Transport Phenomena with Single Aerosol Particles

This is a review of theory and experiments related to single aerosol particle transport processes. The theories of mass, heat, momentum, and charge transfer are outlined, with emphasis on mass transport in the continuum and noncontinuum regimes. Included in the discussion of mass transfer are single and multicomponent droplet evaporation, a comparison of the results of solutions of the Boltzman equations for Knudsen aerosol evaporation and growth and experimental methods for the study of single droplet evaporation or growth. Of particular concern here are the experimental apparati and techniques developed for single particle measurements. These range from the Millikan oil drop experiment through the electrostatic balance (the Millikan condenser with automatic stabilization of the particle) to the electrodynamic balance. The principles and applications of these instruments are reviewed.     

Albedo Measurements and Optical Sizing of Single Aerosol Particles

Aerosols play an important role in global climate change by their interactions with incoming solar radiation and outgoing longwave radiation from the planetary surface. The climate effects of aerosols depend on their scattering and absorption properties. This article describes the development of an instrument (ASTER: Aerosol Scattering To Extinction Ratio) that simultaneously measures the scattering and extinction of single aerosol particles. ASTER uses a high-Q cavity to amplify the extinction signal and innovative optics to collect the scattered light. It can distinguish many partially absorbing particles from a few black ones even if the bulk absorption is the same. Optical sizing and single-scattering albedo measurements were made for laboratory-generated particles with diameters from about 300 nanometers to above one micrometer. Using this prototype instrument, changes in albedo for single particles of 20% or greater were detected by measurement of the scattering and extinction. Optical sizing of the individual particles to within ～ 50 nm was accomplished using the ratio of the forward scattered light to the total scattering. Initial measurements of laboratory air showed a mode of highly absorbing particles. This article reports design and early laboratory tests on ASTER.     

气溶胶单粒子成分和粒径的实时测量

利用气溶胶飞行时间激光质谱仪测定了两种芳香烃气溶胶的质谱,发现芳香族化合物产生的气溶胶粒子容易形成分子离子峰,对苯二酚还发生分子离子反应.对室内外空气中的气溶胶粒子的粒径进行实时检测的结果表明,室内外空气中的气溶胶粒子多以细粒子为主.实验结果表明,该仪器在大气气溶胶污染监测及相关研究领域具有重要的实用价值.     

Airborne Deployment of an Instrument for the Real-Time Analysis of Single Aerosol Particles

An instrument is described that provides real-time chemical analysis of the composition of individual aerosol particles. A differentially pumped aerosol inlet transfers particles from the ambient atmosphere into the source region of a time-of-flight mass spectrometer where they impact on a heated surface and the resulting vapors are ionized by electron ionization prior to mass analysis. Labora tory calibration studies demonstrated that the instrument was capable of detecting particles with diameters greater than approximately 0.4mu m. The instrument was operated on the NASA DC-8 research aircraft as part of the 1996 SUbsonic aircraft: Contrail and Cloud Effects Special Study (SUCCESS) mission with the intent of studying the chemical composition of upper tropospheric particles. More than 25,000 aerosol particle mass spectra were recorded during 19 mission flights. Although approximately 120 of those spectra showed clear evidence of sulfate, nitrate, and other inorganic materials, the remaining spectra contained only mass peaks consistent with water. Moreover, particles were detected only while traversing clouds. These results are not consistent with expectations of the size, quantity, or composition of upper tropospheric particles. It is likely, however, that a subisokinetic aircraft sampling inlet resulted in the collection of only very large ice particles. This situation would account for both the observed preponder ance of water-only spectra and the apparent lack of particles outside of clouds. Despite the sampling problem, the instrument was able to chemically speciate aerosols directly sampled from a medium altitude aircraft. These represent the first examples of aerosol particles chemically speciated in real time from an airborne platform.     

两种粒子计数器相结合测量大气气溶胶粒子折射率虚部

研究了两种粒子计数器,其中以光散射为原理的粒子计数器测量的是光学等效直径,测量结果受折射率的影响很大;以粒子飞行时间为原理的粒子计数器测量的是空气动力学直径,测量结果不受折射率的影响.利用两种仪器的同时测量结果来计算大气的折射率,并给出计算结果.     

Behavior of Compact Nonspherical Particles in the TSI Aerodynamic Particle Sizer Model APS33B: Ultra-Stokesian Drag Forces

The aerodynamic behavior of aggregates consisting of uniform polystyrene latex (PSL) spheres and unaggregated cuboidal Natrojarosite particles in a TSI aerodynamic particle sizer (Model APS33B) has been studied. In initial tests, monodisperse PSL micro-spheres ranging from 0.3 to 7 μm in geometric diameter were generated from aqueous suspensions using a Lovelace nebulizer. APS33B responses for these uniform-sized particles showed multiple peaks. The major (primary) peak, which resulted from the smallest particle, corresponded to the unaggregated single spheres (singlets); the second, third, and fourth peaks were identified as doublets, triangular triplets, and tetrahedral quadruplets, respectively. Both doublets and triplets moved with their long axes in perpendicular (maximum drag) orientation to the flow direction in the APS33B. In contrast, the tetrahedral particles were isometric and had the same dynamic shape factor (drag resistance) for all three primary orientations. The particle Reynolds numbers (Re _p) for these particles were calculated and ranged from 0.2 to 30 in the sensing volume of the APS33B detector (i.e., ultra-Stokesian conditions). Ultra-Stokesian drag forces for all three types of aggregates were, therefore, estimated and expressed as a function of an empirical factor (1 + aRe ^b _p) to the Stokesian drag force. The ultra-Stokesian drag of a Natrojarosite particle was measured in the range 20 Re _p < 50 and could be described with a similar expression. This approach facilitates the study of the dynamic behavior of nonspherical particles and yields new information about the characteristics of drag forces in the ultra-Stokesian regime     

Electrodynamic Focusing of Charged Aerosol Particles

An electrodynamic screen for focusing charged particles carried by an airflow stream has been constructed and tested. The screen has the shape of a cone frustum with half-angle of 8° and is 17 cm long. Entrance and exit diameters are 7.0 and 2.5 cm, respectively. Electrodes forming the screen are three 0.71-mm diameter stainless-steel wires configured as a triple-start helix with 2.0-mm pitch. A three-phase power supply drives each wire with a line-to-ground voltage of 2300 V at 350 Hz with a phase difference of 120° between adjacent wires. Test particles of 5.2-μm aerodynamic diameter with electric charge of 1850–6080 elementary units per particle in an aerosol having a mean flow velocity of 4.3×10^?2 m/s parallel to the vertically oriented cone axis are confined and focused with essentially no particle loss. Minimum distance of approach of the test particles to the screen, as found from computer simulation of particle trajectories, solution of the particle equations of motion by linear approximation, and experimental observation, are in good agreement.     

Bipolar Charging of Ultrafine Aerosol Particles

The stationary bipolar charging characteristics of aerosol particles in the size range between 4.5 and 40 nm have been studied using a new technique whereby the particles neutralized by a ²⁴¹Am radioactive source are enlarged and directly observed in an electric field. The number ratio of charged particles to total particles obtained in this study was found to deviate from the charge distribution obtained from Boltzmann's law and to agree well with that calculated with the bipolar charging theory of Fuchs using his values for the ion properties. The ratio of positively charged to negatively charged particles was found to be approximately 0.35:0.65.

     

Detection of Spatial Correlations among Aerosol Particles

Most approaches for evaluating rates of fundamental processes in aerosol science depend upon the implicit assumption that aerosol particles are independently and identically distributed in space. The validity of this assumption has not been examined in several decades, despite the fact that the presence of correlations can be shown to alter theoretical expressions significantly for such phenomena as attenuation of electromagnetic radiation and coagulation rate. Here, we provide evidence that the classification of the positions of aerosol particles as a Poisson process--even under stationary conditions--is often in error. In particular, small-scale aerosol clustering is experimentally demonstrated. Some consequences of these findings are discussed.     

Drag Force and Slip Correction of Aggregate Aerosols

The drag force on aggregate particles of uniform spheres was measured in a Millikan apparatus as a function of Knudsen number. Our experiment was designed to study the effect of particle orientation on the slip correction factor of nonspherical particles. The velocities of charged particles in a gravitational field with and without an applied electrical field were measured. An electrical field strength of 2000 V/cm was used to align doublet and triplet particles. Results showed that an aggregate particle moved in random orientation while in the gravitational field. The same particle moved with its polar axis parallel to the electric field (doublets) or with its plane of centers parallel to the electrical field (triangular triplets). Using a nonlinear regression method, both the dynamic shape factor and slip correction factor could be determined separately from the data. The dynamic shape factors at different orientations were in good agreement with those obtained previously in a sedimentation tank. The slip correction factor of singlet particles agreed with results previously obtained by Allen and Raabe for latex particles. Slip correction factors of doublets and triangular triplets can also be expressed in the Knudsen-Weber form: 1 + 2λ/d _a [1.142 + 0.558 exp(?0.999 d _a/2λ)]. The adjusted sphere diameter d _a was 1.21 d ₁ (primary diameter) for doublets moving parallel to the flow and 1.31 d ₁ for doublets randomly oriented. These results show that the slip correction factor of a nonspherical particle depends on the orientation and confirm the theory proposed by Dahneke.     

基于变尺度格子气方法的气固相间曳力模型

基于变尺度格子气方法,建立了气固相间曳力模型,对气固两相之间的耦合机制进行了研究,从微观角度出发,得到了气固两相之间相互作用的宏观行为。文章用此模型对气固两相流系统进行了模拟,模拟结果与四种经典曳力模型的模拟结果一致,验证了模型的正确性和有效性。     

Controlled Synthesis of Nanosized Particles by Aerosol Processes

Solid particles in the 1 nm < d_p < 100 nm size range form in gases as a result of gas phase condensation, particle collision processes, and solid-state processes. The relative rates of sintering and collision determine the size and morphology of the spheroidal primary particles. Rapid sintering is equivalent to the classical theory of coagulation with instantaneous coalescence. When the sintering rate is slow compared with the collision rate, fine primary particles form and aggregate into irregularly shaped agglomerates. The growth of primary particles in an aerosol generator that is cooling at a constant rate was studied theoretically. The most important process parameter determining particle diameter is the maximum gas temperature, because the rate of sintering is a sensitive function of temperature. Aerosol volume loading and cooling rate are important when the rate of particle growth is limited by collision processes. Experiments on the formation of alumina particles were made to study these effects. Predictions of primary particle size did not agree well with experimental measurements, which is attributed to an inadequate understanding of solid-state diffusion processes in nanosized particles. Other experiments showed that low concentrations of sodium and potassium additives reduce the primary particle size of silica.     

Hydrolysis of Organonitrate Functional Groups in Aerosol Particles

Organonitrate (ON) groups are thought to be important substituents in secondary organic aerosols (SOAs). Model simulations and laboratory studies indicate a large fraction of ON groups in aerosol particles, but much lower quantities are observed in the atmosphere. Hydrolysis of ON groups in aerosol particles has been proposed recently to account for this discrepancy. To test this hypothesis, we simulated formation of ON molecules in a reaction chamber under a wide range of relative humidity (RH) (0 to 90%). The mass fraction of ON groups (5 to 20% for high-NO_x experiments) consistently decreased with increasing RH, which was best explained by hydrolysis of ON groups at a rate of 4 day^?1 (lifetime of 6 h) for reactions under RH greater than 20%. In addition, we found that secondary nitrogen-containing molecules absorb light, with greater absorption under dry and high-NO_x conditions. This work provides the first evidence for particle-phase hydrolysis of ON groups, a process that could substantially reduce ON group concentration in atmospheric SOAs.

Copyright 2012 American Association for Aerosol Research     

The Effect of Inkjet Operating Parameters on the Size Control of Aerosol Particles

We investigated the effect of inkjet operating parameters on the size control of aerosol particles. Droplets with agglomerates of polystyrene latex particles were generated from an inkjet nozzle and continuously dried up at the temperature of 38°C. When droplets have the size range of 30 to 70 μm in diameter, aerosol particles with the size range of 3.5 to 7.1 μm were generated after the drying process. We controlled the particle size easily within a factor of two by adjusting rising/falling time and voltages of an actuating waveform. Generated particles were shown to have a narrow particle size distribution. Thus, the particle concentration of aqueous suspension does not need to be adjusted precisely in order to control the size of generated aerosol particles.

Copyright © 2015 American Association for Aerosol Research     

Method to Determine the Number of Bacterial Spores Within Aerosol Particles

We describe methodology to reveal the number of microbial spores within aerosol particles. The procedure involves visualization under differential- interference-contrast microscopy enhanced by high-resolution photography and further analysis by computer-assisted imaging. The method was used to analyze spore of Bacillus globigii in aerosols generated by a small (pressured metered-dose inhaler type) generator. Particles consisting in 1 or 2 spores accounted for 85% of all generated particles. This percentage rose to 91% when the same aerosol was collected on an Andersen cascade impactor that collected particles larger than 0.65 μm and was even higher (96%) when particles larger than 3.3 μm were also eliminated. These results demonstrate that the imaging analysis of aerosol particles collected on glass slides is sensitive to even relatively small changes in aerosol particle composition. The accuracy of the enhanced microscopic method described herein (differences between visual and computer analysis were approximately 3% of the total particle counts) seems adequate to determine the spore composition of aerosols of interest in biodefense.     

Dynamic Shape Factor of Nonspherical Aerosol Particles in the Diffusion Regime

Diffusional losses of monodisperse polysterene spheres and doublets from laminar aerosol flows and stagnant aerosols in cylindrical tubes were measured. The losses of spheres were used to determine their diameter (range 69–120 nm) and the volume equivalent diameter of their doublets. The losses of the doublets were used to determine their dynamic shape factor which for randomly oriented doublets was established to be 1.127 ± 0.085 for Knudsen numbers between 1.1 and 2.0.     

Methods of Aerosol Measurement before the 1960s

ABSTRACT

The time period before the 1960s can be described as the classical age of aerosol science and aerosol measurement. The measurement philosophy during this period was different from that of the period after the 1960s. Particle number concentration was considered the most important parameter. Optical microscopy was the determinative procedure for sample evaluation, for particle counting, and for size measurement. The most frequently used sampling methods were impaction and impingement, thermal and electrostatic precipitation, and filtration. Condensation samplers, as well as manual condensation nuclei counters, were used also. Manual and simple light scattering and light absorption methods played an already important role. Ultramicroscopy and nephelometry were commonly used methods. Elutriators and aerosol centrifuges were used for determining aerodynamic particle sizes, particle shape factors, and mass size distributions. Chemical aerosol analysis was in a developing state. Silica, silicates, and heavy metals were the most often detected dust and aerosol components. Titration, colorimetry, photometry, and polarography were the most commonly used analytical procedures. Practically no automatic or computer-supported measurement or analytical equipment was available at this time. Sampling instruments often were made in the laboratory, rather than produced commercially.     

荧光性微粒在气溶胶及微粒技术中应用的研究

以荧色物作标记微粒是微粒技术研究中的一项新方法.荧色物具有很高的分析灵敏度,用通常的荧光光度比色仪,可将其浓度测至1×10~(-10)g/ml;若采用Bergund-Liu气溶胶发生器,就可以得到已知直径及浓度的荧光性标准微粒,这对于研究微粒的空气动力学行为以及对于处理微粒状物料设备的标定是十分有效的.本文介绍了单分散气溶胶发生器,通过实验得出了使用荧光性微粒的一些重要条件,其中包括;溶剂的选择、微粒的发生与捕集、微粒中荧色物的提取及分析.文中还比较了两种常用的荧色物(荧光素钠及若丹明B)用于微粒技术的可能性,并应用这一技术对一种轴流式旋风分离器作了研究,得到了微粒在分离器各部位沉积情况的数据,它表明,该旋风分离器的分高效率和Stokes数间有一定的函数关系.     

Size Measurements of Latex Particles by Laser Aerosol Spectrometer

Size measurements of PSL (polystyrene latex) particles in a size range from 0.109 to 0.330 μm were made by laser aerosol spectrometer (PMS, LAS-X). The results were compared with those by electron microscopy. For example, the geometric standard deviation, σ_g, of nominally 0.176-μm PSL particles was measured as 1.05, assuming that their sizes distribute log-normally. The value of 1.05 was very close to 1.02 measured by electron microscopy. It was found that the spectrometer had very high size resolution, although the size resolution of the light scattering type spectrometer has been said to be poor. For some samples of PSL particles, however, there were large differences between particle sizes measured by LAS-X and those by electron microscopy. It was also found that LAS-X had a problem in calibration of size response curve.