Darryl J. Alofs

Linear inversion method to obtain aerosol size distributions from measurements with a differential mobility analyzer

The use of a differential mobility analyzer to perform aerosol size spectrum measurements requires an inversion method to go from the measured sensor responses to the desired size spectrum information. Here we present a linear inversion method that can be run on a microcomputer or a small minicomputer. It does not use some of the approximations made in the techniques currently available and hence gives better inversion accuracy. The method shows good immunity to both random and systematic experimental error. It is applied to numerous test case aerosols.

Physical characterization of aerosol emissions from a commercial gas turbine engine

This paper discusses the results of the Aircraft Particle Emissions Experiment Project for the physical characterization of total (nonvolatile plus volatile) aerosol emissions (emission factors, hydration properties, and distribution shape parameters) by extractive sampling from an on-wing CFM56-2C 1 engine. Samples were extracted at the engine exit plane (1 m) as well as locations 10 and 30 m downstream. Three different fuels were used in this study: base fuel, high-sulfur fuel, and high-aromatic fuel. For the 1 and 10-m probe locations, strong and sometimes nonlinear dependencies were observed on fuel flow rate and no statistically significant dependencies were observed for fuel composition. At 30 m, the onset of gas-to-particle conversion was apparent for low- to medium-fuel flow rates. The soluble mass fraction was found to increase with distance from the engine exit plane and with increasing fuel aromatic and sulfur content. An intercomparison of gas and particle sampling trains showed that gas-to-particle conversion is a serious sample train artifact for gas sampling trains in which dilution cannot be achieved at the probe tip.

Condensation Coefficient Measurement for Water in the UMR Cloud Simulation Chamber

Abstract A systematic series of condensation coefficient measurements of water have been made using the University of Missouri—Rolla cooled-wall expansion chamber which simulates the thermodynamics of cloud. This coefficient is seen to decrease from a value near unity, at the outset of simulation, to a value in the neighborhood of 0.01 toward the end of a simulation. Final values of this coefficient are sufficiently low as to contribute significantly to the broadening of the drop-size distribution in cloud.

Inversion to obtain aerosol size distributions from measurements with a differential mobility analyzer.

Abstract To measure size distributions of submicrometer aerosols with an electrical differential mobility analyzer (DMA) requires an inversion procedure. The Knutson (1976) and the Hoppel (1978) inversion procedures were numerically investigated for the case of log-normal aerosol size distributions. It was found that the Hoppel procedure converges to the same result as that given by the Knutson procedure. The computational range for geometric mean diameter ( χ g) was 0.025-0.25 μm, and for geometric standard deviation (σg) was 1.1–2.4. The inversion error was found to be greater than 10% in certain “forbidden zones” of χ g and σg values. For the case of an ideal DMA having no lower mobility limit, only one forbidden zone exists, this consisting of small σg values. The boundary of this forbidden zone intercepts the computational range boundaries at σg = 1.25, χ g = 0.025 μ m and σg = 1.62, χ g = 0.25 μ m . These results also apply to an actual DMA when the size distribution of particles larger than the DMA singly charged mobility limit is available a priori. If such information is not available, the concentration of these larger particles is assumed to be zero in performing the inversion. This assumption adds a second forbidden zone, consisting of large σg values and having the intercepts σg = 2.44, χ g = 0.025 μ m and σg = 1.50, χ g = 0.25 μ m . The first forbidden zone remains nearly the same.

A Cloud Nucleus Counter with Long Available Growth Time

Abstract A vertical plate, steady-flow, thermal diffusion chamber designed for use as a cloud condensation nucleus counter is described. In this instrument, phoretic forces are shown to limit the maximum available growth time to a value eight times larger than the growth time available in conventional horizontal plate chambers. This additional growth time is shown to be necessary when operating at supersaturations below 0.2%. Experiments and calculations concerning convection currents in the vertical chamber are also presented.

Insoluble Condensation Nuclei: The Effect of Contact Angle, Surface Roughness and Adsorption

Abstract Condensation of water vapor an various surfaces was studied experimentally. For surfaces with an airwater contact angle θ less than 20° the experimentally determined values of critical supersaturation Sc. agreed with those given by the Volmer theory. At higher θ the experimental values of Sc. were below the Volmer theory values. When the applied supersaturation was less than Sc, condensation was avoided for periods as long as 20 h. Thus, if adsorption of water tends to negate the Volmer theory, the process is a slow one. It was determined both by experiment and theoretical analysis that the effect of surface roughness is to decrease Sc only Slightiy. These results suggest that most insoluble airborne particles are not likely to serve as cloud condensation nuclei.

Nucleation Experiments with Monodisperse NaCl Aerosols

Abstract Nucleation experiments with monodisperse NaCl aerosols showed good agreement with the Kohler theory relating the critical super-saturation Sc to the dry size. Aerosols produced by condensing NaCl showed the same Sc as those produced by evaporating aqueous NaCl solution droplets. This indicates that if there is an energy barrier in going from a dry NaCl particle to a solution droplet, this energy barrier is small. The fact that the evaporation aerosol particles are cubical crystals and the condensation aerosols are amorphous spheres is shown to make no difference in the nucleation threshold. The investigation also gives insights into the performance of the equipment used, especially the commercial electrostatic aerosol classifier and the vertical flow thermal diffusion chamber developed in this laboratory. When operating this chamber in the isothermal mode, a 36% upper limit was found on the uncertainty in Sc due to index of refraction sensitivity in sizing the water drops. Within this range of un...

Performance of a Dual-Range Cloud Nucleus Counter

Abstract An instrument for measuring the concentrations of cloud condensation nuclei as a function of nucleus critical supersaturation (Se) is described. In the range 0.1% ≤ Se ≤ 1%, the instrument is operated as a vertical flow thermal diffusion chamber. In the range 0.016% ≤ Se ≤ 0.1%, the chamber is operated at 100% humidity, and the equilibrium radii of the nuclei are measured with an optical counter. This paper presents a large body of data taken in the laboratory to evaluate the performance of the instrument. The data indicates that the instrument is internally consistent over a wide range of operating conditions.

Measurement of Growth Rate to Determine Condensation Coefficients for Water Drops Grown on Natural Cloud Nuclei

Abstract Growth rate measurements were made for water drops grown on nuclei in atmospheric air samples taken in Rolla, Missouri. Rolla, having a population of 15,000 and very little industry, is relatively free of urban pollutants. The measurements were made in a vertical flow thermal diffusion chamber at supersaturations of 0.5 and 1%. The time to grow from near dry radius to the final radius (6 to 7.5 µm) was measured. If one assumes the thermal accommodation coefficient is unity, the measurements indicate an average value of 0.026 for the condensation coefficient. The temperature ranged from 22.5 to 25.7°C.

Density Distribution Measurements in Rarefied Gases Contained between Parallel Plates at High Temperature Differences

Density distributions were experimentally determined in rarefied gases at rest contained between two parallel plates maintained at widely different temperatures. The temperatures of the hot and cold plates were ~79°K and ~294°K, respectively, resulting in a temperature ratio of about 4:1. Both helium and nitrogen were used as test gases. Particular attention was focused on obtaining data in the temperature jump and transition regimes. The gas densities were measured by observing the luminescence produced by a high-energy electron beam traversed between the plates. In addition to the density measurements, heat transfer measurements were also made in order to estimate the values of the thermal accommodation coefficients. The experimental results for helium were compared with the analytical results of Liu and Lees, and good agreement was found between the theory and data near the cold plate. Near the hot plate the analytical results and the data differed by about 7%.