Real-Time Aerosol Density Determination Utilizing a Modified Scanning Mobility Particle Sizer—Aerosol Particle Mass Analyzer System

Real time secondary organic aerosol (SOA) density evolution for m-xylene photo-oxidation and α-pinene ozonolysis was obtained using an Aerosol Particle Mass Analyzer (APM)/Scanning Mobility Particle Spectrometer (SMPS) setup, which has been modified to achieve higher transmission of particles and improved sampling frequency. The aerosol density of SOA generated from α-pinene ozonolysis was found to be 1.24 ± 0.03 g/cm³ while the aerosol generated from m-xylene photo-oxidation was determined to be 1.35 ± 0.03 g/cm³. These results confirm the measurement approach from a combined SMPS and Aerodyne Aerosol Mass Spectrometer (AMS) system and are found to be within good agreement with the effective density measurements.     

Contribution of Selected Dicarboxylic and ω-Oxocarboxylic Acids in Ambient Aerosol to the m/z 44 Signal of an Aerodyne Aerosol Mass Spectrometer

The Aerodyne aerosol mass spectrometer (AMS) employs flash vaporization (600°C) followed by 70-eV electron impact ionization (EI) to detect organic and inorganic aerosols. The signal at mass-to-charge ratio (m/z) 44 (mainly CO ₂ ⁺ ) is considered the most reliable marker of oxygenated organic aerosol. This study is the first to evaluate the contribution of selected low molecular weight dicarboxylic acids (diacids) and ω-oxocarboxylic acids (ω-oxoacids) to the particle-phase m/z 44 signal of the AMS mass spectrum. Ambient measurements were conducted at a surface site in Tokyo (35°39 ′ N, 139°40 ′ E) during August 3–8, 2003. Diacids and ω-oxoacids were measured using a filter sampling followed by extraction, derivation, and gas chromatograph-flame ionization detector (GC-FID) analysis. The mass concentrations of diacids and ω-oxoacids show tight correlation with the m/z 44 signal (r ² = 0.85–0.94) during the measurement period. Laboratory experiments were also performed to determine the fragment patterns of selected diacids (C₂–C₆ diacids and phthalic acids) and ω-oxoacid (glyoxylic acid) in ambient aerosols. Here, we report for the first time that the selected organic acids could account for 14 ± 5% of the observed m/z 44 signal on average during the measurement period. Oxalic acid (C₂) is the largest contributor, accounting for 10 ± 4% of the observed m/z 44 signal. These results would be useful for interpreting the m/z 44 signals obtained from ambient measurements in various locations.     

Letter to the Editor: On “Inversion” of Aerosol Size Distribution Data

Please click here to find the Original Article to which this Letter refers: http://dx.doi.org/10.1080/02786828208958576     

Analytical–Numerical Solution of the Multicomponent Aerosol General Dynamic Equation—with Coagulation

ABSTRACT

A numerical procedure based on an analytical solution is presented for solution of the full multicomponent aerosol general dynamic equation. The analytical solution for the equation, accounting for growth, removal, and particle sources, is employed in an iterative procedure to account for coagulation. The iterative process is shown to be rapidly convergent, and its performance is validated by comparison with the exact solution for pure coagulation of a single-component aerosol. A simulation is presented of the evolution of a multicomponent coagulating aerosol, where each component grows-evaporates at a different rate.     

On Markowski's Article “Improving Twomey's Algorithm for Inversion of Aerosol Measurement Data”

Please click here to find the Original Article to which this Letter refers: http://dx.doi.org/10.1080/02786828708959153     

Creating Conductive Copper–Silver Bimetallic Nanostructured Coatings Using a High Temperature Reducing Jet Aerosol Reactor

We report production of bimetallic nanostructured copper– silver coatings by in situ deposition and sintering of metal nanoparticles produced as an aerosol. The metal nanoparticles themselves have potential applications in printed electronics, catalysis, antibacterial coatings, and heat transfer fluids. In many applications, nanoparticles are dispersed in an ink, which is then printed or coated onto a substrate and converted into a nanostructured thin film. Direct deposition from the aerosol allows us to produce nanostructured thin films without first dispersing the particles in a solvent. The high temperature reducing jet process allows formation of these metal nanoparticles from low-cost metal salt precursors in the gas phase. In this method, a fuel-rich hydrogen flame provides a low-cost source of energy to drive nanoparticle formation in a reducing environment. The aqueous precursor solution is delivered into the hot combustion product gases within a converging–diverging nozzle. The high-speed gas flow atomizes the precursor and provides exceptionally rapid mixing of the precursor with the hot gases. Here, particles are formed, then immediately quenched and deposited on a glass substrate. The effect of the silver content of the mixed copper–silver films on their electrical conductivity was studied systematically, revealing an abrupt transition from low conductivity to high conductivity between 30 wt.% and 40 wt.% silver.

Copyright 2013 American Association for Aerosol Research     

Ion–Aerosol Flux Coefficients and the Steady-State Charge Distribution of Aerosols in a Bipolar Ion Environment

Fuchs’ theory, as corrected by Hoppel and Frick, is widely used to compute flux coefficients of ions to aerosol particles and the resultant charge distribution. We have identified approximations made in previous works that limit the theory's accuracy. Hoppel and Frick used two characteristic speeds or kinetic energies to calculate the flux coefficients of ions to aerosol particles in lieu of an average of the flux coefficients over the Maxwell–Boltzmann distribution of ion speeds. In the present work, we show that this approximation artificially reduces the number of multiply charged particles. Ion capture may be enhanced by three-body trapping, a process wherein an ion has a collision with a neutral gas molecule and loses sufficient kinetic energy to be captured by the particle. The gas kinetic theory approach to three-body trapping has been refined to better account for the collision between the ion and a neutral gas molecule within the potential presented by the particle. Approximations to the calculation of energy losses and the probability of ion capture have been relaxed. The possibility that an image charge may be induced on the ion as well as on the particle is allowed. While the previous work was limited to electrically conductive particles, both the ion and the particle are allowed to have any dielectric constant in the present work, and the finite size of the ions is taken into account when calculating minimum capture radii for the ion–particle interactions. The resulting ion flux coefficients differ from previous results both in the low nanometer regime and in the continuum regime. We explore the influence of key parameters on the charge distribution, including dielectric constant, temperature, and pressure, to understand how operating conditions may affect the interpretation of differential mobility analyzer measurements of particle size distributions. Finally, an empirical expression for the new charge distribution is given to facilitate rapid calculations.

© 2013 American Association for Aerosol Research     

Size Distributions of 0.5 to 20 μm Aerodynamic Diameter Lead-Containing Particles from Aerosol Sampler Walls and Filters

The study presented here investigates the number weighted particle size distributions of aerosols generated in the laboratory from lead oxide and lead sulfide dusts and sampled by Institute of Occupational Medicine (IOM) and closed face cassette (CFC) samplers as determined by scanning electron microscopy (SEM). The wall deposits and filter deposits from each sampler were characterized separately. A Mann-Whitney statistical analysis revealed that differences in the number weighted distributions of particles captured by the filter and the wall were not significant over the size range (up to 20 μm aerodynamic equivalent diameter) present in these laboratory-generated aerosols. Furthermore, for these samples it was not possible to distinguish an absolute difference between the IOM and CFC filter catches. By comparing direct measurements of aerodynamic equivalent diameter (AED) made by an Aerodynamic Particle Sizer (APS) to AEDs calculated from SEM images, empirical shape factors for lead oxide and lead sulfide were determined. To validate this approach APS and SEM measurements of the AED of 2 μm and 6 μm physical diameter monodisperse glass and polystyrene microspheres were made. Using the shape factors of spheres and the known densities of these materials, it was found that the SEM determinations of AED agreed with the APS results. To demonstrate the reliability of the redeposition method of sample preparation, lead sulfide and lead oxide aerosols were briefly sampled by IOM samplers such that sufficient particles were collected for SEM examination directly on the filter but not so many that particles were likely to touch or overlap. Half of each filter was analyzed in the SEM directly; the other half was ultrasonically removed and re-deposited for analysis by SEM. There were no statistically significant differences in their number weighted size distributions, demonstrating that the sample treatment process does not change the size distribution of these particular aerosols.     

Aerosol Collection Efficiency of a Graded Metal-Fiber Filter at High Airflow Velocity (10 m s–1)

A non-woven graded metal-fiber filter was characterized for its collection efficiency as a function of particle size at an airflow velocity of 10 m s ^?1 . Efficiencies were determined by generating a monodisperse aerosol of polystyrene latex (PSL) spheres and oleic acid liquid particles of measured concentration with and without the filter present. For PSL spheres ≤0.7 μm, the collection efficiencies follow theoretical single-fiber collision efficiency (Ptak and Jaroszczyk 1990 Ptak, T. and Jaroszczyk, T. 1990. “Theoretical-Experimental Aerosol Filtration Model for Fibrous Filters at Intermediate Reynolds Numbers”. In Proceedings 5th World Filtration Congress, 566–572. France: Nice.  [Google Scholar]). Above 0.7 μm diameter (Stokes number 0.8), PSL collection efficiency exhibits a general decreasing trend with increasing particle size suggesting particle bounce. Although this trend is consistent with measured single-fiber collection (collision and adhesion) efficiency for Stokes numbers in the range of 0.2 to 22 (Rembor et al. 1999 Rembor, H.-J., Maus, R. and Umhauer, H. 1999. Measurements of Single Fibre Efficiencies at Critical Values of the Stokes Number. Part. Part. Syst. Charact, 16: 54–59. [Crossref], [Web of Science ®] , [Google Scholar]; Ptak and Jaroszczyk 1990 Ptak, T. and Jaroszczyk, T. 1990. “Theoretical-Experimental Aerosol Filtration Model for Fibrous Filters at Intermediate Reynolds Numbers”. In Proceedings 5th World Filtration Congress, 566–572. France: Nice.  [Google Scholar]), the magnitude of the filter efficiency is less. For 1 μm to 4 μm PSL, the average collection efficiency is 0.43 (expanded uncertainty (U) of 0.08, n = 6). In the case of particles having very different surface adhesion, e.g., oleic acid (sticky) aerosol, the collection efficiency for ≥1 μm diameter particles is nearly 1.00. This is in accordance with the theoretical collision efficiency as expected for particles with high adhesion. PSL tests performed on a filter coated with a synthetic-oil (aerosol) deposit (32.9 μg mm ^?2 ± 0.4 μg mm ^?2 ) improved the collection efficiency for PSL ≥ 0.7 μm to nearly 1.00. Particles that are deposited by interception-inertial impaction at 10 m s ^?1 airflow velocity depend on the particle and fiber adhesion properties.     

Aerosol synthesis of silicon nanoparticles with narrow size distribution—Part 2: Theoretical analysis of the formation mechanism

This work investigates the mechanisms which lead to the formation of silicon nanoparticles with narrow size distributions by means of population balance modeling. The model accounts for the full aerosol process, including chemical reaction, nucleation from supersaturated vapor, growth and agglomeration. The results are in good agreement with experimental data. The effects of the process parameters temperature, silane concentration and reactor total pressure are systematically investigated. The simulation allows an in-depth insight into the particle formation mechanism and reveals the key requirements which are necessary for the generation of narrow particle size distributions. In this mechanism, only a short nucleation burst occurs, while surface growth plays the dominant role in silane precursor consumption. A key role is attributed to condensation, because the numerical calculations can only reflect the experimental observations, if the condensation mechanism is included in the model.     

Mobility Analysis of 2 nm to 11 nm Aerosol Particles with an Aspirating Drift Tube Ion Mobility Spectrometer

We describe the performance of a drift tube-ion mobility spectrometry (DT-IMS) instrument for the measurement of aerosol particles. In DT-IMS, the electrical mobility of a measured particle is inferred directly from the time required for the particle to traverse a drift region, with motion driven by an electrostatic field. Electrical mobility distributions are hence linked to arrival time distributions (ATDs) for particles reaching a detector downstream of the drift region. The developed instrument addresses two obstacles that have limited DT-IMS use for aerosol measurement previously: (1) conventional drift tubes cannot efficiently sample charged particles at ground potential and (2) the sensitivities of commonly used Faraday plate detectors are too low for most aerosols. Obstacle (1) is circumvented by creating a “sample volume” of aerosol for measurement, defined by the streamlines of fluid flow. Obstacle (2) is bypassed by interfacing the end of the drift region with a condensation particle counter. The DT-IMS prototype shows high linearity for arrival time versus inverse electrical mobility (R ² > 0.99) over the size range tested (2.2–11.1 nm), and measurements compare well with both analytical and numerical models of device performance. A dimensionless calibration curve linking drift time to inverse electrical mobility is developed. In less than 5 s, it is possible to measure 11.1 nm particles, while 2.2 nm particles are analyzable on a subsecond scale. The transmission efficiency is found to be dependent upon electrostatic deposition for short drift times and upon advective losses for long drift times.

Copyright 2014 American Association for Aerosol Research     

Ultra-Thin Polymer Layer Deposition by Aerosol–Dielectric Barrier Discharge (DBD) and Electrospray Ionization (ESI) at Atmospheric Pressure

Polyolefins are chemically inert and do not adhere well to metals, polymers or inorganics. To overcome this problem, polyolefin surfaces were modified thermally, plasmachemically, or by flame treatment with different oxygen-containing groups, however, unfortunately, such treatments were accompanied by undesired, adhesion lowering polymer degradation. To solve this dilemma, solutions of synthetic polymers and copolymers were prepared, sprayed into the barrier discharge or electrosprayed without discharge and deposited as thin adhesion-promoting layers. The deposited polymer layers from poly(vinylamine), poly(ethylene glycol)–poly(vinyl alcohol) copolymers and poly(acrylic acid) were endowed with monotype functional groups. Using the aerosol — dielectric barrier discharge only a fraction of functional groups survived the deposition process in contrast to the electrospray in which all functional groups were retained.     

Aqueous film formation on irregularly shaped inorganic nanoparticles before deliquescence,as revealed by a hygroscopic differential mobility analyzer–Aerosol particle mass system

A hygroscopic tandem differential mobility analyzer (H-TDMA) and a hygroscopic coupled DMA and aerosol particle mass (H-DMA-APM) were coupled to examine aqueous film formation and the deliquescence behavior of inorganic nanoparticles. The two systems complement each other because H-DMA-APM measures mass change, while H-TDMA measures mobility diameter (volume) change of nanoparticles upon water uptake. The former mass change was, in particular, more capable to discern minute particle phase changes than the latter size change at moderate RHs. The mass and diameter changes were used to derive the particle effective density for evaluation of aqueous film formation on the nanoparticle surface before and after deliquescence transition. The measurements further showed that approximately 3–5 and 12–20 monolayer equivalents of water molecules formed on the respective surface of 50- and 100-nm inorganic aerosols (ammonium sulfate and sodium chloride) before deliquescence relative humidity (DRH). These findings support the physical basis of the coated-surface model given by Russell and Ming in 2002, and suggest that the phase transition of inorganic nanoparticles near deliquescence is a gradual process instead of an abrupt change. This phenomenon changed the surface energy values, thus confirming the explanation that the DRH of nanoparticles increases as the particle size decreases. This is the first direct observation of nanoparticle deliquescence phase transition using the H-DMA-APM system, and the detailed characterization of aqueous film formation on inorganic nanoparticles is feasible with the presented measurement systems.

© 2016 American Association for Aerosol Research     

An Eddy-Covariance System for the Measurement of Surface/Atmosphere Exchange Fluxes of Submicron Aerosol Chemical Species—First Application Above an Urban Area

Until now, micrometeorological measurements of surface/ atmosphere exchange fluxes of submicron aerosol chemical components such as nitrate, sulfate or organics could only be made with gradient techniques. This article describes a novel setup to measure speciated aerosol fluxes by the more direct eddy covariance technique. The system is based on the Aerodyne quadrupole-based Aerosol Mass Spectrometer (Q-AMS), providing a quantitative measurement of aerosol constituents of environmental concern at a time resolution sufficient for eddy-covariance. The Q-AMS control software was modified to maximize duty cycle and statistics and enable fast data acquisition, synchronized with that of an ultrasonic anemometer. The detection limit of the Q-AMS based system for flux measurements ranges from 0.2 for NO₃ ^? to 15 ng m^?2 s^?1 for hydrocarbon-like organic aerosol (HOA), with an estimated precision of around 6 ng m^?2 s^?1, depending on aerosol loading. At common ambient concentrations the system is capable of resolving deposition velocity values < 1 mm s^?1, sufficient for measurements of dry deposition to vegetation. First tests of the system in the urban environment (6 to 20 June 2003) in Boulder, CO, USA, reveal clear diurnal, presumably traffic related, patterns in the emission of HOA and NO₃ ^?, with indication of fast production of moderately oxygenated organic aerosol below the measurement height, averaging about 15% of the HOA emission. The average emission factor for HOA was 0.5 g (kg fuel)^?1, similar to those found in previous studies. For NO₃ ^? an emission factor of 0.09 g (kg fuel)^?1 was estimated, implying oxidation of 0.5% of the traffic derived NO_x below the measurement height of 45 m. By contrast, SO₄ ^2? fluxes were on average downward, with deposition velocities that increase with friction velocity from 0.4 to 4 mm s^?1.     

Development of a High-Pressure Aerodynamic Lens for Focusing Large Particles (4–10 μm) into the Aerosol Time-of-Flight Mass Spectrometer

A new aerodynamic lens system for an online aerosol time-of-flight mass spectrometer (ATOFMS) has been designed and constructed to transmit and allow the analysis of individual particles in the 4–10-μm-size range. Modeling was used to help design the lens within the bounds of ATOFMS instrumental constraints. The aerodynamic lens operates at a high inlet pressure, 3066 Pa (23 Torr), with a unique tapered relaxation region to improve large particle transmission. Every stage of the lens was tested empirically using a combination of particle deposition and light scattering experiments. The critical orifice was found to significantly impact large particle transmission, with orifices <200 μm in diameter completely suppressing large particle transmission. The addition of a virtual impactor allowed for the use of large orifices without any loss of functionality in the ATOFMS. The detection efficiency of the ATOFMS was >10% for particles from 4–10 μm with a peak efficiency of 74 ± 9% for 6-μm particles. With the extended size range provided by this inlet, the ATOFMS can now be extended to investigate single cell metabolomics.

Copyright 2014 American Association for Aerosol Research     

A New Laser Induced Incandescence–Mass Spectrometric Analyzer (LII-MS) for Online Measurement of Aerosol Composition Classified by Black Carbon Mixing State

We developed a laser induced incandescence–mass spectrometric analyzer (LII-MS) for online measurements quantifying the aerosol chemical compositions with respect to the mixing state of black carbon (BC). The LII-MS is developed as a tandem series comprising an LII chamber to detect and vaporize BC-containing particles and a particle trap laser desorption mass spectrometer (PT-LDMS: Takegawa et al. 2012). The PT-LDMS collects aerosol particles transferred from the LII chamber and quantifies the chemical compositions. A newly designed collection probe, coupled with the sheath-air inlet nozzle of the LII chamber, enables a high throughput of aerosol particles without significant dilution. Total aerosol particles can be analyzed in the PT-LDMS by turning off the laser (MS mode), and the aerosol particles externally mixed with BC can be analyzed by turning on the laser (LII-MS mode). The difference in the PT-LDMS signals between the MS and LII-MS modes yields the chemical composition of materials internally mixed with BC. Performance of the developed instrument was evaluated in the laboratory by generating BC particles internally-mixed with oleic acid (OL) and BC particles externally mixed with ammonium sulfate particles. Preliminary results from ambient measurements are also presented and discussed.

Copyright 2014 American Association for Aerosol Research