William D. Dick

Estimation of water uptake by organic compounds in submicron aerosols measured during the Southeastern Aerosol and Visibility Study

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 μm. 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-a-vis ammonium salts of sulfate.

Elemental composition and morphology of individual particles separated by size and hygroscopicity with the TDMA

Abstract Particles from the Minneapolis atmosphere were segregated according to hygroscopicity using a tandem differential mobility analyzer (TDMA) and collected with a specially designed aerodynamic focusing impactor for elemental analysis. Areal deposit densities obtained using the focusing impactor are up to a factor of 100 greater than those obtained using a conventional single-jet impactor, thereby reducing required sampling times by the same factor. A Philips CM30 scanning transmission electron microscope (STEM) equipped with an EDAX super ultra-thin-window detector was used to analyze for carbon and heavier elements. For the limited sample of 0.3–0.4 μm summertime aerosols examined in this study, less hygroscopic particles included chain agglomerates (∼ 55%), irregular shapes (∼ 33%), spheres and flakes (

Multiangle Light-Scattering Measurements of Refractive Index of Submicron Atmospheric Particles

Multiangle light scattering (MLS) measurements of monodisperse atmospheric particles ranging in diameter from 0.2 to 0.8 μm were made with the DAWN-A optical detector during the Southeastern Aerosol and Visibility Study (SEAVS). The study was conducted on the southwestern edge of the Great Smoky Mountains National Park from July 15 to August 25, 1995. Individual particles were separated into spherical and nonspherical classes on the basis of the variability in elastic light scattering measured at eight azimuthal angles. Values of the real part of the refractive index, n, were then inferred for spherical particles by comparing Lorenz-Mie theory calculations to calibrated light scattering responses from narrow-aperture detectors positioned at seven polar angles ranging from 40–140 degrees. The instrument was calibrated with submicron laboratory particles with n ranging from 1.38 to 1.61 for an illumination wavelength of 488 nm. The overall uncertainty in measured n was estimated to be ±0.02. For hygroscopic particles, refractive index was found to decrease as relative humidities increased due to the addition of water; the study-average value at low humidities (dry particles) was 1.49, while the average value at high humidities (wet particles) was 1.42. The lowest and highest values measured during the study were 1.34 and 1.54. Daily size- and RH-dependent measurements are compared to indices modeled for mixtures of ammoniated sulfate, organic carbon (OC), elemental carbon (EC), and water. Size-dependent mass concentrations of these species, with the exception of water, were measured with MOUDI cascade impactors.

Size- and Composition-Dependent Response of the DAWN-A Multiangle Single-Particle Optical Detector

The performance of the DAWN-A differential light-scattering detector (Wyatt et al., 1988. Appl. Opt. 27:217–221) was characterized in laboratory experiments. Objectives of this work included measurement of size-dependent counting efficiencies and of angular scattering patterns for spherical particles of known size and composition. Counting efficiencies for polystyrene latex (PSL) spheres of nine sizes in the 0.14–0.97-μm diameter range were obtained as a function of the trigger threshold level. Counting efficiencies were found to increase with increasing particle size and decreasing trigger threshold level. Maximum observed counting efficiencies were in the range of 50% to 60%, indicating that the half-width of the laser beam was about a factor of 2 narrower than the width of the particle beam in the scattering volume. A distribution of pulse heights was observed for particles of a given size, reflecting the variability of the illumination intensity. Angular scattering patterns of PSL, dioctyl sebacate (D...

A new method for measuring the dependence of particle size distributions on relative humidity, with application to the Southeastern Aerosol and Visibility Study

A relative humidity-moderated differential mobility optical particle size spectrometer (RH-DMOPSS) was developed to measure particle size distributions at different humidity conditions in the 0.1- to 1.1-μm size range. The system has a high-flow differential mobility analyzer followed by an optical counter and condensation particle counter in parallel. An aerosol dryer or humidity conditioner can be placed in line ahead of the mobility analyzer. A second dryer ahead of the optical counter permits measurements of the decrease in particle size when particles classified at high relative humidity were dried. The RH-DMOPSS data set includes (1) optical counter response to monodisperse ambient aerosols, (2) accumulation mode particle size distributions for dry and humid conditions, (3) selected hygroscopic growth factors for humid diameters of 0.2 to 1.1 μm, (4) inferred volatile particle water volume, and (5) inferred refractive index for dried particles. The RH-DMOPSS was operated in Smoky Mountains National Park as part of the Southeastern Aerosol and Visibility Study and compared with other collocated size and aerosol growth measurements. For dried ambient particles, size distribution parameters were correlated with those from impactor measurements, but volume median diameters were 12% smaller, on average. Growth factors for particles below 0.5-pm diameter were in the same range as those measured by a tandem differential mobility analyzer and, for all sizes, were consistent with the measured differences between dried and humidified particle size distributions. The mean ambient particle real refractive index inferred from the RH-DMOPSS dry-particle calibration data was 1.48, equal to that measured by a multiangle light-scattering instrument.

Distinguishing Between Spherical and Nonspherical Particles by Measuring the Variability in Azimuthal Light Scattering

Azimuthal variabilities in scattering of monochromatic, circularly polarized light by individual spherical and nonspherical particles were measured using the DAWN-A (Wyatt et al. Appl. Opt. 27:2405–2421, 1988) differential light scattering detector. Measured aerosols included polystyrene latex spheres (PSL), quartz, and sodium chloride particles of 0.576, 0.741, 0.966, and 1.250 μm diameter. Signals from eight detectors at different azimuthal angles at a polar angle of 55° showed that variabilities for nonspherical particles significantly exceeded values for the spherical PSL. The probability that a quartz or sodium chloride particle would be incorrectly identified as a sphere are less than about 5% for all sizes investigated.

Comparisons of aerosol properties measured by impactors and light scattering from individual particles: refractive index, number and volume concentrations, and size distributions

Abstract The southeastern aerosol and visibility study (SEAVS) was conducted in Great Smoky Mountains National Park in summer 1995 to investigate variations in ambient aerosol size distributions and their effect on visibility. In this paper, we compare dry aerosol size distribution parameters from a MOUDI impactor and two different optical particle counters (OPCs). Size distributions from the various instruments are expressed in a common measure of size, specifically, optical and aerodynamic diameters are converted to a dry, geometric diameter basis. Comparisons of the real part of particle refractive index obtained directly from light scattering measurements and inferred from aerosol composition measurements are also shown. Real refractive indices from direct measurements and those computed from measured fine aerosol chemical composition were generally within ±0.02. Maximum differences in estimated accumulation mode integrated volume concentrations from all instruments were within ∼22%. Accumulation mode integrated number concentrations and geometric standard deviations from the two OPCs agreed within ∼30% and ∼3%, respectively. Differences between MOUDI- and OPC-derived accumulation mode number concentrations and geometric standard deviations were ∼20% and ∼8%, respectively. The average geometric volume mean diameters derived from the three instruments agreed within 15% or less. The volume median diameters obtained by fitting the CSU number concentration data to a lognormal function were typically the smallest. We show that these discrepancies can be related to the differences and biases in the measurement and data analysis techniques.

White-light Detection for Nanoparticle Sizing with the TSI Ultrafine Condensation Particle Counter

Several of the most common methods for measuring nanoparticle size distributions employ the ultrafine condensation particle counter (UCPC) for detection purposes. Among these methods, the pulse height analysis (PHA) technique, in which the optical response of the UCPC detector is related to initial particle diameter in the 3–10 nm range, prevails in applications where fast sampling is required or for which concentrations of nanoparticles are frequently very low. With the PHA technique, white light is required for particle illumination in order to obtain a monotonic relationship between initial particle diameter and optical response (pulse height). However, the popular, commercially available TSI Model 3025A UCPC employs a laser for particle detection. Here, we report on a novel white-light detection system developed for the 3025A UCPC that involves minimal alteration to the instrument and preserves normal counting operation. Performance is illustrated with pulse height spectra produced by differential mobility analyzer (DMA) – generated calibration aerosols in the 3–50 nm range.

Distinction of coal dust particles from liquid droplets by variations in azimuthal light scattering

Abstract 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 μm 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 mea...

Characterization of nanosized silica size standards

ABSTRACT Nanosized silica size standards produced with a sol–gel synthesis process were evaluated for particle size, effective density, and refractive index in this study. Particle size and effective density measurements were conducted following protocol from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. Particle sizes were measured via electrical mobility analysis using a differential mobility analyzer (DMA) at sheath flow rates (Qsh) of 3.0 and 6.0 L/min and a constant aerosol flow rate (Qa) of 0.3 L/min. The measured mean and mode diameters agreed well with the labeled sizes in the size range 40–200 nm, with differences ranging from 0.03% to 0.8%, well within the labeled expanded uncertainties (95% confidence intervals) of 1.8%–2.2%. The coefficient of variation (CV) of the size distribution was 0.012–0.027 for 40–200 nm. Particle sizes measured for 20 nm and 30 nm standards showed size differences with respect to the certified sizes of 1.7% and 8.3% at Qsh = 6.0 L/min, but the size distributions were narrow, with CV = 0.047–0.064. The average effective density for the range 40–200 nm measured with an aerosol particle mass analyzer (APM) was 1.9 g/cm3. The real component of the refractive index measured with an optical particle counter (OPC) was 1.41 at a wavelength of 633 nm. All properties (size, effective density, and refractive index) were stable and could be measured with good repeatability. From these evaluations, it was found that the nanosized silica size standards have good characteristics for use as size standards and constitute a feasible alternative to PSL particles.