Condensable molecules generated in the gas phase by chemical reaction can either form new particl... more Condensable molecules generated in the gas phase by chemical reaction can either form new particles or condense on existing aerosol particles. Experiments were carried out to study the effect of a preexisting aerosol on aerosol dynamics in such systems. Secondary sulfate aerosol was produced by photochemical tcactions in the SO,-NO,-propylene system. A new theory which predicts the rate at which new particles of a given sixe are formed is also presented. Scavenging of molecular clusters by surrounding aerosols can be important, and is incorporated in the theory. Measured rates of new particle formation are compared with predicted rates, taking into account the minimum parti& size detected experimentally. Agreement between theory and experiment is within experimental error. Based on the theoretical analysis, a crkion for determining whether or not new particle formation is important in such systems is established.
The sorption of vapor molecules onto pre-existing nanometer sized clusters is of importance in un... more The sorption of vapor molecules onto pre-existing nanometer sized clusters is of importance in understanding particle formation and growth in gas phase environments and devising gas phase separation schemes. Here, we apply a differential mobility analyzer-mass spectrometer based approach to observe directly the sorption of vapor molecules onto iodide cluster ions of the form (MI)xM(+) (x = 1-13, M = Na, K, Rb, or Cs) in air at 300 K and with water saturation ratios in the 0.01-0.64 range. The extent of vapor sorption is quantified in measurements by the shift in collision cross section (CCS) for each ion. We find that CCS measurements are sensitive enough to detect the transient binding of several vapor molecules to clusters, which shift CCSs by only several percent. At the same time, for the highest saturation ratios examined, we observed CCS shifts of up to 45%. For x < 4, cesium, rubidium, and potassium iodide cluster ions are found to uptake water to a similar extent, while s...
Recent developments in atmospheric aerosol measurements are reviewed. The topics included complem... more Recent developments in atmospheric aerosol measurements are reviewed. The topics included complement those covered in the recent review by Chow (JAWMA 45: 320–382, 1995) which focuses on regulatory compliance measurements and filter measurements of particulate composition. This review focuses on measurements of aerosol integral properties (total number concentration, CCN concentration, optical coefficients, etc.), aerosol physical chemical properties (density, refractive index, equilibrium water content, etc.), measurements of aerosol size distributions, and measurements of size-resolved aerosol composition. Such measurements play an essential role in studies of secondary aerosol formation by atmospheric chemical transformations and enable one to quantify the contributions of various species to effects including light scattering/absorption, health effects, dry deposition, etc. Aerosol measurement evolved from an art to a science in the 1970s following the development of instrumentation to generate monodisperse calibration aerosols of known size, composition, and concentration. While such calibration tools permit precise assessments of instrument responses to known laboratory-generated aerosols, unquantifiable uncertainties remain even when carefully calibrated instruments are used for atmospheric measurements. This is because instrument responses typically depend on aerosol properties including composition, shape, density, etc., which, for atmospheric aerosols, may vary from particle-to-particle and are often unknown. More effort needs to be made to quantify measurement accuracies that can be achieved for realistic atmospheric sampling scenarios. The measurement of organic species in atmospheric particles requires substantial development. Atmospheric aerosols typically include hundreds of organic compounds, and only a small fraction (∼10%) of these can be identified by state-of-the-art analytical methodologies. Even the measurement of the total particulate organic carbon mass concentration is beset by difficulties including the unknown extent of evaporative losses during sampling, adsorption of gas-phase organic compounds onto sampling substrates, and the unknown relationship between carbon mass and mass of the particulate organics. The development of improved methodologies for such measurements should be a high priority for the future. Mass spectrometers that measure the composition of individual particles have recently been developed. It is not clear that these instruments will provide quantitative information on species mass concentrations, and more work is needed to routinely interpret the vast quantities of data generated during field sampling. Nevertheless, these instruments substantially expand the range of atmospheric aerosol issues that can be explored experimentally. These instruments represent the most significant advance in aerosol instrumentation in recent years.
... j cluster (j integer 1); r, dimensionless time; c#, dimensionless free molecule collision fre... more ... j cluster (j integer 1); r, dimensionless time; c#, dimensionless free molecule collision fre quency ... While the drop above some critical value of L is consistent with the present calculations ... formation tends to be dominated by coalescence of particles that are greatly different in size. ...
A simplified theoretical model is proposed for evaluating the dynamic behavior of photochemically... more A simplified theoretical model is proposed for evaluating the dynamic behavior of photochemically generated aerosols in the SO2-air system. The key simplifying assumption is that evaporation rates are negligible compared with condensation rates for molecular clusters and particles of all sizes. The theory accounts for the formation of condensable molecules (monomer) by chemical reaction, and coagulation of these monomer with clusters of condensed molecules to form larger particles. Cluster coagulation is also included. For a constant rate of monomer production, the equations can be cast in a dimensionless form which is independent of the rate of monomer formation by chemical reaction. These dimensionless equations have been solved numerically, both including and neglecting the effect of London-van der Waals forces on the coagulation rate. Agreement between theory and the data of W. E. Clark ("Measurements of Aerosols Produced by the Photochemical Oxidation of SOs in Air," Ph.D. thesis, University of Minnesota, Minneapolis, 1972) and G. Madelaine, M. Perrin, and A. Renoux (J. Aerosol Sci. 12, 202 (1979)) is good, particularly when London-van der Waals forces are considered. Because the solutions are presented in dimensionless form, direct application of these results for arbitrary rates of aerosol formation and for aerosols with arbitrary physical properties is possible.
Abstract The tandem differential mobility analyzer (TDMA) is useful for studying phenomena that l... more Abstract The tandem differential mobility analyzer (TDMA) is useful for studying phenomena that lead to size changes in submicron aerosol particles. Evaporation, condensation and chemical reactions within airborne droplets are examples of such phenomena. With this method, aerosols of a known size are selected with a differential mobility analyzer (DMA). These monodisperse aerosols then undergo processes that result in growth or shrinkage; the extent of growth or shrinkage is determined with a second DMA. In this paper, a theoretical treatment of a TDMA system is presented, and procedures for laboratory implementation are discussed. The theory is presented in an analytic form that can be encoded for on-line or off-line data analysis. Statistical techniques are used to calculate the precision with which size changes can be determined, and results of these analyses are confirmed experimentally. It is shown that diameter changes can be measured with a precision of about 0.3% using available instrumentation. For aerosols in the 0.01–0.2 μm diameter range where the technique works best, this corresponds to diameter changes of 0.03–0.6 nm, or the characteristic thickness of a monolayer.
Size distributions of urban Atlanta, Georgia, aerosols (0.003-2 microm) were measured from August... more Size distributions of urban Atlanta, Georgia, aerosols (0.003-2 microm) were measured from August 1, 1998 through August 30, 2000 as part of the Aerosol Research Inhalation Epidemiology Study (ARIES). Size distributions were measured five times per hour, and approximately 50,000 size distributions were measured during the 25-month study. This paper focuses on salient features of the sub-100-nm data. We examine concentrations of particles in six equally spaced logarithmic intervals and show that particles of different sizes have distinctly different behaviors. For particles between 10 and 100 nm, average concentrations tended to be highest during winter, during rush hour, and on week days. Concentrations of particles in the 3-10-nm range were elevated in the summer due to photochemically driven nucleation, and also during winter. We hypothesize that the elevated wintertime concentrations of these particles were associated with nucleation that occurs as vehicular emissions mix with the cool ambient air. In any given size range, distributions of concentrations tend to be lognormal, but significant deviations from lognormality were occasionally observed. For particles in the 3.2-5.6-nm diameter range, deviations were apparent in the summer when very high concentrations (up to 10(6) cm(-3)) were produced by photochemically driven nucleation. During 2 months of the study, deviations from lognormality for particles in the 32-56-nm diameter range occurred when anomalously high concentrations of 40-nm particles were observed.
Large smog chambers (-60 m3) constructed of FEP Teflon film are frequently used to study photoche... more Large smog chambers (-60 m3) constructed of FEP Teflon film are frequently used to study photochemistry and aerosol formation in model chemical systems. In a previous paper (6) a theory for aerosol wall loss rates in Teflon film smog chambers was developed; predicted particle loss rates were in good agreement with measured rates. In the present paper, measurements of wall deposition rates and the effects of wall losses on measurements of gas-to-particle conversion in smog chambers are discussed. Calculations indicate that a large fraction (up to 83%; typical values of 33-70%) of the aerosol formed in several smog chamber experiments was on the chamber walls at the end of the experiment. Estimated values for particulate organic carbon yield for several precursor hydrocarbons increased by factors of 1.3-6.0 when wall deposition was taken into account. The theory is also extended to loss rates of gaseous species. Such loss rates are either limited by diffusion through a concentration boundary layer near the surface or by uptake at the surface. It is shown that for a typical 60-m3 Teflon film smog chamber, gas loss rates are limited by surface reaction rates if mass accommodation coefficients are less than 6 X lo4. It follows that previously reported loss rates of several gases in a chamber of this type (12) were limited by surface reactions.
The design and function of a device that regulates the relative humidity of an ambient aerosol sa... more The design and function of a device that regulates the relative humidity of an ambient aerosol sample is described. We use this RH controller upstream of MOUDI impactors to permit sampling at relative humidities in the 70–80% range. Humidity control is achieved by allowing the aerosol to approach equilibrium with a saturated salt solution. Benefits to sampling with impactors in
ABSTRACT The Community Multiscale Air Quality (CMAQ) model is widely used in air quality manageme... more ABSTRACT The Community Multiscale Air Quality (CMAQ) model is widely used in air quality management and scientific investigation. Numerous studies have been conducted investigating how well CMAQ simulates fine particle mass concentrations, but relatively few studies have addressed how well CMAQ simulates fine particle number distribution. Accurate simulation of particle number concentrations is important because particle number and surface area concentrations may be directly related to human health and visibility. Simulated fine particle number concentrations derived using CMAQ are compared to measurements to identify problems and to improve model performance. Evaluation is done using measured particle number concentrations in Atlanta, Georgia, from 1/1/1999 to 8/31/2000. While homogeneous binary nucleation mechanism used in CMAQ needs to be modified for better prediction of particle number concentrations, there are also other factors that affect the predicted particle level. Assumed particle size of the primary emissions in CMAQ causes number concentrations to be significantly underestimated, while particle density has a small impact. Assuming particle size distributions by three lognormal modes cannot accurately simulate particles with size less than 0.01 μ m, particularly during nucleation events. An additional mode that accounts for particles smaller than 0.01 μ m can improve the accuracy of the number concentration simulations. Though, the use of the Expectation-Maximization (EM) algorithm to estimate size distribution parameters of measured particles suggests that assumed parameters for the lognormal modes in CMAQ are generally reasonable.
This is an instruction manual for the aerodynamic lens design tool: the "Aerodynamic Lens Calcula... more This is an instruction manual for the aerodynamic lens design tool: the "Aerodynamic Lens Calculator". We explain how to install and use this software. Examples are provided to use this tool to design or test a lens system. The structures of the source code programming are also provided in this manual. The design tool and its manual mentioned in this article are available in the publisher's online edition of Aerosol Science and Technology. To access this file, click on the link for this issue, then select this article. In order to access the full article on-line, you must either have an institutional subscription or a member subscription accessed through www.aaar.org.
We describe a process in which nanosize particles with a narrow size distribution are generated b... more We describe a process in which nanosize particles with a narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements, by calorimetric energy balances, and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nm. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 nm or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the e.~:periments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiatect by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1996
We have recently built a particle beam mass spectrometer (PBMS) for measuring ultrafine particle ... more We have recently built a particle beam mass spectrometer (PBMS) for measuring ultrafine particle size distributions (0.005–0.25 μm) at low pressures (≳100 mTorr). The PBMS is being used to study nucleation and growth in low pressure chemical vapor deposition processes relevant to the production of semiconductor devices. In this article, the function and performance of the PBMS is summarized, and results of measurements made while depositing polysilicon and silicon dioxide films in tube furnaces are discussed. Measurements made during deposition of polysilicon films showed that there was a critical reactor pressure below which particles were not present; this critical pressure varied in proportion to the residence time in the reactor, and was insensitive to reactor temperature. Above the critical pressure, however, the concentration of particles produced was sensitive to reactor temperature. The average particle size was in the 0.003–0.03-μm-diam range, with concentrations of ∼104 cm...
Journal of the Air & Waste Management Association, 2000
We summarize the results from the various measurements and the inter-sampler comparisons from Sou... more We summarize the results from the various measurements and the inter-sampler comparisons from Southeastern Aerosol and Visibility Study (SEAVS), a study with one of its objectives to test for closure among chemical, gravimetric and optical measurements of atmospheric aerosol particles. Sulfate and organics are the dominant components of the SEAVS fine particles (nominally, particles with aerodynamic diameter &lt; or = 2.5 microns) but between 28 and 42% (range over various samplers) of the gravimetrically measured total fine particle concentration is unidentified by the chemical measurements. Estimates of water associated with inorganic components and measurement imprecision do not totally explain the observed difference between gravimetric and chemical measurements. We examine the theoretical and empirical basis for assumptions commonly made in the published literature to extrapolate total fine particle concentration on the basis of chemical measurements of ions, carbon and elements. We then explore the more general question of closure using the SEAVS data as well as data from other, similar studies reported in the literature. In so combining the SEAVS measurements with other similar studies, we find a strong association between organic carbon and the unidentified component, that is, the fraction of the total fine particle concentration not identified by chemical measurements. We offer several tenable hypotheses for the relationship between the organic and unidentified components that deserve to be tested in future work. Specifically, we hypothesize that (1) errors in the sampling and analysis of organic carbon; (2) estimates of organic mass from measurements of organic carbon; and/or (3) water absorption by organics may all contribute to the observed relationship.
Applied Occupational and Environmental Hygiene, 1996
An experimental study was performed to determine the feasibility of optically distinguishing coal... more An experimental study was performed to determine the feasibility of optically distinguishing coal particles from water droplets for the purpose of reducing the mass concentration artifact from water sprays and steam reported for optical dust monitors. A single-particle multiangle optical detector was used to measure the scattering of monochromatic, circularly polarized light by mixtures of nonspherical bituminous coal dust particles and 0.5 pm dioctyl sebacate (DOS) droplets, which were used for modeling small, spherical water droplets. A total of 20 different mixtures with known coal number fractions ranging from 6 to 90 percent were measured by the detector using two data acquisition methods known as peak-detection and digital signal evaluation (DSE). Variability in azimuthal scattering by single particles, which is an indicator of the degree of sphericity, was used to discern the fraction of coal particles present in each aerosol mixture. Number concentrations of coal and DOS aerosols were measured with an optical particle counter to derive expected coal number &actions. Measured coal number &actions compare well with expected values with average overestimates of 0.040 for peak-detection 0.087 for DSE. A twofold increase in the average peak-detection overestimate resulted when overlap in DOS and coal azimuthal variability distributions was neglected in the inference of coal fractions, but no significant change in the DSE values was observed. Problems to be overcome in developing a single-particle optical dust monitor with shape distinction capability for use in mining and industrial environments are briefly discussed. DICK, W.D.; SACHWEH, B.A.; MURRY, P.H.: D i s n m OF COAL DUST PARncw FROM ID DROP~ETS BY VARIATIONS IN AZIMUIHAL ~H T
Journal of Geophysical Research: Atmospheres, 1998
Extensive airborne measurements of aerosol particles in a pristine marine region were made during... more Extensive airborne measurements of aerosol particles in a pristine marine region were made during the first Aerosol Characterization Experiment (ACE 1) from November 15 to December 14, 1995. During this study, high concentrations of condensation nuclei (CN) were frequently observed both near and within clouds. Near clouds, in the absence of liquid water, Clarke et al. [1998] have reported that high CN levels were from new particle formation by homogeneous nucleation. Here we show, however, that within clouds, elevated CN concentrations were not authentic, but instead a sampling artifact, likely related to fragmentation of cloud drops impacting the aerosol inlet. By themselves, these fragments were often indistinguishable from ambient particles. Spurious CN from fragmenting droplets were observed at temperatures down to roughly-20øC and spanned a broad size range, with diameters down to 3 nm. Comparison of two different sized isokinetic aerosol inlets showed that inlets with smaller openings produce higher droplet fragment concentrations. The mechanism for producing these particles is not completely understood. Although fragmentation appeared to be the primary mechanism, for one instrument, an additional spurious source, correlated with liquid water, was observed when ambient temperatures were below-5øC. These findings show that care must be taken when interpreting airborne aerosol measurements in regions of liquid water. This is particularly pertinent to studies of new particle formation by homogeneous nucleation in the vicinity of clouds.
Journal of Geophysical Research: Atmospheres, 2000
In situ measurements of size-dependent water uptake by atmospheric particles made with a tandem d... more In situ measurements of size-dependent water uptake by atmospheric particles made with a tandem differential mobility analyzer (TDMA) and size-resolved chemical composition of aerosol samples collected with cascade impactors in the Smoky Mountains have been examined in order to ascertain the influence of organic carbon compounds on aerosol hygroscopicity. Particles were dried to-5 % relative humidity (RH) before entering the TDMA, leading us to believe that salts of ammonium and sulfate were in crystalline states for relative humidities below their expected deliquescent points. TDMA-measured water content was found to be in excess of the sulfate-associated water modeled using laboratory data for binary aqueous solutions and the method of Zdanovskii-Stokes-Robinson for multicomponent solutions over a wide range of humidities (RH=5-85%). Furthermore, excess water was observed to increase in proportion to the organic fraction of mass associated with each examined size in the range 0.05 to 0.4 gm. These data are used to obtain an empirical relationship between the amount of water associated with particulate organics and relative humidity. This analysis shows that organic-associated water content is considerably less than that of sulfate compounds, on a volume basis, for high RH, but comparable or greater for low RH. These results are consistent with laboratory data for water absorption by a range of organics vis-h-vis ammonium salts of sulfate. 1994].
Journal of Geophysical Research: Atmospheres, 1997
Ultrafine aerosols, with diameters less than 10 nm, nucleate from gas phase species. The composit... more Ultrafine aerosols, with diameters less than 10 nm, nucleate from gas phase species. The composition of newly formed ultrafine atmospheric aerosols is not known with certainty; new particles have variously been conjectured to be sulfates, organic compounds, and sulfate/organic mixtures. The 1993 Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, provided an opportunity to examine the question of which class of compounds, i.e., sulfates or organics, make the major contribution to new particle formation in the unpolluted troposphere. This study compared the production rates of sulfuric acid (from the oxidation of sulfur dioxide) and oxidized organic compounds to gauge their relative contributions to the formation of ultrafine particles. Potential organic precursor species examined in this study were the naturally occurring terpenes α– and β pinene, and the anthropogenic hydrocarbons toluene, m‐xylene, ethyl benzene, 1,2,4 trimethyl benzene, and methylcyclohexane. The c...
Journal of Geophysical Research: Atmospheres, 1997
Simultaneous measurements of aerosol particles and their expected gas phase precursors were made ... more Simultaneous measurements of aerosol particles and their expected gas phase precursors were made at Idaho Hill, Colorado, a remote continental site. This study used apparatus and techniques similar to those employed in an earlier study at the Mauna Loa Observatory, Hawaii [Weber et al., 1995]. New particle formation, identified by the presence of ultrafine particles (nominally 3 to 4 nm diameter), was commonly observed in downslope (westerly) air and was correlated with high sulfuric acid (H2SO4) concentrations, low relative humidity and low particle surface area concentrations. The data point to H2SO4 as a principle nucleation precursor species with typical daytime concentrations between 106 and 107 molecules cm−3. Particle production was observed at H2SO4 concentrations that are well below predicted values for binary nucleation of H2O and H2SO4, suggesting that another species participated. Particle growth rates were estimated from the data with two independent approaches and in b...
Condensable molecules generated in the gas phase by chemical reaction can either form new particl... more Condensable molecules generated in the gas phase by chemical reaction can either form new particles or condense on existing aerosol particles. Experiments were carried out to study the effect of a preexisting aerosol on aerosol dynamics in such systems. Secondary sulfate aerosol was produced by photochemical tcactions in the SO,-NO,-propylene system. A new theory which predicts the rate at which new particles of a given sixe are formed is also presented. Scavenging of molecular clusters by surrounding aerosols can be important, and is incorporated in the theory. Measured rates of new particle formation are compared with predicted rates, taking into account the minimum parti& size detected experimentally. Agreement between theory and experiment is within experimental error. Based on the theoretical analysis, a crkion for determining whether or not new particle formation is important in such systems is established.
The sorption of vapor molecules onto pre-existing nanometer sized clusters is of importance in un... more The sorption of vapor molecules onto pre-existing nanometer sized clusters is of importance in understanding particle formation and growth in gas phase environments and devising gas phase separation schemes. Here, we apply a differential mobility analyzer-mass spectrometer based approach to observe directly the sorption of vapor molecules onto iodide cluster ions of the form (MI)xM(+) (x = 1-13, M = Na, K, Rb, or Cs) in air at 300 K and with water saturation ratios in the 0.01-0.64 range. The extent of vapor sorption is quantified in measurements by the shift in collision cross section (CCS) for each ion. We find that CCS measurements are sensitive enough to detect the transient binding of several vapor molecules to clusters, which shift CCSs by only several percent. At the same time, for the highest saturation ratios examined, we observed CCS shifts of up to 45%. For x < 4, cesium, rubidium, and potassium iodide cluster ions are found to uptake water to a similar extent, while s...
Recent developments in atmospheric aerosol measurements are reviewed. The topics included complem... more Recent developments in atmospheric aerosol measurements are reviewed. The topics included complement those covered in the recent review by Chow (JAWMA 45: 320–382, 1995) which focuses on regulatory compliance measurements and filter measurements of particulate composition. This review focuses on measurements of aerosol integral properties (total number concentration, CCN concentration, optical coefficients, etc.), aerosol physical chemical properties (density, refractive index, equilibrium water content, etc.), measurements of aerosol size distributions, and measurements of size-resolved aerosol composition. Such measurements play an essential role in studies of secondary aerosol formation by atmospheric chemical transformations and enable one to quantify the contributions of various species to effects including light scattering/absorption, health effects, dry deposition, etc. Aerosol measurement evolved from an art to a science in the 1970s following the development of instrumentation to generate monodisperse calibration aerosols of known size, composition, and concentration. While such calibration tools permit precise assessments of instrument responses to known laboratory-generated aerosols, unquantifiable uncertainties remain even when carefully calibrated instruments are used for atmospheric measurements. This is because instrument responses typically depend on aerosol properties including composition, shape, density, etc., which, for atmospheric aerosols, may vary from particle-to-particle and are often unknown. More effort needs to be made to quantify measurement accuracies that can be achieved for realistic atmospheric sampling scenarios. The measurement of organic species in atmospheric particles requires substantial development. Atmospheric aerosols typically include hundreds of organic compounds, and only a small fraction (∼10%) of these can be identified by state-of-the-art analytical methodologies. Even the measurement of the total particulate organic carbon mass concentration is beset by difficulties including the unknown extent of evaporative losses during sampling, adsorption of gas-phase organic compounds onto sampling substrates, and the unknown relationship between carbon mass and mass of the particulate organics. The development of improved methodologies for such measurements should be a high priority for the future. Mass spectrometers that measure the composition of individual particles have recently been developed. It is not clear that these instruments will provide quantitative information on species mass concentrations, and more work is needed to routinely interpret the vast quantities of data generated during field sampling. Nevertheless, these instruments substantially expand the range of atmospheric aerosol issues that can be explored experimentally. These instruments represent the most significant advance in aerosol instrumentation in recent years.
... j cluster (j integer 1); r, dimensionless time; c#, dimensionless free molecule collision fre... more ... j cluster (j integer 1); r, dimensionless time; c#, dimensionless free molecule collision fre quency ... While the drop above some critical value of L is consistent with the present calculations ... formation tends to be dominated by coalescence of particles that are greatly different in size. ...
A simplified theoretical model is proposed for evaluating the dynamic behavior of photochemically... more A simplified theoretical model is proposed for evaluating the dynamic behavior of photochemically generated aerosols in the SO2-air system. The key simplifying assumption is that evaporation rates are negligible compared with condensation rates for molecular clusters and particles of all sizes. The theory accounts for the formation of condensable molecules (monomer) by chemical reaction, and coagulation of these monomer with clusters of condensed molecules to form larger particles. Cluster coagulation is also included. For a constant rate of monomer production, the equations can be cast in a dimensionless form which is independent of the rate of monomer formation by chemical reaction. These dimensionless equations have been solved numerically, both including and neglecting the effect of London-van der Waals forces on the coagulation rate. Agreement between theory and the data of W. E. Clark ("Measurements of Aerosols Produced by the Photochemical Oxidation of SOs in Air," Ph.D. thesis, University of Minnesota, Minneapolis, 1972) and G. Madelaine, M. Perrin, and A. Renoux (J. Aerosol Sci. 12, 202 (1979)) is good, particularly when London-van der Waals forces are considered. Because the solutions are presented in dimensionless form, direct application of these results for arbitrary rates of aerosol formation and for aerosols with arbitrary physical properties is possible.
Abstract The tandem differential mobility analyzer (TDMA) is useful for studying phenomena that l... more Abstract The tandem differential mobility analyzer (TDMA) is useful for studying phenomena that lead to size changes in submicron aerosol particles. Evaporation, condensation and chemical reactions within airborne droplets are examples of such phenomena. With this method, aerosols of a known size are selected with a differential mobility analyzer (DMA). These monodisperse aerosols then undergo processes that result in growth or shrinkage; the extent of growth or shrinkage is determined with a second DMA. In this paper, a theoretical treatment of a TDMA system is presented, and procedures for laboratory implementation are discussed. The theory is presented in an analytic form that can be encoded for on-line or off-line data analysis. Statistical techniques are used to calculate the precision with which size changes can be determined, and results of these analyses are confirmed experimentally. It is shown that diameter changes can be measured with a precision of about 0.3% using available instrumentation. For aerosols in the 0.01–0.2 μm diameter range where the technique works best, this corresponds to diameter changes of 0.03–0.6 nm, or the characteristic thickness of a monolayer.
Size distributions of urban Atlanta, Georgia, aerosols (0.003-2 microm) were measured from August... more Size distributions of urban Atlanta, Georgia, aerosols (0.003-2 microm) were measured from August 1, 1998 through August 30, 2000 as part of the Aerosol Research Inhalation Epidemiology Study (ARIES). Size distributions were measured five times per hour, and approximately 50,000 size distributions were measured during the 25-month study. This paper focuses on salient features of the sub-100-nm data. We examine concentrations of particles in six equally spaced logarithmic intervals and show that particles of different sizes have distinctly different behaviors. For particles between 10 and 100 nm, average concentrations tended to be highest during winter, during rush hour, and on week days. Concentrations of particles in the 3-10-nm range were elevated in the summer due to photochemically driven nucleation, and also during winter. We hypothesize that the elevated wintertime concentrations of these particles were associated with nucleation that occurs as vehicular emissions mix with the cool ambient air. In any given size range, distributions of concentrations tend to be lognormal, but significant deviations from lognormality were occasionally observed. For particles in the 3.2-5.6-nm diameter range, deviations were apparent in the summer when very high concentrations (up to 10(6) cm(-3)) were produced by photochemically driven nucleation. During 2 months of the study, deviations from lognormality for particles in the 32-56-nm diameter range occurred when anomalously high concentrations of 40-nm particles were observed.
Large smog chambers (-60 m3) constructed of FEP Teflon film are frequently used to study photoche... more Large smog chambers (-60 m3) constructed of FEP Teflon film are frequently used to study photochemistry and aerosol formation in model chemical systems. In a previous paper (6) a theory for aerosol wall loss rates in Teflon film smog chambers was developed; predicted particle loss rates were in good agreement with measured rates. In the present paper, measurements of wall deposition rates and the effects of wall losses on measurements of gas-to-particle conversion in smog chambers are discussed. Calculations indicate that a large fraction (up to 83%; typical values of 33-70%) of the aerosol formed in several smog chamber experiments was on the chamber walls at the end of the experiment. Estimated values for particulate organic carbon yield for several precursor hydrocarbons increased by factors of 1.3-6.0 when wall deposition was taken into account. The theory is also extended to loss rates of gaseous species. Such loss rates are either limited by diffusion through a concentration boundary layer near the surface or by uptake at the surface. It is shown that for a typical 60-m3 Teflon film smog chamber, gas loss rates are limited by surface reaction rates if mass accommodation coefficients are less than 6 X lo4. It follows that previously reported loss rates of several gases in a chamber of this type (12) were limited by surface reactions.
The design and function of a device that regulates the relative humidity of an ambient aerosol sa... more The design and function of a device that regulates the relative humidity of an ambient aerosol sample is described. We use this RH controller upstream of MOUDI impactors to permit sampling at relative humidities in the 70–80% range. Humidity control is achieved by allowing the aerosol to approach equilibrium with a saturated salt solution. Benefits to sampling with impactors in
ABSTRACT The Community Multiscale Air Quality (CMAQ) model is widely used in air quality manageme... more ABSTRACT The Community Multiscale Air Quality (CMAQ) model is widely used in air quality management and scientific investigation. Numerous studies have been conducted investigating how well CMAQ simulates fine particle mass concentrations, but relatively few studies have addressed how well CMAQ simulates fine particle number distribution. Accurate simulation of particle number concentrations is important because particle number and surface area concentrations may be directly related to human health and visibility. Simulated fine particle number concentrations derived using CMAQ are compared to measurements to identify problems and to improve model performance. Evaluation is done using measured particle number concentrations in Atlanta, Georgia, from 1/1/1999 to 8/31/2000. While homogeneous binary nucleation mechanism used in CMAQ needs to be modified for better prediction of particle number concentrations, there are also other factors that affect the predicted particle level. Assumed particle size of the primary emissions in CMAQ causes number concentrations to be significantly underestimated, while particle density has a small impact. Assuming particle size distributions by three lognormal modes cannot accurately simulate particles with size less than 0.01 μ m, particularly during nucleation events. An additional mode that accounts for particles smaller than 0.01 μ m can improve the accuracy of the number concentration simulations. Though, the use of the Expectation-Maximization (EM) algorithm to estimate size distribution parameters of measured particles suggests that assumed parameters for the lognormal modes in CMAQ are generally reasonable.
This is an instruction manual for the aerodynamic lens design tool: the "Aerodynamic Lens Calcula... more This is an instruction manual for the aerodynamic lens design tool: the "Aerodynamic Lens Calculator". We explain how to install and use this software. Examples are provided to use this tool to design or test a lens system. The structures of the source code programming are also provided in this manual. The design tool and its manual mentioned in this article are available in the publisher's online edition of Aerosol Science and Technology. To access this file, click on the link for this issue, then select this article. In order to access the full article on-line, you must either have an institutional subscription or a member subscription accessed through www.aaar.org.
We describe a process in which nanosize particles with a narrow size distribution are generated b... more We describe a process in which nanosize particles with a narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements, by calorimetric energy balances, and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nm. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 nm or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the e.~:periments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiatect by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1996
We have recently built a particle beam mass spectrometer (PBMS) for measuring ultrafine particle ... more We have recently built a particle beam mass spectrometer (PBMS) for measuring ultrafine particle size distributions (0.005–0.25 μm) at low pressures (≳100 mTorr). The PBMS is being used to study nucleation and growth in low pressure chemical vapor deposition processes relevant to the production of semiconductor devices. In this article, the function and performance of the PBMS is summarized, and results of measurements made while depositing polysilicon and silicon dioxide films in tube furnaces are discussed. Measurements made during deposition of polysilicon films showed that there was a critical reactor pressure below which particles were not present; this critical pressure varied in proportion to the residence time in the reactor, and was insensitive to reactor temperature. Above the critical pressure, however, the concentration of particles produced was sensitive to reactor temperature. The average particle size was in the 0.003–0.03-μm-diam range, with concentrations of ∼104 cm...
Journal of the Air & Waste Management Association, 2000
We summarize the results from the various measurements and the inter-sampler comparisons from Sou... more We summarize the results from the various measurements and the inter-sampler comparisons from Southeastern Aerosol and Visibility Study (SEAVS), a study with one of its objectives to test for closure among chemical, gravimetric and optical measurements of atmospheric aerosol particles. Sulfate and organics are the dominant components of the SEAVS fine particles (nominally, particles with aerodynamic diameter &lt; or = 2.5 microns) but between 28 and 42% (range over various samplers) of the gravimetrically measured total fine particle concentration is unidentified by the chemical measurements. Estimates of water associated with inorganic components and measurement imprecision do not totally explain the observed difference between gravimetric and chemical measurements. We examine the theoretical and empirical basis for assumptions commonly made in the published literature to extrapolate total fine particle concentration on the basis of chemical measurements of ions, carbon and elements. We then explore the more general question of closure using the SEAVS data as well as data from other, similar studies reported in the literature. In so combining the SEAVS measurements with other similar studies, we find a strong association between organic carbon and the unidentified component, that is, the fraction of the total fine particle concentration not identified by chemical measurements. We offer several tenable hypotheses for the relationship between the organic and unidentified components that deserve to be tested in future work. Specifically, we hypothesize that (1) errors in the sampling and analysis of organic carbon; (2) estimates of organic mass from measurements of organic carbon; and/or (3) water absorption by organics may all contribute to the observed relationship.
Applied Occupational and Environmental Hygiene, 1996
An experimental study was performed to determine the feasibility of optically distinguishing coal... more An experimental study was performed to determine the feasibility of optically distinguishing coal particles from water droplets for the purpose of reducing the mass concentration artifact from water sprays and steam reported for optical dust monitors. A single-particle multiangle optical detector was used to measure the scattering of monochromatic, circularly polarized light by mixtures of nonspherical bituminous coal dust particles and 0.5 pm dioctyl sebacate (DOS) droplets, which were used for modeling small, spherical water droplets. A total of 20 different mixtures with known coal number fractions ranging from 6 to 90 percent were measured by the detector using two data acquisition methods known as peak-detection and digital signal evaluation (DSE). Variability in azimuthal scattering by single particles, which is an indicator of the degree of sphericity, was used to discern the fraction of coal particles present in each aerosol mixture. Number concentrations of coal and DOS aerosols were measured with an optical particle counter to derive expected coal number &actions. Measured coal number &actions compare well with expected values with average overestimates of 0.040 for peak-detection 0.087 for DSE. A twofold increase in the average peak-detection overestimate resulted when overlap in DOS and coal azimuthal variability distributions was neglected in the inference of coal fractions, but no significant change in the DSE values was observed. Problems to be overcome in developing a single-particle optical dust monitor with shape distinction capability for use in mining and industrial environments are briefly discussed. DICK, W.D.; SACHWEH, B.A.; MURRY, P.H.: D i s n m OF COAL DUST PARncw FROM ID DROP~ETS BY VARIATIONS IN AZIMUIHAL ~H T
Journal of Geophysical Research: Atmospheres, 1998
Extensive airborne measurements of aerosol particles in a pristine marine region were made during... more Extensive airborne measurements of aerosol particles in a pristine marine region were made during the first Aerosol Characterization Experiment (ACE 1) from November 15 to December 14, 1995. During this study, high concentrations of condensation nuclei (CN) were frequently observed both near and within clouds. Near clouds, in the absence of liquid water, Clarke et al. [1998] have reported that high CN levels were from new particle formation by homogeneous nucleation. Here we show, however, that within clouds, elevated CN concentrations were not authentic, but instead a sampling artifact, likely related to fragmentation of cloud drops impacting the aerosol inlet. By themselves, these fragments were often indistinguishable from ambient particles. Spurious CN from fragmenting droplets were observed at temperatures down to roughly-20øC and spanned a broad size range, with diameters down to 3 nm. Comparison of two different sized isokinetic aerosol inlets showed that inlets with smaller openings produce higher droplet fragment concentrations. The mechanism for producing these particles is not completely understood. Although fragmentation appeared to be the primary mechanism, for one instrument, an additional spurious source, correlated with liquid water, was observed when ambient temperatures were below-5øC. These findings show that care must be taken when interpreting airborne aerosol measurements in regions of liquid water. This is particularly pertinent to studies of new particle formation by homogeneous nucleation in the vicinity of clouds.
Journal of Geophysical Research: Atmospheres, 2000
In situ measurements of size-dependent water uptake by atmospheric particles made with a tandem d... more In situ measurements of size-dependent water uptake by atmospheric particles made with a tandem differential mobility analyzer (TDMA) and size-resolved chemical composition of aerosol samples collected with cascade impactors in the Smoky Mountains have been examined in order to ascertain the influence of organic carbon compounds on aerosol hygroscopicity. Particles were dried to-5 % relative humidity (RH) before entering the TDMA, leading us to believe that salts of ammonium and sulfate were in crystalline states for relative humidities below their expected deliquescent points. TDMA-measured water content was found to be in excess of the sulfate-associated water modeled using laboratory data for binary aqueous solutions and the method of Zdanovskii-Stokes-Robinson for multicomponent solutions over a wide range of humidities (RH=5-85%). Furthermore, excess water was observed to increase in proportion to the organic fraction of mass associated with each examined size in the range 0.05 to 0.4 gm. These data are used to obtain an empirical relationship between the amount of water associated with particulate organics and relative humidity. This analysis shows that organic-associated water content is considerably less than that of sulfate compounds, on a volume basis, for high RH, but comparable or greater for low RH. These results are consistent with laboratory data for water absorption by a range of organics vis-h-vis ammonium salts of sulfate. 1994].
Journal of Geophysical Research: Atmospheres, 1997
Ultrafine aerosols, with diameters less than 10 nm, nucleate from gas phase species. The composit... more Ultrafine aerosols, with diameters less than 10 nm, nucleate from gas phase species. The composition of newly formed ultrafine atmospheric aerosols is not known with certainty; new particles have variously been conjectured to be sulfates, organic compounds, and sulfate/organic mixtures. The 1993 Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, provided an opportunity to examine the question of which class of compounds, i.e., sulfates or organics, make the major contribution to new particle formation in the unpolluted troposphere. This study compared the production rates of sulfuric acid (from the oxidation of sulfur dioxide) and oxidized organic compounds to gauge their relative contributions to the formation of ultrafine particles. Potential organic precursor species examined in this study were the naturally occurring terpenes α– and β pinene, and the anthropogenic hydrocarbons toluene, m‐xylene, ethyl benzene, 1,2,4 trimethyl benzene, and methylcyclohexane. The c...
Journal of Geophysical Research: Atmospheres, 1997
Simultaneous measurements of aerosol particles and their expected gas phase precursors were made ... more Simultaneous measurements of aerosol particles and their expected gas phase precursors were made at Idaho Hill, Colorado, a remote continental site. This study used apparatus and techniques similar to those employed in an earlier study at the Mauna Loa Observatory, Hawaii [Weber et al., 1995]. New particle formation, identified by the presence of ultrafine particles (nominally 3 to 4 nm diameter), was commonly observed in downslope (westerly) air and was correlated with high sulfuric acid (H2SO4) concentrations, low relative humidity and low particle surface area concentrations. The data point to H2SO4 as a principle nucleation precursor species with typical daytime concentrations between 106 and 107 molecules cm−3. Particle production was observed at H2SO4 concentrations that are well below predicted values for binary nucleation of H2O and H2SO4, suggesting that another species participated. Particle growth rates were estimated from the data with two independent approaches and in b...
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Papers by Peter McMurry