Characterization and Response Model of the PPS-M Aerosol Sensor

The Pegasor PPS-M sensor is an electrical aerosol sensor based on diffusion charging and current measurement without particle collection. In this study, the role and effect of each component in the instrument is discussed shortly and the results from a thorough calibration measurements are presented. A comprehensive response model for the operation of the PPS-M sensor was developed based on the calibration results and computational fluid dynamics (CFD) modeling results. The obtained response model, covering the effects of the particle charger, the mobility analyzer, and both diffusion and inertial losses, was tested in the laboratory measurements with polydisperse test aerosols, where a good correlation between the model and the measured results was found.

Copyright 2014 American Association for Aerosol Research     

Determination of Arbitrary Moments of Aerosol Size Distributions from Measurements with a Differential Mobility Analyzer

A method to determine arbitrary moments of aerosol size distributions from differential mobility analyzer measurements has been proposed. The proposed method is based on a modification of the algorithm developed by Knutson and Whitby to calculate the moments of electrical mobility distributions. For this modification, the electrical mobility and the charge distribution have been approximately expressed by power functions of the particle diameter. To evaluate the validity of the approximation, we have carried out numerical simulations for typical size distributions. We have found that for typical narrowly distributed aerosols such as polystyrene latex particles and particles that arise in the tandem differential mobility analyzer configuration, the distribution parameters can be accurately determined by this method. For a log-normally distributed aerosol, the accuracy of the distribution parameters determined by this method has been evaluated as a function of the geometric standard deviation. We have also compared the accuracy of the proposed method with other existing methods in the case of the asymmetric Gaussian distribution.     

Characterization of an Ambient Coarse Particle Concentrator Used for Human Exposure Studies: Aerosol Size Distributions,Chemical Composition,and Concentration Enrichment

The goals of the experiments described herein involve determining in real time the size, concentration enrichment, and chemical composition of coarse-mode (<2.5 μm) and fine-mode (>2.5 μm) particles within the nonconcentrated and concentrated flows of a coarse particle concentrator used for human exposure studies. The coarse particle concentrator was intended to concentrate ambient particles in the PM_10–2.5 size range before sending them into a human exposure chamber. The aerodynamic size and chemical composition of particles in the upstream and downstream flows of the concentrator were monitored with an aerosol time-of-f1ight mass spectrometer (ATOFMS) for fixed time intervals over the course of three days. Based on the ATOFMS results, it was found that there was no change in the composition of the ten major particle types observed in the upstream and downstream flows of the concentrator under normal operating conditions. Furthermore, no new particle types were detected downstream that were not detected upstream of the concentrator. A characterization of the aerosol chemical composition and its dependence on sampling conditions is also discussed. Aerosol size distributions were measured with three aerodynamic particle-sizing (APS) instruments sampling simultaneously from different regions of the concentrator. The APS size distributions were used to scale ATOFMS data and measure the ambient concentration factors for the coarse particle concentrator and the exposure chamber. The average concentration factor (ratio of inlet number concentration to the outlet number concentration) for the particle concentrator was 60 + 17 for the 2.5–7.2 μm size range before dilution and transport to the exposure chamber. It was observed that not only were coarse particles being concentrated but fine (<2.5 μm) particles were being concentrated as well, with concentration factors ranging from 2–46 for aerodynamic particle sizes from 0.54–2.5 μm.     

Development and Characterization of a Fast-Stepping/Scanning Thermodenuder for Chemically-Resolved Aerosol Volatility Measurements

Numerous sources of liquid aerosols are to be found in industrial environments. Such aerosols may, for instance, be cutting fluids, pesticides, etc., that are harmful or even toxic to humans. To control and reduce worker exposure to potentially toxic aerosols, these latter are usually filtered through fibrous filters. When non-saturated air traverses a clogged filter, however, the drops deposited on the fibers may evaporate. Consequently, workers are exposed to greater amounts of more concentrated vapors than the initial state of the filtered aerosol. Furthermore, exposure readings are distorted by an artifact that may be significant. This study offers an experimental approach to long-term monitoring of the evaporation of a semi-volatile n-hexadecane liquid aerosol deposited on filters of varying efficiency. Results were modeled using two semi-empirical models for identifying the basic parameters of liquid aerosol evaporation on fibers. For the first time ever it has been demonstrated that the Fick's first law, as previously suggested by models proposed in the literature, does not control evaporation kinetic.     

Development of an Aerosol Focusing-Laser Induced Breakdown Spectroscopy (Aerosol Focusing-LIBS) for Determination of Fine and Ultrafine Metal Aerosols

An Aerosol Focusing-Laser Induced Breakdown Spectroscopy (Aerosol Focusing-LIBS) with a sheath air focusing and an aerodynamic lens focusing was developed to determine elemental composition of fine and ultrafine metal aerosols. Data showed that with a sheath air focusing, the LIBS qualitatively detected various metals (Al, Ca, Cd, Cr, Cu, K, Mg, Na, Ni, Zn) in submicrometer to micrometer aerosols, but that detection of ultrafine particles smaller than 100 nm was not successful due to weak intensity of emitted light. Also, the hitting rate was so low for particles at low number concentration and the single particle detection approach was only valid when aerosol loading is low. Thus, we concentrated aerosols on to a collection substrate by using the aerodynamic lens focusing system, resulting in the strong emission light from the generated plasma even for nanoparticles and the better quantification performance by the LIBS. We found the linear relationship between LIBS signal response and metal mass concentration. For example, as Cu metal concentration increased, peak area of LIBS emission line for Cu increased. The resulting correlation coefficient was 0.94 and the LOD for Cu mass concentration was found to be ～80 ng/m3, which can be further lowered by extending current collection time (～5 min). A similar linear relationship was found for Cd and Ni ultrafine metal aerosols. We also successfully detected internally mixed metal aerosols. When particles were collected on a substrate with the aerodynamic lens for 5 min prior to analysis of the deposit it was possible to analyze particles as small as 60 nm.     

Design and Characterization of a Fluidized Bed Aerosol Generator: A Source for Dry,Submicrometer Aerosol

A fluidized bed aerosol generator has been designed and built for the purpose of generating a constant output of dry, submicrometer particles with a large number density. The output of the fluidized bed for generating aerosol particles from dry soot powder has been characterized using a differential mobility analyzer and a condensation particle counter. The particle size distribution is bimodal, with a mode in the submicrometer diameter size range and a mode in the supermicrometer diameter size range. The larger diameter mode is fully separated from the smaller mode and can thus be easily removed from the aerosol flow using impaction techniques. The distribution in the submicrometer size range is nearly log-normal, with a count median diameter falling between 0.1 and 0.3 micrometers. A number density of greater than 10⁵ particles cm^-3 of soot particles in the submicrometer range can be produced, constant to within 25% (1 standard deviation) over a 4 h time period. The number density of particles produced in the submicrometer range was found to vary with the ratio of soot to bronze beads in the bed mixture, whether or not a feed system was used, and nitrogen flow rate through the fluidized bed and feed system.     

Measurement of Atlanta Aerosol Size Distributions: Observations of Ultrafine Particle Events

As part of EPRI's Aerosol Research Inhalation Epidemiology Study (ARIES), measurements of aerosol size distributions in the 3 nm to 2     

The Nonequilibrium Character of the Aerosol Charge Distributions Produced by Neutralizes

The mechanisms that control the polar ion concentrations downstream of an ionized region are examined and it is shown that the ratio of positive to negative ion concentrations is not constant. The imbalance in the ionic concentrations caused by unequal diffusion of ions to the walls and to aerosol particles is magnified in the ion ratio as ionic recombination rapidly depletes ions of both polarities equally. Consequently, the aerosol charge distribution is not in equilibrium but is evolving in response to the changing ion environment. The conclusions drawn are supported by numerical modeling and by measurements of ionic concentrations and ratios of negatively to positively charged particles downstream of the ionized region. Several existing neutralizers are evaluated and a prototype ionizer which produces an aerosol with a nearly symmetric equilibrium charge distribution is discussed.     

Distortion of Size Distributions by Condensation and Evaporation in Aerosol Instruments

Aerosols that contain volatile species or condensable vapors may be altered by changes in temperature, pressure, and vapor concentration. When such changes occur within aerosol sampling instruments, the measured size distribution can be distorted significantly. The distortion of particle size distributions in a number of commonly used aerosol instruments, including cascade impactors, both conventional and low pressure instruments, and optical particle counters, is explored both theoretically and experimentally in this paper. Ammonium sulfate aerosols in humid atmospheres have been used to test the instruments. In a low pressure impactor in which the pressure is intentionally reduced to facilitate the collection of small particles, a water containing particle may shrink due to evaporation as the pressure is reduced. However, if the sample flow is also accelerated to high velocities, aerodynamic cooling can lead to condensation of water vapor and particle growth. Either of these competing effects may lead to erroneous estimates of the particle size distribution. Optical particle counters generally use a recirculated sheath airflow. Pumps and electrical dissipation heat this air, leading to a temperature increase that shifts the vapor equilibrium, causing a decrease in particle size due to evaporation. Modifications have been made to avoid this distortion in measured size distributions.     

Bipolar Charge Distributions of Aerosol Particles in High-Purity Argon and Nitrogen

The bipolar diffusion charging has been studied for monodisperse sodium chloride and silver particles of 5–100 nm in atmospheric air, argon and nitrogen. The particles were bipolarly charged in a neutralizer by ions, produced by beta-rays from a Kr 85 source. The differently charged particle fractions were separated in a differential mobility analyzer and measured with an aerosol electrometer. The experimentally determined results in atmospheeric air are comparable with earlier measured asymmetric bipolar charge distributions. They show good agreement with the theoretically determined results based on the extended Fuchs model with four-input ion parameters: mobilities and masses of positive and negative ions. The experimentally determined bipolar charge distributions in argon and nitrogen are more asymmetric than in atmospheric air. The theoretically determined distributions, based on the extended Fuchs model, can be fitted to the experimental data. Furthermore, the extended Fuchs model is strongly dependent upon variations of the four input ion parameters. No differences for the experimentally determined bipolar charge distribution could be found between different gas purities and different particle materials.     

Investigation of the Influence of Post-Injection on Diesel Exhaust Aerosol Particle Size Distributions

The effect of double-pulse fuel split injection on the exhaust aerosol particle size distribution emitted by a state-of-the-art Heavy Duty Diesel engine was experimentally investigated. The influence of post-injection fuel quantity and dwell was evaluated at four steady-state engine conditions by analyzing the changes in the accumulation mode particle number, mean diameter and geometric standard deviation, with respect to the baseline case of single injection. Generally, rising post-injection fuel amount was found to increase accumulation mode particle number and mean diameter of the size distributions. Particle number reduction efficiency resulted dependent on the operating conditions and post-injection parameters scheme. Reductions in the particle number around 50% with respect to the baseline single-pulse injection case were observed for 1500 rpm partial load cases. For 1800 rpm 75% load conditions the reduction was lower and was achieved only when the fuel amount was below 20% of the total fuel injected. An increase in the particle number was produced when post-injection was applied at low speed and high load conditions, due to the low accumulation mode particle number emitted at this operation mode. An optimum post-injection fuel amount for particle number reduction was only seen for the 1500 rpm 75% load operation conditions. The results are indicative that further research is necessary in order to explore the existence of optimum post-injection schemes for particle number reduction at the other operating conditions studied.

Geometric standard deviation from post-injection results was higher than that corresponding to the single injection baseline cases, while an increase in the σ _g value was observed for larger dwells in some operating conditions.     

Characterization of an Aerodyne Aerosol Mass Spectrometer (AMS): Intercomparison with Other Aerosol Instruments

The Aerodyne Aerosol Mass Spectrometer (AMS) provides size-resolved chemical composition of non-refractory (vaporized at 600°C under vacuum) submicron aerosols with a time resolution of the order of minutes. Ambient measurements were performed in Tokyo between February 2003 and February 2004. We present intercomparisons of the AMS with a Particle-Into-Liquid Sampler combined with an Ion Chromatography analyzer (PILS-IC) and a Sunset Laboratory semi-continuous thermal-optical carbon analyzer. The temperature of the AMS inlet manifold was maintained at > 10 ^? C above the ambient dew point to dry particles in the sample air (relative humidity (RH) in the inlet < 53%). Assuming a particle collection efficiency of 0.5 for the AMS, the mass concentrations of inorganic species (nitrate, sulfate, chloride, and ammonium) measured by the AMS agree with those measured by the PILS-IC to within 26%. The mass concentrations of organic compounds measured by the AMS correlate well with organic carbon (OC) mass measured by the Sunset Laboratory carbon analyzer (r ² = 0.67–0.83). Assuming the same collection efficiency of 0.5 for the AMS organics, the linear regression slope is found to be 1.8 in summer and 1.6 in fall. These values are consistent with expected ratios of organic matter (OM) to OC in urban air.     

Determination of Volatility Distributions of Primary Organic Aerosol Emissions from Internal Combustion Engines Using Thermal Desorption Gas Chromatography Mass Spectrometry

A new technique for measuring the primary organic aerosol (POA) emissions from internal combustion engines is presented. The method combines thermal-optical OC/EC analysis and thermal desorption gas chromatography mass spectrometry (TD-GC-MS) of quartz filter samples collected using a dilution sampler to quantify the total emissions of low-volatility organics and to distribute them across the volatility basis set. These data can be used in conjunction with partitioning theory to predict the gas-particle partitioning and thus the total amount of POA over the entire range of atmospheric conditions. The approach is evaluated using POA emissions data from two gas-turbine engines and one diesel generator. To evaluate the new method, we directly measured the effects of temperature and concentration on gas-particle partitioning of the emissions from each. Predictions based on the volatility distributions derived from the filter analyses are consistent with the direct partitioning measurements. The new approach represents a major improvement over the traditional assumption of nonvolatile POA emissions, which over predicts actual POA emissions from these sources by a factor of 2–4 at typical ambient concentration and temperature. By using quartz filter samples, this new technique is designed to be applied to routine source test data. Volatility distributions derived using this new approach can also be applied directly to the large catalog of quartz filter data used by existing emission inventories and models. The emissions data derived from this approach are designed for use in the next generation of chemical transport models and emissions inventories that employ the volatility basis set approach to explicitly track the gas-particle partitioning of POA emissions.

Copyright 2012 American Association for Aerosol Research     

Comparison of a Quadrupole and a Time-of-Flight Aerosol Mass Spectrometer during the Feldberg Aerosol Characterization Experiment 2004

The Feldberg Aerosol Characterization Experiment (FACE-2004) took place from July 13–August 4, 2004 at the Taunus Observatory on the “Kleiner Feldberg” (825 m a.m.s.l.) in Central Germany. The experiment included (amongst others) size-resolved chemical characterization of non-refractory aerosol components. One of the experiment's objectives was to better understand and to characterize recently developed aerosol measurement instrumentation by intercomparison with other co-located instruments. One of these instruments was the Aerodyne Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS).

Here we compare the datasets obtained by the ToF-AMS with those obtained by the well-characterized co-located Quadrupole Aerosol Mass Spectrometer (Q-AMS). A good agreement between the recently developed ToF-AMS with the established Q-AMS is reported here for all species measured with the two instruments for a time period where both instruments operated under well-calibrated conditions. During measurements with reduced detector gain after a pump failure changed species concentrations were measured with the ToF-AMS that did not agree with those measured with the Q-AMS. These changes were different for the individual species and could be attributed to the influence of the ion detection threshold as was shown by model calculations.

For efficient and user-friendly processing of ToF-AMS raw data a data processing software package was developed. Since this is the first time this software was used for field data, it is described in some detail here.     

The Los Angeles Aerosol Characterization and Source Apportionment Study: An Overview

The Los Angeles Aerosol Characterization and Source Apportionment Study (LA-ACSAS) is a decade-long laboratory and field study undertaken by the Southern California Edison Company (SCE) to characterize the chemical composition of fine (d _p < 3.5 μm) particles, PM_3,5, in the Los Angeles Basin, in terms of sulfate, nitrate, organic compounds and trace metals and apportion their sources. This paper provides an overview of the study results and summarizes the important conclusions regarding the composition and sources of PMJS in the Los Angeles Basin, especially regarding the organic fraction.     

Joint Method for the Experimental and Computational Characterization of Ammonia Absorption Columns

Well-designed absorption columns are indispensable to industry. A prototypical system is the absorption of gaseous NH₃. This experiment employed the method of transfer units where collection of vapor (V) and liquid (L) flow rates and concentrations enabled the characterization of the number of transfer units (NTU), height of transfer unit (HTU), and overall gas phase mass transfer coefficient (K_y). A nonmonotonic relationship between V, L, NTU, and HTU values was found; K_y showed a monotonic relationship with V and L. Computer-simulated, correlation-based, and experimental NTU, HTU, and K_y values demonstrated close numerical similarities. This experiment highlighted the importance of a combined empirical, theoretical, and simulation approach in achieving more reliable results from absorption column analysis.     

Development and Preliminary Validation of a Modal Aerosol Model for Tropospheric Chemistry: MAM

This paper presents a modal aerosol model (MAM) developed to be used in three dimensional air quality models. MAM, which represents the aerosol distribution with four modes, has the advantage of simplicity and speed efficiency associated to modal models, while mass transfer is modeled with a dynamic approach. To assess the ability of MAM to represent mass transfer, MAM is compared to a size-resolved model based on the dynamic approach and to a version of MAM based on an equilibrium approach. Comparisons are done using measurements of inorganic species made in Japan as initial conditions. Furthermore, it is shown that MAM combined with a well chosen mode splitting scheme is able to deal accurately with the simultaneous occurrence of strong nucleation/condensation and coagulation, as may be observed in high nucleation episodes.     

Determination of the Molecular Weight Distributions of Xylanase-Treated Pulps

The molecular weight distributions (MWD) of unbleached kraft pulps of Pinus radiata and Populus tremuloides were determined by size exclusion chromatography (SEC) of the carbanilated derivatives. The applicability of this method for the study of the effects of xylanase treatments on these substrates is discussed.     

A Dilution Stack Sampler for Collection of Organic Aerosol Emissions: Design,Characterization and Field Tests

A dilution stack sampler specifically intended to collect fine organic aerosol from combustion sources while minimizing sample contamination has been designed and tested. The sampler simulates the cooling and dilution processes that occur in the plume downwind of a combustion source, so that the organic compounds which condense under ambient conditions will be collected as particulate matter. The special features of this sampler are described in detail, and compared with other stack sampling systems. The results of both laboratory and field tests of the system are discussed. Collection of organic aerosol using this sampler is compared with collection using EPA Method 5.     

Method for Identifying and Mapping Flaw Size Distributions on Glass Surfaces for Predicting Mechanical Response

The statistical and critical tensile stresses associated with crack initiation on glass surfaces are dependent on the size and location of pre-existing flaws. The introduction, sizes, concentrations, and distribution of those pre-existing flaws at any moment of time are a direct and cumulative consequence of any glass's manufacturing, packaging, handling, and service histories. A new, nondestructive “High Resolution Flaw Classification System” is under development that rapidly identifies, measures, and maps surface-located flaws on glass. Flaws smaller than 8 × 8 μm are detectable and many square centimeters per second can be scanned. The potential mechanical response of that glass, with that quantified surface-flaw state at that moment of time, can then be predicted using the classical Griffith criterion.