and primary particle diameter based on published Monte-Carlo drag calculations is derived. The relationship shows that the aggregate aerodynamic diameter is independent of the number of primary particles that compose an aggregate, hence the aggregate mass. The analysis applies to aggregates with low fractal dimension and uniform primary particle diameter. This is often a reasonable approximation for the morphology of nanoparticles generated in high temperature gases. An analogy is the use of the sphere as an approximation for compact particles. The analysis is applied to the collection of NCAs by a low-pressure impactor. Our results indicate the low-pressure impactor collects aggregates with a known surface area per unit volume on each stage. Combustion processes often produce particles with aggregate structure. For diesel exhaust aggregates, the surface area per unit volume calculated by our method was about twice that of spheres with diameter equal to the aerodynamic diameter. Measurements of aggregates collected near a major freeway and at Los Angeles International Airport (LAX) were made for two aerodynamic cutoff diameter diameters (d a,50), 50 and 75 nm. (Aerodynamic cutoff diameter refers to the diameter of particles collected with 50% efficiency on a low-pressure impactor stage.) Near-freeway aggregates were probably primarily a mixture of diesel and internal combustion engine emissions. Aggregates collected at LAX were most likely present as a result of aircraft emissions. In both measurements, the aggregate aerodynamic diameters calculated from the primary particle diameter were fairly close to the stage cutoff diameter. The number of primary particles per aggregate varied one order of magnitude for particles depositing on the same stage. The average aggregate surface area per unit volume was 2.41 × 106 cm−1 and 2.59 × 106 cm−1 (50 nm d a,50) and 1.81 × 106 cm−1 and 1.68 × 106 cm−1 (75 nm d a,50) for near-freeway and LAX measurements, respectively. These preliminary measurements are consistent with values calculated from theory.

Introduction Diesel soot from automotive exhaust is a significant constituent of the urban aerosol. Due to the composition of soot particles, a carbonaceous, solid core and adsorbates like polycyclic aromatic hydrocarbons, and their inhalability due to the fine particle size, these particles are cause of concern for public health. To minimize the potential risk, a reduction of the particulate emissions of diesel powered vehicles is sought by means of improved motor technology and by the development of soot traps. On the engine side new technologies have enabled an increase in fuel efficiency and a decrease in overall particulate mass emission. However, to meet future emission standards in the US and Europe, a combination of engine improvement and filter technology is necessary. To enhance the durability of soot traps, the continuous regeneration of the filter element is essential. This is done by the oxidation of the trapped particulate material and is substantially facilitated by the use of catalysts in combination with the soot trap. Many studies have been conducted on the catalytic oxidation of soot, a large number of catalysts and catalyst-support combinations have been addressed. Although a lot of work has been done, the mechanism of the oxidation of soot remains largely unknown. The studies reported in literature so far have been carried out on bulk powder mixtures, an experimental technique that does not allow the definition of the contact between soot and catalyst particles. Purpose of this study was to determine the effects of the particle size of Pt-catalysts and of the inter particle distance between soot and catalyst. Experiments were carried out on a microscopic scale on SiO/SiO2-coated transmission electron microscope (TEM) grids, laden with Pt particles and soot. The particle size, inter-particle distance and the progress of the reaction were obtained by analysis of TEMand SEMmicrographs. These experiments on the microscale enabled the correlation between the examined parameters and the reaction rate.

Mechanical properties of nanoparticle chain aggregates (NCA) including tensile strength and Young's modulus were measured using an instrument incorporating an AFM tip under SEM imaging. The NCA were studied individually and as network films. Carbon NCA were made by laser ablation of graphite, and SnO2 NCA were made by oxidation of a tin compound. The films were deformable and showed elastic behavior. NCA serve as reinforcing fillers in rubber and films of SnO2 NCA for trace gas detection.

The oxidation of soot agglomerates over catalytically active surfaces is of interest for the development of catalytic reactors for the control of soot emissions. The process involves the transport and deposition of nanoparticle aggregates to a surface on which catalyst particles are deposited. To simulate this process, graphitized carbon nanoparticles and platinum nanoparticles were separately deposited on an oxidized silicon wafer by laser ablation and electro hydro dynamic atomization. Changes in particle morphology produced by the reaction were visualized ex situ by scanning electron microscopy. In this way chemical reaction data could be correlated with the local surface coverage and particle size of the catalytically active material and the morphology of the reacting particles, resulting in detailed local information on their interaction, which is not available in studies on bulk samples. The contact between catalyst and soot particles was loose, simulating the behavior of catalyst systems used in practice. The activation energy of the oxidation in air was found to be 40 kJ/mol whereas the activation energy in air/NO was found to be 160 kJ/mol, both in presence of Pt deposited on a SiO2 support. Notwithstanding the higher activation energy, the reaction rate of soot oxidation in air/NO is about two to three orders of magnitude higher than in air. A linear relationship between the relative Pt surface and reaction rate was found for the oxidation in an air/NO atmosphere. In air, the relationship has a minimum which indicates that there are different simultaneous mechanisms of reaction. Although activation energies are different from other studies, the oxidation temperatures are comparable. The EHDA and laser ablation produced platinum catalysts behave similarly and show potential to be used as model catalyst.

As a new scientific discipline, nanoparticle aerosol science and technology (NAST) deals with the formation, properties and behavior of nanoparticles in gases. Driven by its practical applications in many different fields, NAST has been undergoing rapid development. A conceptual framework of the discipline, with its own basic principles, experimental methods and computational techniques, is now taking shape. This paper presents an overview of the current status and research needs of the new discipline. The presentation begins with a discourse on the relationship among various particle systems, which occur frequently in nature and industry. The properties and behavior of nanoparticle aerosols are then discussed, with emphasis on the key roles played by particle size and morphology. Similar to fluid dynamics, NAST is an enabling discipline in the sense that it has provided the concepts and methodology needed for the development of many other fields. Applications of nanoparticle aerosol science and technology are highlighted in three important areas: (1) aerosol processes for synthesis of nanoparticles, (2) atmospheric nanoparticles and global climate, and (3) dosimetry of inhaled nanoparticles. These fields have features in common insofar as nanoparticle aerosols follow the same basic laws of physics and chemistry.

High concentrations of pollution particles, including “soot” or black carbon, exist over the Indian Ocean, but their sources and geographical origins are not well understood. We measured emissions from the combustion of biofuels, used widely in south Asia for cooking, and found that large amounts of carbonaceous aerosols are emitted per kilogram of fuel burnt. We calculate that biofuel combustion is the largest source of black carbon emissions in India, and we suggest that its control is central to climate change mitigation in the south Asian region.

Previous studies in our laboratory have shown that individual nanoparticle chain aggregates (NCAs) exhibit unusual mechanical behaviour when under strain inside the transmission electron microscope. NCAs made of various materials (e.g. carbon, metal oxides and metals) were strained by as much as 100% under tension. The nanoparticles that compose the chains were 5-10 nm in diameter and the chains of the order of 1 mu m in length. Such aggregates are of technological importance in the manufacture of nanocomposite materials (e.g. rubber), aggregate break-up (e.g. sampling diesel emissions) and chemical-mechanical planarization. The goal of this study was to simulate the mechanical behaviour of chain aggregates with morphological properties similar to those of technological interest. Molecular dynamics (MD) and energy minimization computer simulations are employed to investigate, at the atomic scale, the behaviour of short nanoparticle aggregates under strain and to obtain quantitative information on the forces involved in aggregate straining and fracturing. The interaction potential used is that of copper obtained with the embedded atom method (EAM). Two seven-nanoparticle aggregates are studied, one linear and the other kinked. The seven nanoparticles in both aggregates are single crystals and about 2.5 nm in diameter each. The aggregates are strained along their longest dimension, to the breaking point, at strain rates spanning from 2.5 x 10(7) to 8.0 x 10(8) s(-1) (MD simulations). The linear aggregate yield strain is about 0.1. The kinked aggregate elastic limit is also about 0.1, but only one-third of the stress develops along the straining direction compared to the linear aggregate. The kinked aggregate breaks at a strain of about 0.5, five times higher than the breaking strain of the linear aggregate. The ability of the kinked aggregate to straighten through combined nanoparticle interface sliding and rotation accounts for the extra strain accommodation. Simulation strain rates are orders of magnitude higher than the experimental ones. However, aggregate behaviour is independent of strain rates over the range studied here. The MD and energy minimization straining gave very similar results. In the elastic regime, the I,S-11 modulus for the seven-nanoparticle kinked aggregate is about one-fifth of the bulk value. This is due to a combined effect of the small primary particle diameter and the aggregate kinked structure. If this softening behaviour also occurs for nanoparticle aggregates of other materials (e.g. carbon, silica), nanoparticle aggregates, in some cases, may be strained along with the nanocomposite they reinforce.

Aerosol dynamics (AD) is the discipline that deals with changes in particle size distributions in space and time. AD is based on (1) certain fundamental principles embodied in a set of equations, (2) experimental methods and instrumentation and (3) numerical and computational methods. Over the last few decades, AD has emerged as an enabling discipline in the design of aerosol reactors employed in the gas phase synthesis of fine powders, the characterization of particle emissions from sources such as coal-fired power plants and the atmospheric aerosol. The development of basic AD concepts since early in the 20th century is traced to the present. Major gaps that remain in the field and likely advances over the next few years are discussed. Although accurate predictions of particle size from first principles are difficult to make in practical applications, AD principles can be used to explain trends in product properties for flame and laser ablation reactors that operate under very different temperatures and quench rates.

Carbon black in the form of nanoparticle chains is used as a reinforcing filler in elastomers. However, the dynamics of the filler particles under tension and their role in the improvement of the mechanical properties of rubber are not well understood. We have studied experimentally the dynamics of isolated nanoparticle chain aggregates (NCAs) of carbon made by laser ablation, and also that of carbon black embedded in a polymer film. In situ studies of stretching and contraction of such chains in the transmission electron microscope (TEM) were conducted under different maximum values of strain. Stretching causes initially folded NCA to reorganize into a straight, taut configuration. Further stretching leads to either plastic deformation and breakage (at 37.4% strain) or to a partial elastic behavior of the chain at small strains (e.g. 2.3% strain). For all cases the chains were very flexible under tension. Similar reorientation and stretching was observed for carbon black chains embedded in a polymer film. Such flexible and elastic nature of NCAs point towards a possible mechanism of reinforcement of rubber by carbon black fillers.