Inter-Comparison of a Fast Mobility Particle Sizer and a Scanning Mobility Particle Sizer Incorporating an Ultrafine Water-Based Condensation Particle Counter

An Ultrafine Water-based Condensation Particle Counter (UWCPC), a Scanning Mobility Particle Sizer (SMPS) incorporating an UWCPC, and a Fast Mobility Particle Sizer (FMPS) were deployed to determine the number and size distribution of ultrafine particles. Comparisons of particle number concentrations measured by the UWCPC, SMPS, and FMPS were conducted to evaluate the performance of the two particle sizers using ambient particles as well as lab generated artificial particles. The SMPS number concentration was substantially lower than the FMPS (FMPS/SMPS = 1.56) measurements mainly due to the diffusion losses of particles in the SMPS. The diffusion loss corrected SMPS (C-SMPS) number concentration was on average ～ 15% higher than the FMPS data (FMPS/C-SMPS = 0.87). Good correlation between the C-SMPS and FMPS was also observed for the total particle number concentrations in the size range 6 nm to 100 nm measured at a road-side urban site (r² = 0.91). However, the particle size distribution measured by the C-SMPS was quite different from the size distribution measured by the FMPS. An empirical correction factor for each size bin was obtained by comparing the FMPS data to size-segregated UWCPC number concentrations for atmospheric particles. The application of the correction factor to the FMPS data (C-FMPS) greatly improved the agreement of the C-SMPS and C-FMPS size distributions. The agreement of the total particle concentrations also improved to well within 10% (C-FMPS/C-SMPS = 0.95).     

Causes of Concentration Differences Between a Scanning Mobility Particle Sizer and a Condensation Particle Counter

Accurate aerosol concentration measurement is important in many applications of aerosol science. Here we compare aerosol concentration measurements of classified NaCl aerosol in the size range of 20 to 80 nm (diameter) between a scanning mobility particle sizer (SMPS) and a condensation particle counter (CPC). The SMPS systematically measured higher concentrations than the CPC, with the difference increasing with decreasing particle size. Experiments suggest several causes for the discrepancy. First, the factory calibration of the SMPS impactor flow was incorrect for the study site at 780 mbar. Second, the neutralizer used in the SMPS was inefficient in bringing the classified aerosol to charge equilibrium, and third, there were significant losses of charged aerosol within the CPC. The comparisons were improved with proper impactor flow calibration and proper charge neutralization of the classified aerosol before measurement by the SMPS and CPC. The results of this study point to the importance of proper conditioning of aerosol below about 100 nm for measurement with the SMPS and condensation-based particle counters.     

Online Nanoparticle Mass Measurement by Combined Aerosol Particle Mass Analyzer and Differential Mobility Analyzer: Comparison of Theory and Measurements

A combination of a differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM) is used to measure the mass of NIST Standard Reference Materials (SRM®) PSL spheres with 60 and 100 nm nominal diameter, and NIST traceable 300 nm PSL spheres. The calibration PSL spheres were previously characterized by modal diameter and spread in particle size. We used the DMA to separate the particles with modal diameter in a narrow mobility diameter range. The mass of the separated particles is measured using the APM. The measured mass is converted to diameter using a specific density of 1.05. We found that there was good agreement between our measurements and calibration modal diameter. The measured average modal diameters are 59.23 and 101.2 nm for nominal diameters of 60 and 100 nm (calibration modal diameter: 60.39 and 100.7 nm) PSL spheres, respectively. The repeatability uncertainty of these measurements is reported. For 300 nm, the measured diameter was 305.5 nm, which is an agreement with calibration diameter within 1.8%.

The effect of spread in particle size on the APM transfer function is investigated. Two sources of the spread in “mono-dispersed” particle size distributions are discussed: (a) spread due to the triangular DMA transfer function, and (b) spread in the calibration particle size. The APM response function is calculated numerically with parabolic flow through the APM and diffusion broadening. As expected from theory, the calculated APM response function and measured data followed a similar trend with respect to APM voltage. However, the theoretical APM transfer function is narrower than the measured APM response.     

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.     

Investigation of Surface Properties of Ultrafine Particles by Application of a Multistep Condensation Nucleus Counter

The fate of atmospheric ultrafine particles is determined by their size, chemical composition, and especially by their physical and chemical surface properties. To characterize the surface of ultrafine particles, their behavior as condensation nuclei can be used. Monodisperse ultra-fine particles with different surface structures were investigated by observing the onset of droplet formation at a fixed electrical mobility diameter. Droplet growth was detected by application of a multistep condensation nucleus counter (CNC). The particles were generated under well-controlled conditions and monodisperse fractions were obtained using an electrostatic classifier. For studying the influence of changes in the surface structure, ultrafine sulfuric acid droplets were coated with different organic materials. Different surface films required different supersaturations for droplet growth depending on the molecular structure and layer thickness of the material used for coating. Therefore it was concluded that certain compounds, enriched on the particle surface, affect condensation of water vapor in such a way that higher supersaturations are required in comparison to the particle core material. Additionally, it was observed that remarkably high supersaturations of water vapor were required for condensation on particles consisting of the following materials: metals, carbon, and Aerosil (spherical silica particles).     

Diffusion Distortions in a Differential Mobility Analyzer: The Shape of Apparent Mobility Spectrum

The effect of molecular and or small-scale turbulent diffusion in a differential mobility analyzer (DMA) is described in terms of apparent mobility spectrum. Without restricting generality, the normalized apparent spectrum, i.e., the apparent spectrum of unimobile particles with unity charge density is introduced. An approach based on the calculation of the probabilities of the random displacements of particles around their regular trajectories enables us to derive analytical expressions for the normalized apparent spectrum. A particular derivation is carried out for the case of a second-order DMA with one collecting electrode and variable electric field strength. Explicit analytical equations of various approximation degrees have been derived. The normalized apparent spectrum of the particular DMA shows a remarkable asymmetry; its mobility mode is shifted toward lower mobilities. The derived equations serve as a basis for the estimation of the spectral resolution of the DMA. The equations can also be used for a proper design of the DMA, reducing the effect of diffusion. Once the normalized apparent spectrum is known, a possibility appears to improve the resolution of the DMA by solving a relevant equation and eliminating the effect of diffusion in such a way.     

Measurements of Particle Loss Functions in a Differential Mobility Analyzer (TSI,Model 3071) for Different Flow Rates

Particle losses in a differential mobility analyzer (TSI, Model 3071) caused by diffusive deposition and Brownian diffusion are measured for particles in the diameter size range between 3 and 100 nm. For small sampling and aerosol flow rates (0.3 liters/min) at 20 nm, nearly 50% of the primary particles are lost; and for 2 liters/min, the particle losses have to be considered in the diameter size range below 30 nm (50% at 7 nm). From the measured penetration values, an effective tube length is derived which is useful to calculate particle losses for other flow rates through the analyzer.     

Development of a Pulsed-Field Differential Mobility Analyzer: A Method for Measuring Shape Parameters for Nonspherical Particles

For a nonspherical particle, a standard differential mobility analyzer (DMA) measurement yields a mobility-equivalent spherical diameter, but provides no information about the degree of sphericity. However, given that the electrical mobility for nonspheres is orientation-dependent, and that orientation can be manipulated using electric fields of varying strength, one can, in principle, extract some type of shape information through a systematic measurement of mobility as a function of particle orientation. Here, we describe the development of a pulsed-field differential mobility analyzer (PFDMA) which enables one to change the peak E-field experienced by the particle to induce orientation, while still maintaining the same time-averaged field strength as a standard DMA experiment. The instrument is validated with polystyrene latex (PSL) spheres with accurately known size, and gold rods with dimensions accurately determined by transmission electron microscopy (TEM). We demonstrate how the instrument can be used for particle separation and extraction of shape information. In particular, we show how one can extract both length and diameter information for rod-like particles. This generic approach can be used to obtain dynamic shape factors or other multivariate dimensional information (e.g., length and diameter).

Copyright 2014 American Association for Aerosol Research     

Accurate Determination of Aerosol Activity Coefficients at Relative Humidities up to 99% Using the Hygroscopicity Tandem Differential Mobility Analyzer Technique

Aerosol water content plays an important role in aqueous phase reactions, in controlling visibility, and in cloud formation processes. One way to quantify aerosol water content is to measure hygroscopic growth using the hygroscopicity tandem differential mobility analyzer (HTDMA) technique. However, the HTDMA technique becomes less reliable at relative humidity (RH) >90% due to the difficulty of controlling temperature and RH inside the second DMA. For this study, we have designed and implemented a new HTDMA system with improved temperature and RH control. Temperature stability in the second DMA was achieved to ±0.02°C tolerance by implementing active control using thermoelectric heat exchangers and PID control loops. The DMA size resolution was increased by operating high-flow DMA columns at a sheath:sample flow ratio of 15:0.5. This improved size resolution allowed for improving the accuracy of the RH sensors by interspersing ammonium sulfate reference scans at high frequency. We present growth factor data for pure compounds at RH up to 99% and compare the data to theoretical values and to available bulk water activity data. With this HTDMA instrument and method, the osmotic coefficients of spherical, nonvolatile aerosols of known composition between 30 and 200 nm in diameter can be determined within ±20%. We expect that data from this instrument will lead to an improvement of aerosol water content models by contributing to the understanding of aerosol water uptake at high RH.

Copyright 2013 American Association for Aerosol Research     

Rapid,Size-Resolved Aerosol Hygroscopic Growth Measurements: Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe (DASH-SP)

We report on a new instrument developed to perform rapid, size-resolved aerosol hygroscopicity measurements. The differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP) employs differential mobility analysis in-concert with multiple humidification and optical sizing steps to determine dry optical size and hygroscopic growth factors for size-selected aerosols simultaneously at three elevated relative humidities. The DASH-SP has been designed especially for aircraft-based measurements, with time resolution as short as a few seconds. The minimum particle diameter detected with 50% efficiency in the optical particle counters (OPCs) is 135 ± 8 nm, while the maximum detectable particle diameter is in excess of 1 μm. An iterative data processing algorithm quantifies growth factors and “effective” refractive indices for humidified particles using an empirically derived three-dimensional surface (OPC pulse height–refractive index–particle size), based on a calculated value of the “effective” dry particle refractive index. Excellent agreement is obtained between DASH-SP laboratory data and thermodynamic model predictions for growth factor dependence on relative humidity for various inorganic salts. Growth factor data are also presented for several organic acids. Oxalic, malonic, glutaric, and glyoxylic acids grow gradually with increasing relative humidity up to 94%, while succinic and adipic acids show no growth. Airborne measurements of hygroscopic growth factors of ship exhaust aerosol during the 2007 Marine Stratus/Stratocumulus Experiment (MASE II) field campaign off the central coast of California are presented as the first report of the aircraft integration of the DASH-SP.     

Letter to the Editor: Reply to C. N. Davies and N. Egilmez Concerning the Nolan-Pollak Nucleus Counter

Please click here to find the Letter to the Editor to which this Response refers: http://dx.doi.org/10.1080/02786828608959082

In response to “Letter to the Editor Lagrangian Stochastic Particle Tracking” by Aliabadi and Rogak in Aerosol Science and Technology 45:313–314, 2011.     

Measurement of Ambient Aerosols by the Differential Mobility Analyzer Method: Concepts and Realization Criteria for the Size Range Between 2 and 500 nm

Requirements for aerosol spectrometer based on the differential electrical mobility method are deduced from the principles of aerosol classification and concentration determination. Data reduction and method-inherent sources of errors are discussed in detail. Criteria for the components of the arrangement to be used for ambient aerosol size distribution measurements are given with respect to size range, concentration range and time resolution.     

Design and Validation of a Volatility Hygroscopic Tandem Differential Mobility Analyzer (VH-TDMA) to Characterize the Relationships Between the Thermal and Hygroscopic Properties of Atmospheric Aerosol Particles

The nature of atmospheric aerosols is extremely complex and often requires advanced analytical tools for the determination of its physical and chemical properties. In particular, the interaction of particles with atmospheric water is a complex function of both particle size and composition. The ability of a particle to grow in a humid environment can be measured by humidity tandem differential mobility analyzing techniques (H-TDMA). In this article, we present a new development combining thermo-desorption and humidification aerosol conditioning in series that allows to measure changes in the hygroscopic behavior of aerosol at 90% relative humidity (RH) after conditioning of the particle by thermo-desorption to a temperature between 25°C and 300°C. The main feature of this system, named Volatility Hygroscopic—Tandem Differential Mobility Analyzer (VH-TDMA), is to allow for rapid (10 minutes) series of scans to control particle response to 1-thermal conditioning, 2- RH increase to 90% and 3—a combination of both thermal and RH conditioning. The VH-TDMA is, therefore, suited to investigate particle ageing through a simple coupling of H-TDMA and V-TDMA performances.

The aim of the present article is to describe the instrument design and to validate its performances by focusing on the measurement of hygroscopic behavior of pure inorganic particles such as sodium chloride or ammonium sulfate, as well as internally mixed organic-inorganic particles. Based on laboratory experiments and applications to natural aerosols, we show that the VH-TDMA system can be used to investigate the hygroscopic properties of the non-volatile fraction of ambient sub-micrometer aerosols in the range of 20 to 150 nm and the influence of the more volatile fraction of the particle on hygroscopic growth.     

Particle Morphology and Density Characterization by Combined Mobility and Aerodynamic Diameter Measurements. Part 2: Application to Combustion-Generated Soot Aerosols as a Function of Fuel Equivalence Ratio

Composition, shape factor, size, and fractal dimension of soot aerosol particles generated in a propane/O₂, flame were determined as a function of the fuel equivalence ratio (φ). Soot particles were first size-selected by a differential mobility analyzer (DMA) and then analyzed by an Aerodyne aerosol mass spectrometer (AMS). The DMA provides particles of known mobility diameter (d_m ). The AMS quantitatively measures the mass spectrum of the nonrefractory components of the particles and also provides the vacuum aerodynamic diam eter (d_va ) corresponding to the particles of known mobility diameter. The measured d_m, d_va , and nonrefractory composition are used in a system of equations based on the formulation presented in the companion article to estimate the particle dynamic shape factor, total mass, and black carbon (BC) content. Fractal dimension was estimated based on the mass-mobility relationship. Two types of soot particles were observed depending on the fuel equivalence ratio. Type 1: for φ < 4 (lower propane/O₂), d_va ; was nearly constant and independent of d_m . The value of d_va increased with increasing φ. Analysis of the governing equations showed that these particles were highly irregular (likely fractal aggregates), with a dynamic shape factor that increased with d_m and φ. The fractal dimension of these particles was approximately 1.7. These particles were composed mostly of BC, with the organic carbon content increasing as φ increased. At φ = 1.85, the particles were about 90% BC, 5% PAH, and 5% aliphatic hydrocarbon (particle density = 1.80 g/cm³). Type 2: for φ > 4 (high propane/O₂), d_va was linearly proportional to d_m . Analysis of the governing equations showed that these particles were nearly spherical (likely compact aggregates), with a dynamic shape factor of 1.1 (versus 1 for a sphere) and a fr actal dimension of 2.95 (3 for a sphere). These particles were composed of about 50% PAH, 45% BC, and 5% aliphatic hydrocarbons (particle density = 1.50 g/cm³). These results help interpret some measurement s obtained in recent field studies.     

Interpretation of Volatility Tandem Differential Mobility Analyzer (V-TDMA) data for accurate vapor pressure and enthalpy measurement: Operational considerations,multiple charging,and introduction to a new analysis program (TAO)

Abstract

Significant evaporation of pure aerosols in a Volatility Tandem Differential Mobility Analyzer (V-TDMA) creates two Condensation Particle Counter (CPC) response peaks. Two hypotheses for the observed peaks have been proposed: the existence of two phases or the separation of the singly charged experimental size distribution from the remaining experimental size distributions with charges greater than 1 (charge separation). To explore this observation, we atomized pure levoglucosan aerosol and evaporated the aerosol until two peaks formed. We used an additional classifier and neutralizer to select particles from each of the two peaks and assessed the number of charges on the particles. The smaller diameter peak contained singly charged particles, and the larger diameter peak contained the remaining charges. The charge separation hypothesis alone accounts for the two-peak observations. We used a new V-TDMA model named TAO and show that charge separation should occur in other pure components as well. The TAO model was then used to display the impact of different DMA transfer functions, different inlet size distributions, and different oven residence time distributions (RTDs) on the CPC response. Large errors are possible when direct measurement of the RTD is not performed or when wide RTDs are used. We recommend use of narrow transfer functions with narrow RTDs to detect charge separation. When the singly charged CPC response is isolated (smaller diameter peak in the two peak response), accurate estimations of vapor pressure can be recovered, assuming accurate values for gas phase diffusivity, surface energy, particle density, etc. are used.

Copyright © 2020 American Association for Aerosol Research     

Fast Batch to Continuous Transposition: Application to the Extraction of Andrographolide from Plants

A fast development method for batch to continuous process transposition is proposed. This method is based on transient regime experiment analyses and is applied to a solid‐liquid extraction. The application under consideration is the extraction of an active principle from a plant in a non‐sinusoidal pulsed column. Typically, the proposed signal is composed of two different periods: firstly, a classical sinusoidal pulsation step is used to mix the liquid and solid phases in the active part of the column and allow an optimal mass transfer and, secondly, an impulsion phase, used generally for the transport of solids. The extraction is carried out in a disc and doughnut column of 54 mm diameter and 3.5 m height. Liquid and solid are flowing co‐currently and downwardly. This technological improvement has been implemented to solve the difficulties due to the significant heterogeneity of the matter: one part tends to float and other to sink, which always leads to a definitive flooding in classical operations. The effects of the solid flow rate and the solvent characteristics on the hydrodynamic behavior of the column are studied. The mean residence time and the total solid holdup are calculated by using a transient regime mass balance on the experimental results. These experiments allow the identification and quantification of opposite effects of the operating parameters. Mass transfer experiments have been performed and the results fit calculated values obtained by coupling the hydrodynamic and batch extraction results. Despite the simplifications made, this validates the fast development method proposed to help batch to continuous transposition.     

Force Curve Measurements with the AFM: Application to the In Situ Determination of Grafted Silicon-Wafer Surface Energies

The atomic force microscope (AFM) can be used to perform surface force measurements in the quasi-static mode (cantilever is not oscillating) to investigate nanoscale surface properties. Nevertheless, there is still a lack of literature proposing a complete systematic and rigorous experimental procedure that enables one to obtain reproducible and significant quantitative data. This article focuses on the fundamental experimental difficulties arising when making force curve measurements with the AFM in air. On the basis of this AFM calibration procedure, quantitative assessment values were used to determine, in situ, SAM (or Self Assembled Monolayer)-tip thermodynamic work of adhesion at a local scale, which have been found to be in good agreement with quoted values. Finally, determination of surface energies of functionalised silicon wafers (as received, CH₃, OH functionalised silicon wafers) with the AFM (at a local scale) is also proposed and compared with the values obtained by wettability (at a macroscopic scale). In particular, the effect of the capillary forces is discussed.     

Force Curve Measurements with the AFM: Application to the In Situ Determination of Grafted Silicon-Wafer Surface Energies

The atomic force microscope (AFM) can be used to perform surface force measurements in the quasi-static mode (cantilever is not oscillating) to investigate nanoscale surface properties. Nevertheless, there is still a lack of literature proposing a complete systematic and rigorous experimental procedure that enables one to obtain reproducible and significant quantitative data. This article focuses on the fundamental experimental difficulties arising when making force curve measurements with the AFM in air. On the basis of this AFM calibration procedure, quantitative assessment values were used to determine, in situ, SAM (or Self Assembled Monolayer)-tip thermodynamic work of adhesion at a local scale, which have been found to be in good agreement with quoted values. Finally, determination of surface energies of functionalised silicon wafers (as received, CH₃, OH functionalised silicon wafers) with the AFM (at a local scale) is also proposed and compared with the values obtained by wettability (at a macroscopic scale). In particular, the effect of the capillary forces is discussed.     

Technique for Measuring Grain-Boundary Mobility: Application to MgO-Doped Al2O3

An experimental technique employing localized laser heating has been developed to seed abnormal grain growth in Mg-doped Al₂O₃ permitting the migration behavior of individual grain boundaries to be investigated. Nonuniform growth of both naturally nucleated and laser-nucleated abnormal grains was observed. Calculated boundary mobilities exceeded those characterizing grain growth in both doped and undoped porous Al₂O₃).     

Extensometric and Thermo-Elasticimetric Measurements to Characterize the Damage Evolution in Adhesively Bonded Joints: Application for the Adhesively Bonded Double Scarf Joint

In this article, two nondestructive experimental methods are compared in order to evaluate the influence of a singularity, created by geometry, on the damage evolution in the adhesively bonded joints. The first experimental method uses the “back face technique” to measure the disturbances of the strain fields induced by the cracks. The second experimental method uses an infrared focal plane array camera to measure the thermo-elastic field in the geometrical singularity. First of all, both methods are carried out independently: the analysis of the stress/strain and the thermo-graphic analysis. Then, these two methods enabled us to generate a database, the use of which improves the diagnosis of the damage state of the adhesively bonded joints.