Conditions for Cloud Settling and Rayleigh-Taylor Instability

Most aerosol motion can be analyzed by individual particle motion or by the motion of the suspending gas. There are, however, two related situations in which an aerosol can exhibit bulk motion: cloud settling and Rayleigh-Taylor instability. In both cases, the aerosol particles move faster as a cloud than they do as individual particles. In the case of cloud settling, the aerosol is usually a spheroidal cloud surrounded by clean air. Rayleigh-Taylor instability occurs when a dense aerosol layer overlies a layer of clean air. This instability is characterized by abrupt breakthrough of the aerosol layer into the clean air layer at multiple points. High-concentration, submicrometer test aerosols were generated in two experimental systems that permitted observation of the transition from particle-dominated motion to cloud, or bulk, dominated motion and measurement of cloud settling velocities and characteristics. In both systems aerosol concentration could be controlled over two orders of magnitude. One system used commercial ventilation smoke tubes to release a dense stream of aerosol into a low velocity wind tunnel. The other used diluted mainstream cigarette smoke from a smoking machine in an aerosol centrifuge. Based on these experiments, theoretical equations for cloud settling predict cloud settling velocity within an order of magnitude. The transition from individual particle motion to observable bulk motion occurs when predicted cloud settling velocity is from 0.01 to 0.05 m/s. Cloud settling appears to be initiated from an aerosol stream or layer by Rayleigh-Taylor instability. The ratio of cloud settling velocity to particle settling velocity does not appear to be a reliable predictor of the transition from particle to bulk motion.     

Experimental Study of Particle Deposition on Semiconductor Wafers

A sensitive method for detecting particle deposition on semiconductor wafers has been developed. The method consisted of generating a monodisperse fluorescent aerosol, depositing the known-size monodisperse aerosol on a wafer in a laminar flow chamber, and analyzing the deposited particles using a fluorometric technique. For aerosol particles in the size range of 0.1–1.0 μm, the mobility classification-inertial impaction technique developed by Romay-Novas and Pui (1988) was used to generate the monodisperse test aerosols. Above a particle diameter of 1.0 μm, monodisperse uranine-tagged oleic acid aerosols were generated by a vibrating-orifice generator. The test wafer was a 3.8-cm diameter silicon wafer placed horizontally in a vertical laminar flow chamber which was maintained at a free stream velocity of 20 cm/s. A condensation nucleus counter and an optical particle counter were used to obtain the particle concentration profile in the test cross section and to monitor the stability of aerosol concentration during the experiment. The results show that the measured particle deposition velocities on the wafers agree well with the theory of Liu and Ahn (1987) in the particle size range between 0.15 and 8.0 μm. The deposition velocity shows a minimum around 0.25 μm in particle diameter and increases with both smaller and larger particle size owing to diffusional deposition and gravitational settling, respectively.     

Absorption and Scattering Properties of Dust Clouds at 10.5-μm

Mass-density-normalized absorption and extinction coefficients for arid region soil-based dust were measured at a wavelength of 10.5 μm using photoacoustical techniques, short-path transmissometry, and aerosol dosimetry. An environmental chamber incorporating strong circulation, as well as the various aerosol sampling systems, was specifically designed for aerosol size distributions with particles as large as 40 μm in radius.

The mass extinction coefficient was found to be 0.22 m²/g, while the single scattering albedo, determined from the absorption and extinction coefficients, was 0.5. Calculations of these properties were based on two approaches: analyses of size distributions from photomicrographs of filter samples and analyses of the results obtained using a mixed-medium settling theory. In both cases, Mie theory was applied despite the clearly irregular particle forms. Agreement was close to the measured value for both approaches. The expected overestimation of the optical properties for the former model did not occur. Larger particles in this range are included because of their relevance to arid region dust clouds.     

Evolution of atmospheric aerosol particle size distributions via Brownian coagulation numerical simulation

Current atmospheric observations tend to support the view that continental tropospheric aerosols, particularly urban aerosols, show multimodal mass distributions. One of the obvious mechanisms causing the multimodality is the mixing of different primary sources. Other modes involve dissimilar aerosol formation processes in the atmosphere. Fine aerosol particles are generated from secondary processes such as nucleation, condensation and chemical reaction, whereas coarse particles usually consist of dust, fly ash and mechanically generated aerosols. With the use of a newly developed computer code GROWTH in our laboratory, we report here the simulated results of Brownian coagulation dynamics involving multimodal mass density functions for long periods of time. In our model calculations we assume that the aerosol particles are well mixed in an atmospheric volume so that spatial variation in the distribution is negligible. Our accurate numerical simulation of the Brownian coagulation dynamics indicates that once formed, an atmospheric multimodal aerosol distribution in the range 0.1 to 100 μm will maintain its identity for a very long period of time (at least hours) unless “atmospheric perturbations” such as meteorological instabilities, rain-washout and gravitational settling occur. It is our belief that understanding the complex domain of atmospheric aerosols requires systematic investigation of each process. This paper is a continuation of such an investigation.     

Improving the accuracy of the moments method for solving the aerosol general dynamic equation

The General Dynamic Equation for aerosol evolution is converted into a set of ordinary differential equations for the moments M_m by multiplying by v^m and integrating over particle volume, v. Closure of these equations is achieved by assuming a functional form for the moments, instead of the usual assumption of a functional form for the size distribution itself. Specifically, it is assumed that In(M_m) can be expressed as a pth-order polynomial in m. The time-dependent coefficients in the polynomial are found by solving (p + 1) differential equations numerically. The case p = 2 corresponds to the assumption that the size distribution is always log-normal but comparison with accurate solutions shows that increasing p increases the accuracy of the method for all processes considered (removal, condensation and Brownian coagulation). Particle loss during evaporation and achievement of a self-preserving form for Brownian coagulation are also considered. Inversion of the moment expression to obtain the size distribution using the Mellin inversion formula is discussed.     

Design of a Novel Open-Path Aerosol Extinction Cavity Ringdown Spectrometer

We describe a new open-path aerosol extinction cavity ringdown spectrometer (CRDS), which measures extinction coefficients as aerosol is drawn transversely through the optical cavity. With no inlet tubing, particle losses in the open-path CRDS due to impaction of coarse particles or evaporation of highly humidified particles in transfer lines are minimized, improving aerosol extinction measurements in dusty and humid environments. This report presents the key elements of the new open-path CRDS design as well as comparisons with a conventional closed-path CRDS and data obtained during a field study at NOAA's instrumented 300 m tower in Erie, Colorado. The open-path CRDS's 1σ detection limit for one second averaged data was 0.05 Mm^?1, comparable to the limit of our closed-path CRDS.

Copyright 2015 American Association for Aerosol Research     

Measuring the Mass Extinction Efficiency of Elemental Carbon in Rural Aerosol

Previous measurements of the mass absorption efficiency of ambient elemental carbon (EC) indicate that EC optical properties vary with location and imply that the variations may be due to different particle size distributions and composition at different locations (Liousse et al. 1993). For this reason, optical properties appropriate to regional characteristics of EC, determined over the wavelengths of light significant for aerosol extinction, are needed to adequately model the radiative impact of this species. Here we present a method for measuring one of these properties, the mass extinction efficiency (m 2 g -1 ) of EC, as a function of particle size and wavelength of light. In this method, size segregated atmospheric aerosol particles are collected on Nucleopore filters. The filter samples are extracted in a mixture of 30% isopropanol and 70% deionized distilled water to form a suspension of insoluble EC particles. Transmission of light through the extraction liquid is measured over wavelengths from 300 to 800 nm using a spectrophotometer. The transmission measurements taken through the liquid extract are mathematically converted to EC extinction coefficients in air. Although the conversion is a function of a parameter determined from Mie theory, which assumes monodisperse, spherical particles with a known density and refractive index relative to the medium, the method is shown to be reasonably insensitive to these assumptions. Using EC mass concentration obtained from a parallel sample, the EC mass extinction efficiency (in air) is calculated from the extinction coefficient (in air). This method is applied to a rural Midwestern, midcontinental aerosol. In general, the EC mass extinction efficiency in air is highest at lower wavelengths and for smaller particles. For particles with diameters between 0.09 and 2.7 w m and an assumed density of 1.9 g cm -3 , the measured EC mass extinction efficiency at 550 nm ranges from 7.3 to 1.7 m 2 g -1 .     

Development and Validation of a Simple Numerical Model for Estimating Workplace Aerosol Size Distribution Evolution Through Coagulation,Settling, and Diffusion

Recent research has indicated that the toxicity of inhaled ultrafine particles may be associated with the size of discrete particles deposited in the lungs. However, it has been speculated that in some occupational settings rapid coagulation will lead to relatively low exposures to discrete ultrafine particles. Investigation of likely occupational exposures to ultrafine particles following the generation of aerosols with complex size distributions is most appropriately addressed using validated numerical models. A numerical model has been developed to estimate the size-distribution time-evolution of compact and fractal-like aerosols within workplaces resulting from coagulation, diffusional deposition, and gravitational settling. Good agreement has been shown with an analytical solution to lognormal aerosol evolution, indicating good compatibility with previously published models. Validation using experimental data shows reasonable agreement when assuming spherical particles and coalescence on coagulation. Assuming the formation of fractal-like particles within a range of diameters led to good agreement between modeled and experimental data. The model appears well suited to estimating the relationship between the size distribution of emitted well-mixed ultrafine aerosols, and the aerosol that is ultimately inhaled where diffusion loses are small.     

A Calibration Technique for Improving Refractive Index Retrieval from Aerosol Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy (CRDS) is a technique that is commonly used to measure the extinction of light by aerosol particles in situ. This extinction, when normalized to particle concentration, yields the extinction cross section, a measure of a single particle's ability to scatter and absorb light. The complex index of refraction can then be retrieved by comparison of the extinction cross sections at several particle diameters with those predicted by Mie theory. This approach requires accurate determination of particle diameter and concentration as well as the length of the extinction region in the cavity, but it is often difficult to quantify the systematic errors in the measurements of these quantities. Here, we introduce a calibration technique using particles of a reference compound to account for these systematic errors. The two calibration parameters are: C_f , which scales the measured extinction cross sections, and Δd, which shifts the particle diameters. It is found that C_f correlates strongly with the condensation particle counter (CPC) used to measure particle concentration and that Δd is associated with the differential mobility analyzer (DMA) used to select particle diameters. Calibration is shown to reduce errors of subsequently-measured extinction cross sections of a test aerosol from 11% to with a concomitant improvement in the accuracy of the retrieved complex index of refraction and corresponding atmospheric radiative forcing estimates.

Copyright 2013 American Association for Aerosol Research     

A Computational Fluid Dynamics Study of Particle Penetration through an Omni-Directional Aerosol Inlet

Computational fluid dynamics (CFD) was used to study aerosol penetration through the entrance section of a bell-shaped omni-directional ambient aerosol sampling inlet. The entrance section did not include either an insect screen or a large-particle pre-separator. Simulations of the flow field were carried out for wind speeds of 2, 8, and 24 km/h and a fixed exhaust flow rate of 100 L/min; and, particle tracking was performed for 2 to 20 μ m aerodynamic diameter particles. Penetration calculated from CFD simulations was in excellent agreement with experimental results from previous studies with the root mean square relative error between simulation and experimental data being 3.8%. CFD results showed that the most significant regional particle deposition occurred on the upwind side of a curved flow passage between two concentric axisymmetric shells of the inlet housing and that deposition at the leading edges of the shells and within the exhaust tube was far less significant. At a wind speed of 2 km/h, penetration was affected by gravitational settling, e.g., penetration of 20 μ m particles was 71.9% when gravity was included and 80.4% without gravity. At higher wind speeds gravity had little effect. An empirical equation was developed to relate aerosol penetration to a Stokes number, a gravitational settling parameter, and a velocity ratio. Good fits of the correlation curves to experimental data and numerical results were obtained.     

Simulation of the influence of coarse mode particles on the properties of fine mode particles

The effects of coarse mode particles on aerosol dynamics with simultaneous processes are investigated by applying the modal approach. The Brownian coagulation and condensation processes are considered for an aerosol system with three modal aerosol distributions to simulate how the distribution changes when the coarse mode number concentration increases. Simulation results show that during the Brownian coagulation process, the number concentration in the nuclei and accumulation mode decrease due to inter-modal coagulation of the nuclei, accumulation and coarse modes particles. This inter-modal coagulation process is enhanced when the number concentration of the coarse mode increases, such as in dust storm events. It means that the coarse mode can influence on the nuclei and accumulation mode significantly, especially, when the number concentration of the coarse mode becomes large. For the case of gravitational coagulation, the intra-modal coagulation of coarse mode particles makes the number concentration to decrease, while the inter-modal gravitational coagulation between fine modes and coarse mode is negligible.     

Treatment of size-dependent aerosol transport processes using quadrature based moment methods

The use of moment methods for simulation of aerosol settling and diffusion phenomena in which the settling velocity and diffusion coefficient are functions of the size of the particles leads to difficult computational problems, especially if the moment equations need to be closed. In this study, a simple one dimensional problem of aerosol diffusion and gravitational settling is carried out using quadrature method of moments (QMOM) and the direct quadrature method of moments (DQMOM). Analytical solutions can be obtained for the number density function, and issues related to the integration of the solutions to get the moments are discussed. Comparison of the solutions of the moment equations to the moments obtained from the analytical solutions reveals that solutions depend on the initial choice of moments. Results also indicate that the proper choice of moments of the initial number density function may be a significant factor in obtaining more accurate solutions from QMOM or DQMOM.     

COSIMA—a computer program simulating the dynamics of fractal aerosols

The sectional aerosol behavior code COSIMA simulates the time evolution of the structural, dynamical, and optical properties of airborne agglomerate particles as well as their heterogeneous chemical interactions with reactive trace gases utilizing a formalism based on fractal scaling laws. The modeled processes include diffusion to the walls and sedimentational deposition, Brownian and gravitational coagulation, molecular transport from the gas phase to the accessible particle surface, surface adsorption and reactions, gas phase reactions, and dilution effects due to sampling (e.g. during aerosol chamber experiments). The effect of hydrodynamic interactions and shielding on particle mobility is considered within the framework of the Kirkwood–Riseman theory. Rayleigh–Debye–Gans theory is used to deal with light absorption and scattering. The code is validated against new experimental data on the dynamics of Diesel and graphite spark soot as well as recent theoretical and simulation results. Applying the Kirkwood–Riseman formalism to compute the mobility of fractal like agglomerates significantly enhances coagulation rates as well as wall and depositional loss but does not affect the form of the self preserving size distributions attained in the long time regime if Brownian coagulation dominates the aerosol dynamics.     

The Behavior of Constant Rate Aerosol Reactors

An aerosol reactor is a gaseous system in which fine particles are formed by chemical reaction in either a batch or flow process. The particle sizes of interest range from less than 10 Å (molecular clusters) to 10μm. Such reactors may be operated to study the aerosol formation process, as in a smog reactor, or to generate a product such as a pigment or a catalytic aerosol. Aerosol reactors can be characterized by three temporal or spatial zones or regions of operation for batch and flow reactors, respectively. In zone I, chemical reaction results in the formation of condensable molecular products which nucleate and form very high concentrations of small particles. The number density depends on the concentration of preexisting aerosol. Zone II is a transition region in which the aerosol number concentration levels off as a result of hetergeneous condensation by the stable aerosol. In zone III coagulation becomes sufficiently rapid to reduce the particle number concentration. There may be a zone IV in which agglomerates form. Chemical reaction may continue to generate condensable material throughout the various zones. This paper deals with reactors in which aerosol material is generated at a constant rate. Design parameters of interest are the particle size distribution, number density, surface area, and mass loadings. For ideal systems composed of spherical coalescing particles, these can be predicted theoretically for certain limiting cases. However, the irregular agglomerates which may form in zone IV are more difficult to characterize theoretically.     

A new splitting wavelet method for solving the general aerosol dynamics equation

In this paper, a new and robust splitting wavelet method has been developed to solve the general aerosol dynamics equation. The considered models are the nonlinear integro-partial differential equations on time, size and space, which describe different processes of atmospheric aerosols including condensation, nucleation, coagulation, deposition, sources as well as turbulent mixing. The proposed method reduces the complex general aerosol dynamic equation to two one-dimensional splitting equations in each time interval, and further the wavelet method and the upstream finite difference method are proposed for solving the particle size directional and the spatial directional splitting equations. By the method, the aerosol size spectrum is represented by a combination of Daubechies’ wavelets and substituted into the size-directional splitting equation at each time step. The class of Daubechies’ wavelets in the wavelet-Galerkin scheme as trial and weight functions has the advantages of both compact support and orthonormality which can efficiently simulate the sharp shape distribution of aerosols along the particle size direction. Numerical experiments are given to show the efficient performance of the method.     

The University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC): A Simple Design for Ice Nucleation Studies

A new instrument, the University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC), has been designed to study ice nucleation at low temperatures. Based on previous continuous flow instruments, it is a parallel plate model that minimizes convective instabilities by operating horizontally with the warmer plate on top. A variable position sample injector can account for effects arising from gravitational settling of ice particles that form. The residence time in the chamber can vary between 2.6 to 25 s and ice particle formation is monitored with a two-channel optical particle counter. Observation of homogeneous freezing of 100 nm sulfuric acid aerosols was used to verify the accuracy of the calculated relative humidities (RHs) in the chamber to be ±4%, where we report onset RHs for 0.1% of the particles freezing in the temperature range of 218 to 243 K. We also show that the chamber accurately establishes conditions of water saturation by conducting water uptake studies onto sulfuric acid aerosol at 243 K. The two channel OPC allows for ice and water droplet formation to be distinguished under such conditions. The chamber is a simple, cheap, and small design that can be readily assembled for laboratory studies.     

Error Analysis and Uncertainty in the Determination of Aerosol Optical Properties Using Cavity Ring-Down Spectroscopy,Integrating Nephelometry,and the Extinction-Minus-Scattering Method

Despite the substantial improvements in the measurements of aerosol physical and chemical properties and in the direct and indirect radiative effects of aerosols, there is still a need for studying the properties of aerosols under controlled laboratory conditions to develop a mechanistic and quantitative understanding of aerosol formation, chemistry, and dynamics. In this work, we present the factors that affect measurement accuracy and the resulting uncertainties of the extinction-minus-scattering method using a combination of cavity ring-down spectroscopy (CRDS) and integrating nephelometry at a wider range of optical wavelengths than previously attempted. Purely scattering polystyrene latex (PSL) spheres with diameters from 107–303 nm and absorbing polystyrene spheres (APSL) with 390 nm diameter were used to determine the consistency and agreement, within experimental uncertainties, of CRDS and nephelometer values with theoretical calculations derived from Mie theory for non-absorbing spheres. Overall uncertainties for extinction cross-section were largely 10%–11% and dominated by condensation particle counter (CPC) measurement error. Two methods for determining σ_ext error are described, and they were found to produce equivalent results. Systematic uncertainties due to particle losses, RD cell geometry (R_L), CPC counting efficiency, ring-down regression fitting, blank drift, optical tweezing, and recapturing of forward scattered light are also investigated. The random error observed in this work for absorbing spheres is comparable to previous reported measurements. For both absorbing and non-absorbing spheres, a statistical framework is developed for including the contributions to random error due to CPC measurement uncertainty, R_L, statistical fluctuations in particle counts, fluctuations in the blank, and mass flow controller flow error.

Copyright 2014 American Association for Aerosol Research     

Modeling of the monomer role and the coalescence limitation in primary particle growth

The general dynamic equation (GDE) has been numerically solved to simulate the growth of ultrafine particles (UFPs) in a tubular aerosol reactor, approximating the particle size distribution by a lognormal function. The GDE includes all the terms describing diffusion, thermophoresis, nucleation, condensation and coagulation. We have also considered the efficiency of liquid-like coagulation to primary particles. The data calculated from our model were compared with those from the previous model and also with some experimental results from a TiO₂ UFP generator. The condensation term, which we split from a single coagulation term in the previous model, well described the monomer contribution to the particle growth. Introduction of one adjustable parameter, the efficiency of coagulation, was successful in limiting the growth of primary particles and fit the experimental data.     

OPTICAL PROPERTIES OF AEROSOLS

ABSTRACT

The evolution of small aerosol particles accompanying the combustion of straw for energy production is investigated. A sampling equipment specially designed for field measurements is described and characterized. The aerosol is studied by low-pressure cascade impactor and scanning mobility particle sizer, the particle morphology by transmission electron microscopy, and the chemical composition by energy dispersive x-ray analysis. The combustion gas contains 3–500 mg/Nm³ of submicron particles with a mean diameter of approximately 0.3 μm. The particles consist of almost pure potassium chloride and sulphate. The formation mechanism is analyzed by a theoretical simulation of the chemical reactions and the aerosol change during cooling of the flue gas. It is concluded that some sulphation of KC1 occurs in the gas phase although the sulphate concentration is much lower than predicted by an equilibrium assumption. The theoretical simulation proves that the fine mode particles can be formed by homogeneous nucleation of either KCl or K2SO₄ as the first step and further growth occurs by coagulation and diffusive condensation of both KC1 and K₄SO₄ on existing particles.     

Mean field theory for condensation on aerosols and application to multi-component organic vapours

The timescale is calculated for a particle to equilibrate by vapour condensation from a surrounding volume equal to the volume per particle in an aerosol, and is compared to the timescale to transport vapour by diffusion to neighbouring particle volumes. For practically all aerosols, the diffusive timescale is much smaller, showing that vapour diffusion, and, in the same way heat conduction, ensure that the vapour concentration and temperature in the vapour–gas mixture assume mean field values with which the whole of the aerosol equilibrates. Recent claims that individual particles equilibrate with the mixture are refuted. The concept is applied to obtain equations for the condensation of organic vapours whose equilibrium with condensate is governed by absorption partitioning coefficients together with a Kelvin term at small sizes. These dynamic equations, which contain vapour production and condensational loss terms, have steady-state solutions when these terms are changing slowly with time. Such solutions are obtained for non-volatile and semi-volatile constituents, their difference being defined to be that the equilibrium concentration is small compared to the actual concentration in the non-volatile case. For semi-volatile material, the concentration will generally be maintained at a value close to equilibrium over plane surfaces, so that it cannot contribute to nucleation and growth at small sizes. As for water condensation in the atmosphere, particles need to reach a certain size to be activated for growth by condensation of semi-volatile organics.