Fog and cloud droplet size spectra measured with a low-cost optical particle counter

This study explores the possibilities of using a Climet CI-8060 optical particle counter as a fog and cloud droplet counter in ambient conditions. Intercomparisons with an AERAS cascade impactor (modified for fog measurements) show that the performance of the instrument is excellent and yields accurate droplet size distributions in the range of 0.54 to 25 μm in one-minute time resolution. The CI-8060 was applied successfully in a field experiment investigating in-cloud scavenging of air pollutants. It is shown that the cloud droplet size distribution is a key parameter for the depletion of pollution by wet deposition. Being aware of the fact that commercial instruments built particularly for the purpose of cloud droplet sizing exist, the CI-8060 has some advantages: it is inexpensive, and it is rugged and easy to use in the field.     

Optical particle counters: Response, resolution and counting efficiency

Five types of commercial optical particle counters have been studied. The response curves of each counter were determined using spherical, non-absorbing particles (DOP, PSL). The response curves of all OPCs were in a reasonable agreement with theoretical curves and those given by the manufacturers. In the case of absorbing particles (methylene blue) there was a significant decrease in the response of all counters. The resolution of the counters was studied by measuring the widths of the pulse height distributions for monodisperse DOP-particles. Noticeable differences between the counters in this respect were observed. The counting efficiencies of the instruments for solid particles in calm air were determined with the aid of filter samples. Most of the counters gave relatively reliable readings up to about 8 μm. One of the counters (Climet 250) gave apparently too low readings obviously because of the particle loss in the sampling system. Another counter (Royco 218) gave too high readings when large particles were measured using a low discriminator setting. The maximum measurable concentrations were estimated by measuring the pre-amplifier pulse length distributions for each counter. The effect of long pulses, probably caused by the recirculation, were found to be significant. The concentrations for counters without a sheath air system were considerably smaller compared to the concentrations calculated from the detection volumes given by the manufacturers.     

Response Characteristics of the Particle Measuring Systems Active Scattering Aerosol Spectrometer Probes

Predictions of the size response of various light-scattering aerosol counters manufactured by Particle Measuring Systems are reported. Models that exploit the high intensity of light available within the cavity of a He-Ne gas laser (generically referred to by the manufacturer as ''active scattering aerosol spectrometer probes'') are considered. The new response function properly averages over particle trajectories through nodes, antinodes, and intermediate regions of the intracavity laser beam. Our studies address probes having two basic scattering geometries: those that collect light scattered over a relatively narrow solid angle (subtending angles between 4° and 22° from the laser beam axis, as in the model ASASP-300 and ASASP-300X probes) and those that collect light over a rather large solid angle (between 35° and 120° , as in the ASASP-X, ASASP-100X, LAS-250X, LAS-X, and HS-LAS probes). The theoretical response predictions for both narrow-angle and wide-angle probes are compared to previous measurements of monodisperse test aerosols of polystyrene latex, dyoctylphthalate, nigrosin dye, and carbon black. The new response function predicts smoother dependence on particle size than the previous response function of Pinnick and Auvermann (1979) and is in better agreement with measurement. Response calculations for common atmospheric aerosol (water, sulfuric acid, ammonium sulfate, and black carbon) reveal the considerable sensitivity of the response to particle dielectric properties. Response functions for internal mixtures (black carbon inclusions in water droplets, quartz in sulfuric acid, carbon in ammonium sulfate, and metal in sulfuric acid) are somewhat different than those for homogeneous particles. Comparison of response calculations with the manufacturer's calibration reveal conditions for which the manufacturer's calibration is most appropriate and the potential for errors (as much as a factor of two in sizing) when it is blindly applied. Finally, response functions for multiline laser operation, as the manufacturer suggests might be appropriate for the HS-LAS and LAS-X probes, are nearly the same as for single-line lasing. These results should help the user of these instruments to more realistically interpret size distribution measurements.     

Response of Knollenberg light-scattering counters to non-spherical doublet polystyrene latex aerosols

Doublet particles of polystyrene, polyvinyltoluene and styrene vinyltoluene latex were measured with the Knollenberg CSASP and ASASP light-scattering aerosol counters. These instruments respond to these non-spherical particles as they do spheres of equal volume and refractive index, providing particles have equivalent radii r 0.4 μm. However, their response to larger nonspherical particles having equivalent radii in the range 0.4 μm r 1.7 μm cannot be approximated by that for spheres of equal volume or equal cross-section. For even larger particles with equivalent radii 1.7 μm r 4 μm the CSASP response is about the same as for spheres of equal cross-section and refractive index. The counting efficiency of the CSASP and ASASP instruments was checked by measurement of monodisperse aerosol particles of known concentration. These instruments measure aerosol concentrations correctly to within a factor 2 for near-micrometer sized particles.     

Excimer Laser Induced Removal of Particles from Hydrophilic Silicon Surfaces

A pulsed KrF excimer laser was used to remove several types of submicron-sized particles from silicon surfaces. Polystyrene latex particles, 0.1 μm and larger, were removed from silicon surfaces by dry laser cleaning (no water layer condensed on the surface) but SiO₂ particles could not be so removed. However, during steam laser cleaning, in which a thin film of water is deposited on the surface as both an energy transfer medium and an adhesion force reduction agent, these 0.1-0.2 μm SiO₂ particles were almost entirely removed. Calculations of the various forces contributing to adhesion indicate that hydrogen bonds are the major contributor to the adhesion of inorganic particles to substrate surfaces. Photoacoustic detection, using piezoelectric transducers, monitored the surface vibrations induced by the laser pulses.     

Aerosol measurement by laser doppler spectroscopy—II. Operational limits, effects of polydispersity, and applications

The theoretical basis and the results of a computer simulation are presented which describe the operational limits of size and concentration for aerosol sizing by laser Doppler spectroscopy LDS,. This analysis suggests that a state of the art LDS system has the capability of sizing 0·03 μm diameter particles when the number concentration is 10⁸ cm⁻³ or greater and 0·2 μm diameter for coocentrations as low as 100 particles cm⁻³. An evaluation of the effect on the laser Doppler spectroscopy measurements of a polydisperse aerosol having a log normal size distribution is presented and methods for combining these measurements with other averaged measurements to determine both count median diameter (CMD) and geometric standard deviation (δ_g) are proposed. For aerosols having log normal distributions with 0·3 < CMD < 3 μm and 1·0 < δ_g < 2·0, laser Doppler spectroscopy is able to measure the surface area median diameter within ± 15 per cent, independent of polydispersity. Applications of LDS to aerosol sizing are evaluated and its advantages and disadvantages relative to other sizing methods are discussed.     

Liquid Explosive Evaporative Removal of Submicron Particles from Hydrophilic Oxidized Silicon Surfaces

A pulsed CO₂ laser-based system, operating at a wavelength of 10.6 μm, was used as a cleaning tool to remove particles as small as 0.1 μm from hydrophilic, oxidized silicon surfaces. The laser beam served as a fast heating source to induce the explosive evaporation of a water film deposited on the particle-contaminated surface. The resulting explosive forces were high enough to expel particles from the surface efficiently. The contaminant particles used were 0.1 μm alumina, 0.1-0.2 μm silica, and 0.1 μm polystyrene latex.

For each of these, the cleaning efficiency was monitored as a function of the laser fluence, the thickness of the deposited water film and the number of cleaning cycles. Whatever the nature of the particles, the cleaning efficiency was characterized by an upper limit of the energy density, determined to be 1.5 J/cm², at which substrate damage occurred. At all lower laser fluences, the removal efficiency was particle-dependent.

The thickness of the deposited water film was varied by changing the time of exposure of the surface to water vapor, the vapor flow being fixed at 4700 ml/min. An exposure time of 1.5 s was found to be the most effective. Increasing the number of cleaning cycles permitted the evaluation of the effect of the zeta potentials of the particles with respect to that of the surface.     

Microstructure and flexure creep behaviour of SiC-particle reinforced Al2O3 matrix composites

The flexure creep behaviour of monolithic Al₂O₃ and 10 vol% SiC-particle reinforced Al₂O₃ matrix composites was investigated in air atmosphere at 1160 to 1400 °C and under a stress of 40 to 125 MPa. Two kinds of SiC particles with different particle sizes and oxygen contents were used in the composites, one having an average size of 0.6 μm with 1.7 vol% SiO₂ impurities and the other of average size 2.7 μm with 3.4 vol% SiO₂ impurities. Compared with the creep behaviour of monolithic Al₂O₃ the strain rate of the composites with 0.6 μm SiC particles did not decrease; however, the composites with 2.7 μm SiC particles exhibited excellent creep resistance. Microstructure analysis showed that the Al₂O₃ grains in the composites with 0.6 μm SiC particles were mainly equiaxed with most of the SiC particles lying at the grain boundaries or triplegrain junctions, whereas the grain features of the composites with 2.7 μm SiC particles were irregular and elongated and most of the SiC particles were entrapped into Al₂O₃ matrix grains. It was revealed that the entrapment of 2.7 μm SiC particles into Al₂O₃ matrix grains was related to the high SiO₂ impurity content on SiC particle surfaces, and the change of grain morphology and the good high-temperature oxidation resistance were responsible for the creep resistance increase of the composites with 2.7 μm SiC particles.     

A data processing system for the Royco aerosol particle counter 225 and determination of the properties of the modified system

A data processing system, consisting of a peak detector, an AD-converter and a minicomputer was developed, which enables the simultaneous counting of particles with different diameters. A computer program was written to handle the output of data in groups of 1, 2, …, 1024 channels. The maximum number of particles that can be counted in a single channel (1024 mode) is 16777215. The resolving time of the system was determined from oscilloscope photographs of pulses generated by monodisperse PSL and DOP particles and amounts to about 15 μsec. The optical counter was calibrated for the size range 0.3–9 μm with monodisperse PSL and DOP aerosols. The response curve is non-singular in between 0.8 and 1.4 μm. The ratio between the standard deviation (σ_K) and the median (K_g) of the channel number distribution for monodisperse aerosols decreases continuously with increasing K_g from 0.7 (at 8) to 0.04 (at 922). Count losses due to coincidence and pulse processing were calculated. Completely ambiguous results have to be expected for concentrations higher than about 450 particle cm⁻³.     

Inlet characteristics of bioaerosol samplers

The inlet sampling characteristics of several commercial bioaerosol samplers operating in indoor and outdoor environments have been analyzed by use of available and newly developed equations for sampling efficiency. With a focus on the physical aspects of sampling efficiency, the aspiration and transmission efficiencies have been calculated for the bioaerosol particle size range 1–30 μm, which represents single bacteria, bacteria aggregates, bacteria carrying particles, fungal spores, yeast, and pollen. Under certain sampling conditions, the bioaerosol concentration was found to be significantly over- or underestimated. At wind velocities between 0 and 500 cm s⁻¹, calculations show that the AGI-30 would sample 1–10 μm particles with an inlet sampling efficiency of 20–100%. The entrance efficiency of the 6-stage Andersen viable sampler is 90–150% when sampling isoaxially with respect to horizontal aerosol flows, and 8–100% when oriented vertically at a right angle to the horizontal aerosol flow. For the Burkard portable air sampler, an even wider range of deviation may occur. The bioaerosol samplers used for large particles such as pollen are even less accurate: e.g. 10 times the ambient concentration of Lycopodium spores has been calculated to be aspirated by the Lanzoni sampler when operated at 0.5 1 min⁻¹ facing the wind at wind velocity of about 500 cm s⁻¹.

The actual bioaerosol concentration can be calculated from the measured data by use of the indicated procedures. The sampling efficiency graphs presented can be used to bracket the sampling conditions that enable the investigator to avoid or minimize significant sampling biases for each sampler. The findings can also be used for the design of new samplers or for improving commercially available samplers.     

Size Measurements of Latex Particles by Laser Aerosol Spectrometer

Size measurements of PSL (polystyrene latex) particles in a size range from 0.109 to 0.330 μm were made by laser aerosol spectrometer (PMS, LAS-X). The results were compared with those by electron microscopy. For example, the geometric standard deviation, σ_g, of nominally 0.176-μm PSL particles was measured as 1.05, assuming that their sizes distribute log-normally. The value of 1.05 was very close to 1.02 measured by electron microscopy. It was found that the spectrometer had very high size resolution, although the size resolution of the light scattering type spectrometer has been said to be poor. For some samples of PSL particles, however, there were large differences between particle sizes measured by LAS-X and those by electron microscopy. It was also found that LAS-X had a problem in calibration of size response curve.     

AN EXPERIMENTAL STUDY OF A TURBULENT ROUND TWO-PHASE JET

The effect of solid particles on the flow structure of an axisymmetric turbulent incompressible jet has been studied. A two-color laser anemometer was used to measure the mean and fluctuating velocity components along the axial and radial directions as well as the corresponding component of the shear stress. 50 μ and 200 μ glass beads were used. Results for the 200 μ case are presented for moderate mass loadings (mass ratio = 0.8, volumetric ratio = 3.5 × 10^-4)which indicates ignificant influence of the solid phase on the flow structure. Results for the 50 μ. case are reported elsewhere.1 When compared with the single-phase measurement, the spread rate of the two-phase jet was found to be smaller and a lower level of turbulence was observed.     

Vertical distribution of aerosol particles near the air-sea interface in coastal zone

Measurements of vertical aerosol concentration profiles in the Mediterranean coastal zone are presented in the size band 0.1 to 20 μm, with focus on aerosol particles smaller than 10 μm. The results show that the profile shape depends mainly on turbulent processes induced by the wind rather thanthe whitecap production of freshly generated aerosols from the wind-wave interaction. Therefore, the data recorded during weak and moderate wind speeds show the prevalence of turbulent mixing processes induced by the wind for profiles in the size range 0.1–1 μm. For the size range 1–10 μm, in addition to the effect of wind velocity and direction (i.e. the coastal influence), we also noted the importance of the relative humidity gradient (very close to the interface). Good agreement was found between vertical concentration profiles recorded for winds of marine origin and the marine mixed layer model (Davidson and Shutz, 1983). However, we show that the model cannot be used for winds of continental origin. For wind speeds above >9 ms⁻¹, a maximum occurs in the concentration profiles (near 2–3 m height) for all particles larger than 1 μm, which confirms the predominant influence of an eddy which appears in the lee of the crest, referred to as the “wave rotor” model (De Leeuw, 1986).     

Cleaning of fine coals by dense-medium hydrocyclone

The efficiency of separation for fine coal in a 150 mm dia. dense-medium hydrocyclone has been determined. The partition curves have been measured for particles in the size ranges −500 μm +425 μm, −300 μm +250 μm, −150 μm +125 μm and −90 μm +75 μm. The cut point for separation increases with a decrease in particle size and the efficiency of separation decreases as particle size decreases. The cut point of separation varies with medium specific gravity but the efficiency of separation does not. Neither the cut point of separation nor the efficiency of separation is greatly influenced by the cyclone feed rate, provided that overloading does not occur.

The results lead to an accurate predictive model for the calculation of the partition curve as a function of coal particle size and medium specific gravity. The model allows the prediction of the performance of a dense-medium hydrocyclone for the washing of fine coal having arbitrary washability characteristics and particle size distribution. The model is used to demonstrate that a two-stage dense-medium hydrocyclone configuration can significantly improve the cleaning performance for a coal that has good ash-liberation characteristics. However, multi-stage dense-medium hydrocyclones do not offer any real performance advantages for coal that has poor ash-liberation characteristics.     

Hydrodynamic behavior of a liquid-solid fluidized-bed reactor for the bioconversion of coal particles to liquid products

A predictive mathematical model based on particle convection and dispersion is presented for a liquid fluidized bed of coal particles. The numerical representation can follow transient behavior of liquid fluidized beds that contain a defined particle-size distribution. The calculations exhibited excellent agreement when compared to experimental transient pressure-drop data from a column containing particles of Illinois No. 6 bituminous coal in the size range of 20–120 μm. In addition, the model was used to simulate the effect of periodic (hourly) liquid velocity step changes on elutriation of small particles from a given particle-size distribution of 34.5–75.5 μm. For the situation tested, the calculations indicate that (a) the column does not reach a steady state between velocity changes and (b) that higher initial particle elutriation rates decay to lower values until no more particles elutriate from the column.     

On physical properties of aerosol at Ross Island, Antarctica

We report on the physical characteristics of Aitken nuclei and their relation to air mass type at Ross Island, Antarctica. The size distribution spectrum of the particles was derived with a diffusion battery and non-linear mathematical inversion and is considered to be quite accurate over the particle diameter range 0.01 < d < 0.2 μm.

The Aitken nuclei concentration, n_o, decreased at Ross Island by about an order of magnitude from summer (n_o = 600 cm⁻³) to winter (n_o = 100 cm⁻³). Particle size varied with air mass type: Maritime polar air masses had a geometric mean particle diameter d_g = 0.011 μm, while continental Antarctic, cA, air masses possessed somewhat smaller particles (i.e. d_g 0.005 μm). The anomalously small particles associated with cA air suggests the presence of an upper tropospheric or partly stratospheric origin of nuclei perhaps associated with mixing formed by breaking waves over the Ellsworth Range in Marie Byrd Land.     

OCM in a fixed-bed reactor: limits and perspectives

The oxidative coupling of methane (OCM) over a La₂O₃/CaO catalyst was studied in a poly tropic fixed-bed reactor (I.D. = 15 mm, W/F= 0.15 g · s/ml). Reaction conditions for stable operation were determined. (1) A minimum inlet temperature of 580°C was necessary to initiate the reaction. (2) The maximum hot-spot temperature of 1000°C limited the highest oxygen inlet concentration to 20%. The temperature gradients in the bed amounted to 250 K. The influence of the reaction conditions on the C₂₊ selectivity was investigated by testing the effects of temperature (T_inlet = 580–860°C), oxygen concentration (C_O2 = 5–20%) and particle diameter (d_p = 250–350 μm, and pellets of h_p = 4 mm and d_p = 4 mm). The C₂₊ selectivity ran through a maximum with increasing temperature and decreased with rising inlet oxygen concentration. Mass-transfer limitations, which occurred when applying pellets, resulted in a drop of C₂₊ selectivity. Highest C₂₊ yields amounted to 15.5% (X_CH4 = 31%, S ₂₊ = 51%). Distributed feed of oxygen was tested as a means to cope with the high temperature gradients and to increase C₂₊. selectivity. Upon applying this mode of operation, oxygen concentrations up to 30% could be converted. However, no improvement of C₂₊ selectivity and yield compared to cofeed operation was achieved.     

Measurements of the total and regional deposition of inhaled particles in the human respiratory tract

The total and regional deposition of monodisperse aerosols in the human respiratory tract has been measured in 12 healthy subjects breathing through the mouth. Radioactively labelled polystyrene particles in the aerodynamic diameter range 3.5–10.0 μm were employed. The total deposition results are similar to those reported by Stahlhofen et al. (1980), showing only a slight progressive increase with particle size, from a mean fraction of 0.79 of the inhaled aerosol at 3.5 μm, to 0.88 for 10 μm particles. The extrathoracic airways show a very marked deposition at all sizes, predominantly in the throat. The throat values rise rapidly from a mean of 0.09 at 3.5 μm to 0.36 at 10 μm particle diameter. Two intrathoracic fractions were also obtained by the widely accepted method of measuring the relative amounts of activity cleared from the thorax as a function of time. Alveolar deposition was apparently still some 0.06 of the inhaled aerosol at 10 μm particle diameter. Tracheo-bronchial deposition showed little change at any particle size except at 3.5 μm, when it was 0.24 of the inhaled aerosol.     

Chlorination kinetics of pyrite mineral in two Turkish lignites

The kinetics of the chlorination of pyrite in two Turkish lignites in water and water-carbon tetrachloride media at ambient pressure ( 610 mm Hg) are investigated. The effects of speed of stirring (5–20 s⁻¹), particle size (74–88, 150–180 and 250–425 μm), temperature (13–70°C) and reaction time (0–18 000 s) were studied. The experimental data were analyzed on the basis of the unreacted shrinking core model. The fine pyrite particles are assumed to be embedded inside the coal particles. The rate-controlling step was found to be diffusion of chlorine through the ash (the coal matrix). The activation energies were calculated as 25.1 kJ mol⁻¹ for Dada i coal in water medium and 25.0 kJ mol⁻¹ for Mengen coal in water-carbon tetrachloride medium.     

Synthesis of platelike CaTiO3 particles by a topochemical microcrystal conversion method and fabrication of textured microwave dielectric ceramics

Platelike CaTiO₃ particles with an orthorhombic perovskite structure have been synthesized by topochemical microcrystal conversion (TMC) from platelike precursor particles of the layer-structured CaBi₄Ti₄O₁₅ at 950 °C. The CaTiO₃ particles inherited and retained the shape of the precursor particles with a thickness of approximately 0.3 μm, and a width of 2–6 μm. XRD analysis showed that in the TMC reaction, the crystallographic {0 0 1} plane of CaBi₄Ti₄O₁₅ is converted into the {1 0 0} plane of CaTiO₃. Using the platelike CaTiO₃ particles as templates in the templated grain growth method, dense {1 0 0} grain-oriented CaTiO₃ ceramics having a {1 0 0} orientation could be fabricated at sintering temperatures between 1350 and 1500 °C. The maximum orientation factor reached 99.7% at 10% of template. It was found that texturing improves microwave dielectric low-loss properties, providing a 1.55 times higher Q_f value of 9310 GHz in textured ceramics compared to that of 6005 GHz in non-textured ceramics.