RAMAN STUDIES OF AEROSOL CHEMICAL REACTIONS

Since Professor Matijevité and his colleagues published pioneering work on aerosol chemical reactions, based on experiments with monodisperse aerosol generators and laminar flow reactors, there has been considerable progress in the chemical characterization of aerosol particles and the study of their chemical reactions. This paper surveys recent developments and new research on the application of Raman spectroscopy to gas/liquid and gas/solid aerosol reactions. Of particular interest are applications of the vibrating orifice aerosol generator and electrodynamic and optical levitators coupled to Raman spectrometers to explore aerosol chemistry. The systems examined include the production of polymeric microsphcrcs, the generation of metal oxide particles from alkoxide droplets, SQ2/sorbent particle reactions used for demilitarization of stick gases, chemical characterization of particle arrays, and reactions following collisions of dissimilar particles. The complications associated with the interpretation of Raman data introduced by morphology-dependent resonances in the elastically scattered light are also examined.     

Deliquescence and Efflorescence Processes of Aerosol Particles Studied by in situ FTIR and Raman Spectroscopy

Deliquescence and efflorescence are the two most important physicochemical processes of aerosol particles. In deliquescence and efflorescence cycles of aerosol particles, many fundamental problems need to be investigated in detail on the molecular level, including ion and molecule interactions in supersaturated aerosols, metastable solid phases that may be formed, and microscopic structures and deliquescence mechanisms of aerosol particles. This paper presents a summary of the progress made in recent investigations of deliquescence and e2orescence processes of aerosol particles by four common spectral techniques, which are known as Raman/electrodynamic balance, Fourier transform infrared/aerosol flow tube, Fourier transforminfrared/attenuated total reflection, and confocal Raman on a quartz substrate.     

Thermophoresis of spherical and non-spherical particles: a review of theories and experiments

Thermophoresis is an important mechanism of micro-particle transport due to a temperature gradient in the surrounding medium and has found numerous applications, especially in the field of aerosol technology. Extensive studies, both theoretical and experimental, have been done to understand the nature of this phenomenon. However, it is clear that a lot more of work needs to be done before we can predict thermophoresis accurately for any given gas-particle system as well as particle shape and orientation in any flow regime. This paper reviews the existing theories and data in two major categories, for spherical particles and for non-spherical particles, as well as the various techniques in making thermophoresis measurements. The current state of development for thermophoresis studies is that for spheres the theories and experimental data agree with each other fairly well but for non-spherical particles in the transition regime the theories are yet to be developed and experimental data showing the effect of particle shape are much needed in all Knudsen number range. The best techniques of thermophoretic force measurements involve the use of electrodynamic balances to work on single micro-particles and the use of microgravity to minimize the effect of convection. A combination of the above two has not been attempted and should provide the most accurate data.     

Exploring the complexity of aerosol particle properties and processes using single particle techniques

The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.     

Determination of evaporation rates and vapor pressures of very low volatility compounds: a study of the C4-C10 and C12 dicarboxylic acids

A method for the measurement of evaporation rates and vapor pressures of low volatility compounds was developed and applied to the homologous series of C4-C10 and C12 dicarboxylic acids. Proton-transfer chemical ionization mass spectrometry was used to follow directly the temperature-dependent evaporation rates of aerosol samples collected on a cold plate that could be heated at a known rate. The vapor pressures of the deposited compounds were derived from observed evaporation rates through application of the Hertz-Knudsen equation. Temperature programmed desorption allowed for quantification of the enthalpy (DeltaHsub) and entropy (DeltaSsub) of sublimation of the diacids and is described. A strong odd-even dependence with respect to the total carbon number was observed in the derived diacid vapor pressures, consistent with previous measurements. However, the vapor pressures from this method were systematically lower than previous measurements. Though seen in the vapor pressure, no odd-even carbon chain length dependence was readily discernible in the measured values of DeltaHsub and DeltaSsub. Perhaps most importantly, these experimental results also suggest that residual solvent molecules (from the aerosol generation process) trapped in the diacid samples can have a considerable influence on the measured thermodynamic parameters and, if not properly accounted for, may give erroneous results.     

Electronic excitations in many-particle systems: a quantitative analysis

Small particles, produced in usual experiments, commonly form many-particle systems. Interactions of various kinds among the particles influence the properties of such systems. We consider here the optical properties of noble metal particle systems with separated nearly spherical particles and with aggregates formed by induced coagulation of the single particles. In order to describe their optical extinction we apply electrodynamic interaction models on particles and aggregates. We perform a quantitative analysis of the extinction spectrum of one selected sample with the electrodynamic interaction model.     

Probing the evaporation of ternary ethanol-methanol-water droplets by cavity enhanced Raman scattering

Cavity enhanced Raman scattering is used to characterise the evolving composition of ternary aerosol droplets containing methanol, ethanol and water during evaporation into a dry nitrogen atmosphere. Measurements made using non-linear stimulated Raman scattering from these ternary alcohol-water droplets allow the in situ determination of the concentration of the two alcohol components with high accuracy. The overlapping spontaneous Raman bands of the two alcohol components, arising from C-H stretching vibrational modes, are spectrally-resolved in stimulated Raman scattering measurements. We also demonstrate that the evaporation measurements are consistent with a quasi-steady state evaporation model, which can be used to interpret the evaporation dynamics occurring at a range of pressures at a particular evaporation time.     

In-situ atomic force microscopy investigation of aerosols exposed to different humidities

In-situ atomic force microscopy (AFM) studies were performed on aerosol samples showing the potential of a topochemical approach for gaining information on chemical and physical aerosol properties. The behavior of single sub-micron particles has been investigated with respect to changing humidity in the surrounding atmosphere. Volume calculations allowed monitoring of these changes on a quantitative basis. As expected these in-situ experiments showed the restructuring of particles with highly agglomerated chain-like structures induced by condensation and evaporation on a nanometer scale. The particle volumes decreased as the branched chain-like structure changed into a more regular clump-like structure. The degree of restructuring was clearly depending on the chemical surface properties as could be proven for soot-like test aerosol particles. The collapse of the chain-like structure on a nanometer scale was found to be significantly more pronounced for soot particles previously exposed to ozone. Furthermore, in-situ studies were performed on ammonium sulfate test aerosol. Though a distinct deliquescence point typical for salts could not be detected, neither in the topography nor in the phase image, ammonium sulfate test aerosol particles seemed to partially dissolve in humid atmosphere and hence to decrease in volume. Thus, the volume decrease induced by purging with humid nitrogen and subsequent drying which was also observed for a considerable fraction of urban aerosol, could be interpreted in terms of composition and surface properties considering the geometrical structure (i.e. state of agglomeration) of the particles.     

In-situ atomic force microscopy investigation of aerosols exposed to different humidities

In-situ atomic force microscopy (AFM) studies were performed on aerosol samples showing the potential of a topochemical approach for gaining information on chemical and physical aerosol properties. The behavior of single sub-micron particles has been investigated with respect to changing humidity in the surrounding atmosphere. Volume calculations allowed monitoring of these changes on a quantitative basis. As expected these in-situ experiments showed the restructuring of particles with highly agglomerated chain-like structures induced by condensation and evaporation on a nanometer scale. The particle volumes decreased as the branched chain-like structure changed into a more regular clump-like structure. The degree of restructuring was clearly depending on the chemical surface properties as could be proven for soot-like test aerosol particles. The collapse of the chain-like structure on a nanometer scale was found to be significantly more pronounced for soot particles previously exposed to ozone. Furthermore, in-situ studies were performed on ammonium sulfate test aerosol. Though a distinct deliquescence point typical for salts could not be detected, neither in the topography nor in the phase image, ammonium sulfate test aerosol particles seemed to partially dissolve in humid atmosphere and hence to decrease in volume. Thus, the volume decrease induced by purging with humid nitrogen and subsequent drying which was also observed for a considerable fraction of urban aerosol, could be interpreted in terms of composition and surface properties considering the geometrical structure (i.e. state of agglomeration) of the particles. Received: 14 January 1999 / Revised: 1 March 1999 / Accepted: 4 March 1999     

Measurement of vapor pressures and heats of sublimation of dicarboxylic acids using atmospheric solids analysis probe mass spectrometry

Vapor pressures of low volatility compounds are important parameters in several atmospheric processes, including the formation of new particles and the partitioning of compounds between the gas phase and particles. Understanding these processes is critical for elucidating the impacts of aerosols on climate, visibility, and human health. Dicarboxylic acids are an important class of compounds in the atmosphere for which reported vapor pressures often vary by more than an order of magnitude. In this study, atmospheric solids analysis probe mass spectrometry (ASAP-MS), a relatively new atmospheric pressure ionization technique, is applied for the first time to the measurement of vapor pressures and heats of sublimation of a series of dicarboxylic acids. Pyrene was also studied because its vapor pressures and heat of sublimation are relatively well-known. The heats of sublimation measured using ASAP-MS were in good agreement with published values. The vapor pressures, assuming an evaporation coefficient of unity, were typically within a factor of ～3 lower than published values made at similar temperatures for most of the acids. The underestimation may be due to diffusional constraints resulting from evaporation at atmospheric pressure. However, this study establishes that ASAP-MS is a promising new technique for such measurements.     

Determination of Thermodynamic Parameters from Evaporation of Binary Microdroplets of Volatile Constituents

An experimental technique based on a modified vibrating orifice aerosol generator has been employed to study unsteady evaporation of linear streams of highly monodisperse binary microdroplets of volatile constituents over short time periods (i.e., <1 ms), such that the droplet composition remains nearly constant. The droplet size and temperature (i.e., refractive index) have been determined with high temporal resolution from the resonances observed in the simultaneous elastic and Raman light scattering spectra obtained by varying the droplet size through modulation of droplet generation frequency. By using this technique we show that thermodynamic parameters of binary systems, such as activity coefficients as well as vapor pressures of the constituents as functions of temperature, can be determined. We have applied the procedure to study unsteady evaporation rates of pure ethanol and methanol droplets as well as binary droplets containing various ratios of ethanol and methanol. We have obtained vapor pressures of ethanol and methanol as functions of temperature as well as activity coefficients of ethanol and methanol as functions of composition, and the results show excellent agreements with the values reported in the literature. The technique presented in this paper is applicable to any binary system containing at least one volatile constituent. Copyright 2000 Academic Press.     

The effect of humidity on the ozonolysis of unsaturated compounds in aerosol particles

Atmospheric aerosol particles are important in many atmospheric processes such as: light scattering, light absorption, and cloud formation. Oxidation reactions continuously change the chemical composition of aerosol particles, especially the organic mass component, which is often the dominant fraction. These ageing processes are poorly understood but are known to significantly affect the cloud formation potential of aerosol particles. In this study we investigate the effect of humidity and ozone on the chemical composition of two model organic aerosol systems: oleic acid and arachidonic acid. These two acids are also compared to maleic acid an aerosol system we have previously studied using the same techniques. The role of relative humidity in the oxidation scheme of the three carboxylic acids is very compound specific. Relative humidity was observed to have a major influence on the oxidation scheme of maleic acid and arachidonic acid, whereas no dependence was observed for the oxidation of oleic acid. In both, maleic acid and arachidonic acid, an evaporation of volatile oxidation products could only be observed when the particle was exposed to high relative humidities. The particle phase has a strong effect on the particle processing and the effect of water on the oxidation processes. Oleic acid is liquid under all conditions at room temperature (dry or elevated humidity, pure or oxidized particle). Thus ozone can easily diffuse into the bulk of the particle irrespective of the oxidation conditions. In addition, water does not influence the oxidation reactions of oleic acid particles, which is partly explained by the structure of oxidation intermediates. The low water solubility of oleic acid and its ozonolysis products limits the effect of water. This is very different for maleic and arachidonic acid, which change their phase from liquid to solid upon oxidation or upon changes in humidity. In a solid particle the reactions of ozone and water with the organic particle are restricted to the particle surface and hence different regimes of reactivity are dictated by particle phase. The potential relevance of these three model systems to mimic ambient atmospheric processes is discussed.     

Experimental manifestations of the correlation between the local structure of silver nanoparticle aggregates and their absorption spectra

Experimental data testifying in favor of the correlation between electrodynamic characteristics of fractal aggregates and the structure of the local environment of particles are discussed. One of the possible variants of the control of local structure of disordered silver nanoparticle aggregates placed into a polymer matrix via a significant decrease in its volume is proposed and realized. An indirect method for the detection of the variations in the local structure with the use of plasmon absorption spectra is proposed and substantiated. The evolution of local anisotropy of the environment of the particles of loose aggregates during their compaction is studied. Differences in the absorption spectra of silver nanoaggregates in hydrosols with various concentrations of water-soluble polymer that determine the properties of the local structure of aggregates are discussed, these differences being manifested upon the formation of fragments with quasi-ordered arrangement of particles and in the absence of such fragments.     

气溶胶粒子通过填充柱的保留时间分布测定

采用亚微米单分散聚苯乙烯球形硬气溶胶粒子和脉冲进样技术,测定了气溶胶粒子通过无规则石英砂填充柱的保留时间分布,从保留时间分布曲线得到了气溶胶粒子在填充柱中的平均保留时间和穿透率.研究了平均保留时间和穿透率与流体流速、填充柱的长度、填料粒度和气溶胶粒子大小之间的关系.研究发现,流速越大,保留时间分布曲线越尖锐,流速越小,保留时间分布曲线越平坦;气溶胶粒子的穿透率随着柱长的增加而降低,随流速、气溶胶粒子粒径和石英砂颗粒大小的减小而减小;平均保留时间随柱长增加而增大,随流速增大而减小,随气溶胶粒子粒径减小而减小,而与石英砂颗粒大小几乎无关.     

Ultra-slow water diffusion in aqueous sucrose glasses

We present measurements of water uptake and release by single micrometre-sized aqueous sucrose particles. The experiments were performed in an electrodynamic balance where the particles can be stored contact-free in a temperature and humidity controlled chamber for several days. Aqueous sucrose particles react to a change in ambient humidity by absorbing/desorbing water from the gas phase. This water absorption (desorption) results in an increasing (decreasing) droplet size and a decreasing (increasing) solute concentration. Optical techniques were employed to follow minute changes of the droplet's size, with a sensitivity of 0.2 nm, as a result of changes in temperature or humidity. We exposed several particles either to humidity cycles (between ～2% and 90%) at 291 K or to constant relative humidity and temperature conditions over long periods of time (up to several days) at temperatures ranging from 203 to 291 K. In doing so, a retarded water uptake and release at low relative humidities and/or low temperatures was observed. Under the conditions studied here, the kinetics of this water absorption/desorption process is controlled entirely by liquid-phase diffusion of water molecules. Hence, it is possible to derive the translational diffusion coefficient of water molecules, D(H(2)O,) from these data by simulating the growth or shrinkage of a particle with a liquid-phase diffusion model. Values for D(H(2)O)-values as low as 10(-24) m(2) s(-1) are determined using data at temperatures down to 203 K deep in the glassy state. From the experiment and modelling we can infer strong concentration gradients within a single particle including a glassy skin in the outer shells of the particle. Such glassy skins practically isolate the liquid core of a particle from the surrounding gas phase, resulting in extremely long equilibration times for such particles, caused by the strongly non-linear relationship between concentration and D(H(2)O). We present a new parameterization of D(H(2)O) that facilitates describing the stability of aqueous food and pharmaceutical formulations in the glassy state, the processing of amorphous aerosol particles in spray-drying technology, and the suppression of heterogeneous chemical reactions in glassy atmospheric aerosol particles.     

A Method For Measuring The Vapour Pressures Of Pah'S Adsorbed On The Surface Of Soot

Abstract

A new method is reported that utilizes a low-temperature quartz microbalance in combination with Knudsen cells to obtain adsorption isotherms for substances with very low equilibrium pressures (< 10^?5 Pa). The method allows for the first time to measure directly thermodynamic properties of important aerosol/pollutant systems at typical environmental temperatures. Application is demonstrated by an investigation of fluoranthene and pyrene adsorbed on carbon aerosol particles (T = 293.15 K?302.13 K). Both PAHs are unable to form extended multilayers on the surface. Slightly above monolayer coverage the three-dimensional crystalline solid phase is thermodynamically more stable.     

Calibration of Polarization-Sensitive and Dual-Angle Laser Light Scattering Methods Using Standard Latex Particles

A polarization-sensitive laser light scattering (PSLLS) method and a dual-angle laser light scattering (DALLS) method have been studied for in situ measurement of submicrometer hydrosol and aerosol particles. By using standard monodisperse polystyrene latex particles suspended in water and air as test particles, calibration of systems built based on the above methods have been performed. The effects of light scattered by agglomerated aerosol particles (multiplets) were corrected by considering the fraction of multiplets as determined with an aerosol measurement technique using a differential mobility analyzer. The change in the measured intensities of scattered light with particle diameter was then determined by calculations based on Mie theory. It was shown that the PSLLS system can determine particle diameters as small as approximately 60 nm for the test hydrosol particles and approximately 100 nm for test aerosol particles, respectively. The DALLS system can determine smaller diameters than the PSLLS system for test particles with no light absorption. The change in scattered light intensities with particle diameter was also investigated by theoretical calculations with various refractive indexes and scattering angles. The PSLLS and DALLS systems promise to become routine measurement tools for absorbing and nonabsorbing particles, respectively. Copyright 2001 Academic Press.     

Kinetic Features of Aerosol Formation in the Branched Chain Process of Dichlorosilane Oxidation under Static Conditions: I. Initiated Ignition

It is found experimentally that the initial pressure of aerosol formation progressively decreases with an increase in the initial concentration of dichlorosilane in the initiated ignition of dichlorosilane mixtures with oxygen at 293 K. The dependence of the maximal aerosol concentration on the total pressure is S-shaped. A generalized kinetic scheme is proposed that qualitatively describes the regularities observed in the experiments. The most important calculated parameters are the heat evolved in the chain process and the dependence of the pressure of the saturated vapor of a new phase on temperature. It is shown that the specific features of branched-chain processes under nonisothermic conditions determine the kinetic regularities of new phase formation. The optimal range of pressures is recommended for obtaining particles with as low dispersivity as possible.     

Implementation of dynamical nucleation theory effective fragment potentials method for modeling aerosol chemistry

In this work, the dynamical nucleation theory (DNT) model using the ab initio based effective fragment potential (EFP) is implemented for evaluating the evaporation rate constant and molecular properties of molecular clusters. Predicting the nucleation rates of aerosol particles in different chemical environments is a key step toward understanding the dynamics of complex aerosol chemistry. Therefore, molecular scale models of nanoclusters are required to understand the macroscopic nucleation process. On the basis of variational transition state theory, DNT provides an efficient approach to predict nucleation kinetics. While most DNT Monte Carlo simulations use analytic potentials to model critical sized clusters, or use ab initio potentials to model very small clusters, the DNTEFP Monte Carlo method presented here can treat up to critical sized clusters using the effective fragment potential (EFP), a rigorous nonempirical intermolecular model potential based on ab initio electronic structure theory calculations, improvable in a systematic manner. The DNTEFP method is applied to study the evaporation rates, energetics, and structure factors of multicomponent clusters containing water and isoprene. The most probable topology of the transition state characterizing the evaporation of one water molecule from a water hexamer at 243 K is predicted to be a conformer that contains six hydrogen bonds, with a topology that corresponds to two water molecules stacked on top of a quadrangular (H(2)O)(4) cluster. For the water hexamer in the presence of isoprene, an increase in the cluster size and a 3-fold increase in the evaporation rate are predicted relative to the reaction in which one water molecule evaporates from a water hexamer cluster.     

The effect of multi-component aerosol particles on quantitative laser-induced breakdown spectroscopy: Consideration of localized matrix effects

Spectral measurements were performed in a laser-induced plasma to assess the changes in sodium or magnesium analyte emission response from particle-derived sources with the addition of concomitant mass to the aerosol particles. Temporally resolved measurements revealed up to a 50% enhancement in analyte emission with the addition of the elements copper, zinc or tungsten at mass ratios from 1:9 to 1:19, although the enhancement generally diminished by delay times of 60 μs. Additional measurements in magnesium–cadmium aerosol particles were performed to assess the temporal profile of plasma temperature in the spatial vicinity of the aerosol particles using the ion-to-neutral emission ratios. These measurements revealed a general increase in localized plasma temperature with increasing delay time, which is attributed with an initial suppression of plasma temperature about the aerosol particles as plasma energy is required to vaporize and ionize the aerosol particle mass. These measurements provide direct evidence of a matrix effect for aerosol particles, which is attributed primarily to perturbations in the localized plasma properties. These perturbations are minimized at longer plasma delay times; hence quantitative LIBS analysis of aerosol particles should be performed with careful attention given to the temporal plasma evolution. The data further elucidate the complex interactions between the plasma gas and the aerosol particles, during which the finite time-scales of particle dissociation, and heat and mass transfer are equally important.