Rectangular Slit Atmospheric Pressure Aerodynamic Lens Aerosol Concentrator

A rectangular slit micro-aerodynamic-lens (μADL) aerosol concentrator operating at atmospheric pressure has been developed. A single stage version has shown concentration ratios of up to 40:1 for 1 μm aerosol particles while particles larger than 2 μm can be concentrated by more than 100:1 in a single stage. The design of this device has been guided by unsteady 3D CFD modeling using detached eddy simulations (DES), and has been validated experimentally using polystyrene spheres and salt crystals of known aerodynamic diameters. The pressure drop in the device does not exceed 1.5 kPa in the major flow and 0.3 kPa in the minor flow at a total flow of 10 slpm.

Copyright 2014 American Association for Aerosol Research     

Comparative performance of a thermal denuder and a catalytic stripper in sampling laboratory and marine exhaust aerosols

The performance of a thermal denuder (thermodenuder—TD) and a fresh catalytic stripper (CS) was assessed by sampling laboratory aerosol, produced by different combinations of sulfuric acid, octacosane, and soot particles, and marine exhaust aerosol produced by a medium-speed marine engine using high sulfur fuels. The intention was to study the efficiency in separating non-volatile particles. No particles could be detected downstream of either device when challenged with neat octacosane particles at high concentration. Both laboratory and marine exhaust aerosol measurements showed that sub-23 nm semi-volatile particles are formed downstream of the thermodenuder when upstream sulfuric acid approached 100 ppbv. Charge measurements revealed that these are formed by re-nucleation rather than incomplete evaporation of upstream aerosol. Sufficient dilution to control upstream sulfates concentration and moderate TD operation temperature (250°C) are both required to eliminate their formation. Use of the CS following an evaporation tube seemed to eliminate the risk for particle re-nucleation, even at a ten-fold higher concentration of semi-volatiles than in case of the TD. Particles detected downstream of the CS due to incomplete evaporation of sulfuric acid and octacosane aerosol, did not exceed 0.01% of upstream concentration. Despite the superior performance of CS in separating non-volatile particles, the TD may still be useful in cases where increased sensitivity over the traditional evaporation tube method is needed and where high sulfur exhaust concentration may fast deplete the catalytic stripper adsorption capacity.

Copyright © 2018 American Association for Aerosol Research     

Effect of gas temperature on the capture of charged particles by oppositely charged water droplets

This work reports experimental results on the effects of temperature (25, 45, and 65°C at different relative humidity) on the scrubbing of charged submicron particles by means of cold (25°C) droplets charged with opposite polarity. The aim of the study is to experiment how the capture of particles is influenced by the simultaneous presence of electrostatic and phoretic forces related to the occurrence of thermal and water vapor gradients close to the droplet surface. This information plays an important role in the development of wet electrostatic scrubbing (WES), an emerging technology for submicron and ultrafine particle capture. Tests were performed in a lab-scale system in which the particle laden-gas was scrubbed by a train of identic droplets. Particles were charged by a corona source while droplets are generated by electrospraying. Experiments revealed that for particles larger than about 250–300 nm, there were higher removal efficiencies in nonisothermal conditions, with limited differences between 45 and 65°C tests. For particles finer than about 150 nm, we sometimes observed lower removal efficiencies for higher gas temperatures, probably due to the difficulties in controlling particle charging for these particles. The experiments were interpreted with a consolidated stochastic model that predicted successfully the data at isothermal conditions, but was less effective for tests at higher gas temperatures. In our opinion, this discrepancy relies on synergies among the fluid dynamic field induced by droplet evaporation/condensation, the phoretic and the electrostatic forces, which are not considered in the model.

Copyright © 2016 American Association for Aerosol Research     

Design and Performance of a Low-Cost Micro-Channel Aerosol Collector

Aerosol sampling and identification is vital for assessment and control of particulate matter pollution, airborne pathogens, allergens and toxins, and their effect on air quality, human health, and climate change. Assays capable of accurate identification and quantification of chemical and biological airborne components of aerosol provide very limited sampling time resolution and relatively dilute samples. A low-cost micro-channel collector (μCC) which offers fine temporal and spatial resolution, high collection efficiency, and delivers highly concentrated samples in very small liquid volumes was developed and tested. The design and optimization of this μCC was guided by computational fluid dynamics (CFD) modeling. Collection efficiency tests of the sampler were performed in a well-mixed aerosol chamber using aerosolized fluorescent microspheres in the 0.5–6 μm diameter range. Samples were collected in the μCC and eluted into 100 μL liquid aliquots; bulk fluorescence measurements were used to determine the performance of the collector. Typical collection efficiencies were above 50% for 0.5 μm particles and 90% for particles larger than 1 μm. The experimental results agreed with the CFD modeling for particles larger than 2 μm, but smaller particles were captured more efficiently than predicted by the CFD modeling. Nondimensional analysis of capture efficiencies showed good agreement for a specific geometry but suggested that the effect of channel curvature needs to be further investigated.

Copyright 2014 American Association for Aerosol Research     

Mass,charge, and radius of droplets in a linear quadrupole electrodynamic balance

Single particle levitation is a key tool in the analysis of the physicochemical properties of aerosol particles. Central to these techniques is the ability to determine the size of the confined particle or droplet, usually achieved via optical methods. While some of these methods are extremely accurate, they are not suitable for all applications and sample types, such as solid or optically absorbing particles. In this work, measurements of the radius, mass, and charge of droplets in a linear quadrupole electrodynamic balance (LQ-EDB) are reported. Using the elastic light scattering pattern produced by laser illumination, a method to determine the radius is described, with an accuracy of as good as ±60?nm and a sensitivity to changes on the order of 10?nm. The effect of refractive index on these measurements is explored by application of the technique to simulated data using Mie theory. In addition to radius, the relative and absolute mass and charge of droplets in the trap is measured from the voltage required to stabilize their vertical position. These measurements are facilitated by stacking multiple droplets in the LQ-EDB and solving the force balance equations to yield both parameters. These approaches are demonstrated through measurements of the evaporation of pure ethylene glycol and pure water droplets, the change in density of an aqueous glycerol solution as water evaporates, and the mass and charge of pure glycerol droplets.

Copyright © 2019 American Association for Aerosol Research     

Preliminary investigation of a water-based method for fast integrating mobility spectrometry

A water-based condensational growth channel was developed for imaging mobility-separated particles within a parallel plate separation channel of the Fast Integrated Mobility Spectrometer (FIMS). Reported are initial tests of that system, in which the alcohol condenser of the FIMS was replaced by a water-based condensational growth channel. Tests with monodispersed sodium chloride aerosol verify that the water-condensational growth maintained the laminar flow, while providing sufficient growth for particle imaging. Particle positions mapped onto particle mobility, in accordance with theoretical expectations. Particles ranging in size from 12 nm to 100 nm were counted with the same efficiency as with a butanol-based ultrafine particle counter, once inlet and line losses were taken into account.

Copyright © 2017 American Association for Aerosol Research     

Water-based single-flow mixing condensation particle counter

A novel water-based condensation particle counter has been developed using a patented, single-flow mixing (SFM) condenser that permits a conventional thermal approach of using a hot saturator followed by a cold condenser to activate and grow particles for counting with an optical detector. A computational fluid dynamics (CFD) model of the internal flow, temperature, and vapor profiles was used to predict the effectiveness of the SFM condenser. Using the results from the CFD model, the counting efficiency was numerically calculated for pure water droplets, and the CPC cut-point (i.e., 50% counting efficiency) was predicted to be 8.3 nm. The experimental performance of the new CPC was measured with differential mobility analyzer-classified, monodisperse particles. The measured cut-points were 8.2 nm for Ag particles and 3.9 nm for NaCl particles. The reduction in the cut-point for NaCl is the result of a compound effect: water uptake by NaCl particles, which increases their size before entering into the growth section (condenser), and the reduction of the equilibrium vapor pressure of water over NaCl-water droplets, resulting in a decrease of the activation diameter.

Copyright © 2016 American Association for Aerosol Research     

Influence of surfactants on growth of individual aqueous coarse mode aerosol particles

Understanding the links between aerosol and cloud and radiative properties remains a large uncertainty in predicting Earth's changing energy budget. Surfactants are observed in ambient atmospheric aerosol particles, and their effect on cloud droplet growth is a mechanism that was, until recently, neglected in model calculations of particle activation and droplet growth. In this study, coarse mode aqueous aerosol particles were created containing the surfactant Igepal CA-630 and NaCl. The evaporation and condensation of these individual aqueous particles were investigated using an aerosol optical trap combined with Raman spectroscopy. For a relative humidity (RH) change from 70% to 80%, droplets containing both Igepal and NaCl at atmospheric concentrations exhibited on average more than 4% larger changes in droplet radii, compared to droplets containing NaCl only. This indicates enhanced water uptake in the presence of surfactants, but this result is unexpected based on the standard calculation of the effect of surfactants, using surface tension reduction and/or hygroscopicity changes, for particles of this size. One implication of these results is that in periods with increasing RH, surfactant-containing aqueous particles may grow larger than similarly sized aqueous NaCl particles without surfactants, thus shifting atmospheric particle size distributions, influencing particle growth, and affecting aerosol loading, visibility, and radiative forcing.

Copyright © 2018 American Association for Aerosol Research     

Modeling the thermodynamics and kinetics of sulfuric acid-dimethylamine-water nanoparticle growth in the CLOUD chamber

Dimethylamine (DMA) has a stabilizing effect on sulfuric acid (SA) clusters, and the SA and DMA molecules and clusters likely play important roles in both aerosol particle formation and growth in the atmosphere. We use the monodisperse particle growth model for acid-base chemistry in nanoparticle growth (MABNAG) together with direct and indirect observations from the CLOUD4 and CLOUD7 experiments in the cosmics leaving outdoor droplets (CLOUD) chamber at CERN to investigate the size and composition evolution of freshly formed particles consisting of SA, DMA, and water as they grow to 20 nm in dry diameter. Hygroscopic growth factors are measured using a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA), which combined with simulations of particle water uptake using the thermodynamic extended-aerosol inorganics model (E-AIM) constrain the chemical composition. MABNAG predicts a particle-phase ratio between DMA and SA molecules of 1.1–1.3 for a 2 nm particle and DMA gas-phase mixing ratios between 3.5 and 80 pptv. These ratios agree well with observations by an atmospheric-pressure interface time-of-flight (APi-TOF) mass spectrometer. Simulations with MABNAG, direct observations of the composition of clusters <2 nm, and indirect observations of the particle composition indicate that the acidity of the nucleated particles decreases as they grow from ～1 to 20 nm. However, MABNAG predicts less acidic particles than suggested by the indirect estimates at 10 nm diameter using the nano-HTDMA measurements, and less acidic particles than observed by a thermal desorption chemical ionization mass spectrometer (TDCIMS) at 10–30 nm. Possible explanations for these discrepancies are discussed.

Copyright © 2016 American Association for Aerosol Research     

Investigation of vehicle exhaust sub-23 nm particle emissions

A solid particle number limit was applied to the European legislation for diesel vehicles in 2011. Extension to gasoline direct injection vehicles raised concerns because many studies found particles below the lower size limit of the method (23 nm). Here we investigated experimentally the feasibility of lowering this size. A nano condensation nucleus counter system (nCNC) (d_50% = 1.3 nm) was used in parallel with condensation particle counters (CPCs) (d_50% = 3 nm, 10 nm and 23 nm) at various sampling systems based on ejector or rotating disk diluters and having thermal pre-treatment systems consisting of evaporation tubes or catalytic strippers. An engine exhaust particle sizer (EEPS) measured the particle size distributions. Depending on the losses and thermal pre-treatment of the sampling system, differences of up to 150% could be seen on the final detected particle concentrations when including the particles smaller than 23 nm in diameter. A volatile artefact as particles with diameters below 10 nm was at times observed during the cold start measurements of a 2-stroke moped. The diesel vehicles equipped with the Diesel Particulate Filter (DPF) had a low solid sub-23 nm particles fraction (<20%), the gasoline with direct injection vehicles had higher (35–50%), the gasoline vehicles with port fuel injection and the two mopeds (two and four-stroke) had the majority of particles below 23 nm. The size distributions peaked at 60–80 nm for the DPF equipped vehicles, at 40–90 nm for the gasoline vehicles with a separate nucleation mode peak at approximately 10 nm sometimes. Mopeds peaked at sizes below 50 nm when their aerosol was thermally pre-treated.

© 2017 American Association for Aerosol Research     

Evaporation rate of particles in the vaporizer of the Aerodyne aerosol mass spectrometer

We present calculations for evaporation rates of particles collected on the vaporizer of the Aerodyne aerosol mass spectrometer (AMS). These calculations provide insight on certain observed phenomena associated with the size-resolved mass spectrum (MS), because the time width of the MS signal from a particle can be limited by its evaporation rate upon contact with the vaporizer. We show that the counterintuitive weak dependence of observed MS signal widths (evaporation rates) on particle volatility is due to suppression of evaporation rates induced by latent heat release, which is more prominent at high volatilities. The same physics is responsible for the observed diminishing returns associated with increasing the vaporizer temperature to achieve narrower single particle pulses. We also show that the vaporizer typical operating temperature of 600°C is sufficient to evaporate extremely low volatility organic compounds (ELVOCs) rapidly enough to obtain reliable measurements for particles smaller than approximately 600 nm. However, the sizing resolution is compromised for large (near-micron) sizes regardless of particle volatility. Finally, our calculations indicate that the observed delayed particle signals, which lead to an artificial tail in AMS mass distributions, are not due to slow evaporation of particles deposited on a surface with lower temperature than the vaporizer, but particles bouncing in the ionizer cage and finally depositing on the vaporizer.

Copyright © 2017 American Association for Aerosol Research     

Mathematical modeling and numerical simulation of surfactant delivery within a physical model of the neonatal trachea for different aerosol characteristics

Surfactant aerosol delivery in conjunction with a noninvasive respiratory support holds the potential to treat neonatal respiratory distress syndrome in a safe manner. The objective of the present study was to gain knowledge in order to optimize the geometry of an intracorporeal inhalation catheter and improve surfactant aerosol delivery effectiveness in neonates. Initially, a mathematical model capable of predicting the aerosol flow generated by this inhalation catheter within a physical model of the neonatal trachea was implemented and validated. Subsequently, a numerical study was performed to analyze the effect of the aerosol liquid droplet size and mass flow rate on surfactant delivery and on the required aerosolization time period. Experimental validation of the mathematical model showed a close prediction of the air axial velocity at the distal end of the physical model, with an absolute error between 0.01 and 0.15 m/s. Furthermore, an admissible absolute error between 0.2 and 2 µm was attained in the prediction of the aerosol mean aerodynamic diameter and mass median aerodynamic diameter in this region. The numerical study highlighted the beneficial effects of generating an intracorporeal aerosol with a mass median aerodynamic diameter higher than 4 µm and a surfactant mass flow rate above 8.93 mg/s in order to obtain effective surfactant delivery in neonates with minimal airway manipulation.

Copyright © 2017 American Association for Aerosol Research     

Aerosol Measurement by Induced Currents

We introduce a new electrical measurement technique for aerosol detection, based on pulsed unipolar charging followed by a non-contact measurement of the rate of change of the aerosol space charge in a Faraday cage. This technique, which we call “aerosol measurement with induced currents,” has some advantages compared to the traditional method of collecting the charged particles on either an electrode or with a particle filter. We describe the method and illustrate it with a simple and miniature (shirt-pocket-sized) instrument to measure lung-deposited surface area. Aerosol measurement by induced currents can also be applied to more complex devices.

Copyright 2014 American Association for Aerosol Research     

Comparison of Particle Number Counts Measured with an Ink Jet Aerosol Generator and an Aerodynamic Particle Sizer

Aerodynamic particle sizer (APS) users typically calibrate the particle sizing capabilities, but not the counting efficiency upon which aerosol concentration results are based. Herein, comparisons were made between the counts provided by an ink jet aerosol generator (IJAG) with those measured by an APS. Near-monodisperse (geometric standard deviation of about 1.06) liquid or solid aerosols in the size range of 0.95 to 13.3 μm aerodynamic diameter (AD) generated with an IJAG were released into the inner inlet-tube of the APS in a manner that rendered APS wall and aspiration losses negligible. For most experiments, the IJAG generated 75 particles/s, which rate was maintained by the IJAG system through control of electrical pulses applied to its ink jet cartridge. For particles in the size range of 2–13.3 μm AD, the ratio of relative detection efficiency (ratio of the number of particles counted by the APS to the number reported as generated by the IJAG) was 99.3 ± 1.4%; however, for test particles between 0.95 and 2 μm AD, the relative detection efficiency was somewhat lower, but the drop off was less than about 2%. This slight drop off is likely associated with the light scattering detection approach and corresponding counting algorithm of the APS. Tests were conducted where the IJAG produced 7.0 μm AD particles at rates of 1 to 500 s^-1 and the results showed essentially a 1:1 correspondence between IJAG and APS counts. The presence of smaller-sized background particles did not affect the measured APS counts of larger-sized challenge particles.

Copyright 2014 American Association for Aerosol Research     

Effect of Aerosol Volatility on the Sizing Accuracy of Differential Mobility Analyzers

Differential mobility analyzers (DMAs) are widely used for calibrating other instruments and measuring aerosol size distributions. DMAs classify aerosol particles according to their electrical mobility, which is assumed to be constant during the classification process. However, particles containing semivolatile substances can change their size in the DMA, leading to sizing errors. In this article, the effect of particle size changes during the classification process on the sizing accuracy of DMAs is discussed. It is shown that DMAs select particles whose time-of-flight-averaged electrical mobility is equal to that of stable particles that are selected under given operating conditions. For evaporating particles, this implies that DMAs select particles that are originally larger than the reported size. At the exit of the DMA, selected particles are smaller than the reported size. Particle evaporation and growth inside DMAs was modeled to study the effect of particle size changes on the sizing accuracy and the transfer function of DMAs in constant- and scanning-voltage modes of operation. Modeling predictions were found to agree well with the results of experiments with ammonium nitrate aerosol. The model was used to estimate sizing errors when measuring hygroscopic and other volatile aerosols. Errors were found to be larger at smaller sizes and low sheath flow rates. Errors, however, are fairly small when saturation concentration is below 10 μg/m³, assuming an evaporation coefficient of 0.1. Particles size changes during classification lead to distortion of the DMA transfer function. In voltage scanning mode, errors are generally larger, especially at high scan rates.

Copyright 2014 American Association for Aerosol Research     

Anisokinetic Shrouded Nozzle System for Constant Low-Flow Rate Aerosol Sampling from Turbulent Duct Flow

An anisokinetic shrouded nozzle system was designed for sampling particles at a constant low flow rate from a ventilation duct to an aerodynamic particle sizer (APS). Shrouded anisokinetic nozzles are a means for sampling from a moving airstream with higher particle transmission than with unshrouded isokinetic nozzles. This shrouded nozzle sampling system was evaluated in an experimental ventilation duct system. Aspiration and transport efficiency measurements were made for five particle sizes in the range 1–13 μm at each of three duct air speeds in the range 2.2–8.8 m/s. Under these conditions, the shrouded nozzle system showed improved performance compared to buttonhook isokinetic nozzles, especially for larger particles and higher air speeds. Measured transmission efficiencies through the shrouded nozzle sampling system were generally higher and more reliably predictable than those through buttonhook isokinetic nozzles. Model predictions of transport and aspiration efficiencies of the shrouded nozzle system showed good agreement with measurements over the entire range of experimental conditions. The shrouded nozzle sampling system could be used to measure concentrations in ventilation ducts with an APS for particles in the diameter range 1–13 μm.

Copyright 2014 American Association for Aerosol Research     

Air Entrainment During Flame Aerosol Synthesis of Nanoparticles

Enclosed flames typically produce substantially larger particles than open flames under identical reactant flows and composition. The enclosure hinders air entrainment to the flame and reduces heat losses by radiation and convection, facilitating particle coagulation and coalescence. Here the effect of natural air entrainment on flame aerosol synthesis is investigated by lifting off the enclosing tube from the burner surface and utilizing tracer gas (Ne) analysis after calibration with forced air entrainment. That way the effect of air entrainment on product primary particle diameter and mobility size distribution dynamics is investigated by microscopy, scanning mobility particle sizing, and N₂ adsorption, while temperature is measured by Fourier-transform infrared spectroscopy. So air entrainment during flame spray pyrolysis is examined here for its versatility in scalable manufacture of an array of material compositions, while copper oxide (CuO) is used for its electro-chemical applications (e.g., battery electrodes). It is shown that natural air entrainment facilitates rapid gas-to-particle conversion and high process yields by minimizing vortex recirculation and particle deposition on the enclosing tube walls and burner surface. For example, the average primary particle diameter of CuO can be controlled from 42 to 10 nm and the yield from 40 to 90% by gradually lifting off the enclosing tube, resulting in up to 250 L/min natural air entrainment at the present CuO synthesis conditions.

Copyright 2014 American Association for Aerosol Research