Aerosol mass spectrometers allow particles to be counted on the basis of size and chemical composition. In most instruments, individual particles are ablated with a pulsed laser to obtain a mass spectrum. Using this method to characterize ambient aerosols requires an understanding of biases induced by the measurement process. For particles less than 200 nm diameter, the efficiency of detection is shown to be dependent on both size and composition. These dependencies arise from the transmission characteristics of the particle inlet and the intrinsic ability of a particle to be vaporized and ionized. The relative contributions of each are determined for a series of atmospherically relevant test aerosols. Small particles are generally more difficult to detect and analyze than large particles because they are more difficult to focus through the inlet into a tight beam and because they are more difficult to ablate. Particles composed of polycyclic aromatic hydrocarbons, ammonium nitrate, and alkali metal ions are efficiently ablated by laser ablation. Aliphatic organics are less efficiently ablated, and ammonium sulfate is very difficult to detect in a positive ion spectrum. The mass spectra of ultrafine particles show extensive fragmentation, making it difficult to distinguish aliphatic and aromatic components. However, organic particles may be classified on the basis of inorganic impurities. Initial field measurements of ambient ultrafine particles are consistent with these conclusions.

Single-particle mass spectrometers are now commonly used to analyze atmospheric particles and generate tens of thousands of spectra from typical measurement campaigns. The ART-2a spectrum algorithm has been used to classify these spectra. In this work, we generate a range of particles that are models of those that are common in the atmosphere. A single-particle mass spectrometer is used to analyze these known particles, and the spectra are classified using ART-2a. The optimum vigilance parameter is approximately 0.5 while the optimum learning rate is approximately 0.05. The classifications elucidate limitations in generation of test particles, their analysis by single-particle techniques, and their classification by ART-2a.

) states that the film pinholes in the evaporated device give rise to the growth of dark spots. However, physical lamination by soft, conformable electrodes has an advantage in that film pinholes will not make contact with the conformable electrodes. Although we did not observe the symptoms of pinholes from the current density at low voltages of our evaporated devices, pinholes could be a source for growth of dark spots in evaporated devices. The ultimate goal of carbon nanotube research is the fabrication of functional macroscopic structures that can fully utilize the individual nanotube properties. Although ordered nanotube assemblies [1,2] are required for making integrated devices, simpler macroscopic constructs that reflect the intrinsic random fiber morphology of the nanotubes offer potential applications such as media for aerosol filtration. Although gas-phase filtration properties of nanotube macrostructures have never been studied, there have been reports on nanotube pore structure [3,4] and gas permeability of nanotube membranes. [5] In this communication, we present the first investigation of the use of multiwalled carbon nanotubes (MWNTs) as highly efficient, airborne particulate filter media. Filter efficiencies in excess of 99 % were achieved from films of MWNTs deposited onto cellulose fiber filters. The MWNT-coated filters exhibited low pressure drops and better filter quality than cellulose filters even for very low MWNT coverages (0.07 mg cm ±2 ). The MWNT filter performance was comparable to the highest efficiency HEPA (high efficiency particulate air) filter standards. Filtration of airborne particulate matter is essential in many instances, including air purifiers, respiratory protection equipment, and clean rooms. Fibrous filters are the most common type of filter media used for such applications, and are usually made from cellulose, glass, or polymer fibers. Fibrous aerosol filters do not work like sieves (that allow only particles smaller than the holes to pass through), rather, the mechanisms of interception of particles by the fiber surface, inertial impaction of a particle on a fiber, and Brownian diffusion of particles in the filter pores are mainly responsible for particle retention in the filter. These three mechanisms each dominate for different particle sizes. Fiber diameter is an important parameter that affects filter performance. Typically filters have fiber diameters on the order of 10 lm, therefore filters made from nanotubes with diameters in the range of 20±50 nm should display unique properties due to their small dimensions. Figure 1 shows scanning electron microscopy (SEM) images of the MWNT-coated filter morphology. We also thank George Karles, Vicki Baliga, Randall Baren, and Prof. P. M. Ajayan (RPI) for valuable assistance and fruitful discussions relating to this work. We are grateful to NanoTechLabs Inc., for the MWNT samples.

No abstract

No abstract

It is known that puffing conditions such as puff volume, duration, and frequency vary substantially among individual smokers. This study investigates how these parameters affect the particle size distribution and concentration of fresh mainstream cigarette smoke (MCS) and how these changes affect the predicted deposition of MCS particles in a model human respiratory tract. Measurements of the particle size distribution made with an electrical low pressure impactor for a variety of puffing conditions are presented. The average flow rate of the puff is found to be the major factor effecting the measured particle size distribution of the MCS. The results of these measurements were then used as input to a deterministic dosimetry model (MPPD) to estimate the changes in the respiratory tract deposition fraction of smoke particles. The MPPD dosimetry model was modified by incorporating mechanisms involved in respiratory tract deposition of MCS: hygroscopic growth, coagulation, evaporation of semivolatiles, and mixing of the smoke with inhaled dilution air. The addition of these mechanisms to MPPD resulted in reasonable agreement between predicted airway deposition and human smoke retention measurements. The modified MPPD model predicts a modest 10% drop in the total deposition efficiency in a model human respiratory tract as the puff flow rate is increased from 1050 to 3100 ml/min, for a 2-s puff.

The critical supersaturations required for the homogeneous nucleation ͑rate of 1 drop cm Ϫ3 s Ϫ1 ͒ of ethylene glycol, propylene glycol, trimethylene glycol and glycerol vapors have been measured over wide temperature ranges ͑e.g., 280-400 K͒ using an upward thermal diffusion cloud chamber. At lower temperatures the experimental nucleation rates are much higher than the predictions of the classical nucleation theory. Glycerol shows the best agreement between experiment and theory in the temperature range of 340-370 K. An apparent increase in the critical supersaturation of glycerol is observed with increasing carrier gas ͑helium͒ pressure and this effect is more pronounced at lower temperatures. The results from corresponding states and scaled nucleation models indicate that the nucleation behavior of glycerol is quite different from other glycols. Glycerol requires higher critical supersaturations compared to the other glycols at the same reduced temperatures. This implies quite small critical clusters for glycerol ͑20-50 molecules͒ in the temperature range 300-380 K. The discrepancy between experiment and theory at lower temperatures may be explained by considering that the surface tension of the critical clusters is lower than the bulk surface tension. It is, however, surprising that a Tolman type correction for the curvature dependent surface tension could be applicable for such small critical clusters. Further theoretical work is required in order to fully understand the observed higher nucleation rates at lower temperatures of glycols and glycerol.

A general solution for the steady-state ion-induced nucleation kinetics has been derived, considering the differences between ion-induced nucleation and homogeneous nucleation. This solution includes a new effect for nucleation kinetics, the interaction of charged clusters with vapor molecules. Analytical expressions for the ion-induced nucleation rate have been obtained for the limiting cases of high and low thermodynamic barriers. The physical explanation of the so-called sign effect is proposed based on multipole expansion of an electric field of the cluster ion. This theory gives good agreement with experiments and is used to elucidate experimentally observed phenomena.