Measured Size Range Research Articles

THE SCANNING FLOW DMA

A new method of DMA operation has been implemented in which the flow rates are continuously changed in conjunction with the applied voltage. By optimizing the flow and voltage ramps, improvements can be made in the DMA's measurable size range, counting statistics, resolution, or a partial combination of each of these. Detailed modeling of this technique suggests that errors on the order of 2 to 5% result from incorrect assumptions concerning the flow profile within the DMA. The experimental system enabled accurate control of flows that were varied by an order of magnitude in as little as 30 s . Excellent agreement was obtained between mobility distributions recovered from a voltage ramp, a flow ramp, and a combined voltage and flow ramp. Slight deviations were apparent in the recovered data as the flow scan time was reduced from 60 to 30 s .

Single aerosol particle sizing and identification using a coupled-cavity diode laser

A new instrument for aerosol particle size determination and identification by shape is presented. The instrument is based on a diode laser in a coupled cavity. An extinction loss determines the size of a single aerosol particle entering the coupled-cavity diode laser, and a Fourier transformed diffraction image of near-forward scattered light is used for shape and orientation analysis. We describe a model of the coupled-cavity diode laser using light scattering based on Mie theory. This model relates the extinction loss directly to the cross-sectional area of the aerosol particle and shows good agreement with experimental results on aerosol particles and fibers. It is demonstrated that the changes in extinction loss are essentially independent of refractive index of the aerosol particles. The measurable size range of the aerosol particles is 4–170 μm with an accuracy of ±0.5 μm. The lower limit of the range is set by the extinction loss S/ N ratio, the upper limit is given by the laser beam cross-section, and the accuracy is imposed by the oscillations in the extinction efficiency factor.

The hot plate impactor: A novel instrument for continuous characterization of liquid sprays

A method is described to continuously characterize sprays of volatile liquids. Liquid drops are impacted inertially from a flowing gas stream onto collectors of different sizes, hence having differing collection characteristics. Electric heating of the collectors both removes the liquid by evaporation and also allows measurement of its rate of deposition from the heat required to maintain a given temperature of the collector. By using collectors of various sizes, different components of the drop size spectrum are impacted on different collectors thus allowing an indication of the distribution of drop sizes to be obtained. Experiments have been conducted using water sprays having mass median diameters in the range 5–20 μm at liquid fluxes from 10−4 to 0.1 kg/m2s in air at a velocity of 3 m/s. The measurable drop size range may be altered by selecting appropriately sized collectors.

Design and evaluation of a nanometer aerosol differential mobility analyzer (Nano-DMA)

A nanometer aerosol differential mobility analyzer, Nano-DMA, has been developed for measuring the size distribution of nanometer aerosols in the particle size range of 3–50 nm. The design is based on a cylindrical configuration and is optimized by means of the numerical model of Chen and Pui (1997, J. Aerosol Sci. 28, 985–1004). Important design features include high particle penetration (low loss) through the Nano-DMA and high sizing resolution. For reducing particle loss in the aerosol transport passage, the aerosol residence time in the Nano-DMA is reduced by shortening the inlet transport passage. An optional feature of high inlet flow (up to 16.5 lpm) is designed in order to further reduce the residence time between the aerosol inlet and the slit in the classifying region of the Nano-DMA. A new entrance slit is designed to have optimal aerosol and sheath flow matching at a flow ratio of 1 : 10, and has a wide dynamic flow-ratio range (up to aerosol/sheath flow ratio of 1/70) compared with the TSI-standard DMA design. This slit improvement makes the Nano-DMA suitable for high resolution particle sizing and classification. For reducing the effect of Brownian diffusion broadening on the transfer function of the Nano-DMA, the collector tube length is shortened to 5.0 cm compared to the TSI-Standard DMA of 44.44 cm and TSI-Short DMA of 11.11 cm. At the design flow condition of 1.5 lpm aerosol (or 16.5 lpm aerosol high inlet flow case) and 15.0 lpm sheath flow rates, the measurable size range is from 3–50 nm. The lower detection limit of 3.0 nm coincides with the lower detection limit of the TSI UCPC (Ultrafine Condensation Particle Counter). The base of the Nano-DMA is completely re-designed to avoid particle loss due to the undesirable electrostatic effect observed by Kousaka et al. (1986, J. Chem. Eng. Japan 19, 401), and to obtain an uniform electric field in the entire classifying region. The overall performance of the Nano-DMA is then evaluated by the numerical model of Chen and Pui (1997, J. Aerosol Sci. 28, 985–1004) before its construction and experimental evaluation. By comparing with the experimental results obtained using the Tandem DMA technique described in Hummes et al. (1996, J. Aerosol Sci. 27, S135–S136; Part. Part. Syst. Charact. 5, 327–332), it is concluded that the Nano-DMA is performing well in the designed size range and its transfer function agrees well with the numerical prediction.

In situ determination of the activation energy for restructuring of nanometer aerosol agglomerates

A new approach to the kinetics of the rearrangement of aerosol agglomerates is presented. This analysis is based on the change in free energy per primary particle during restructuring. The excess free energy compared with the final state drives the agglomerate to become more compact which results in an increase in the coordination number, as estimated from the changing fractal dimension of the aerosol agglomerates The analysis was tested on agglomerates of nanometer silver and copper particles produced by an evaporation-condensation technique and by laser ablation. The assumption that the rearrangement is an activated process was supported by the results. It was concluded that the interaction between the primary particles occurs over a short range. The activation energies were one order of magnitude lower than the bond energies expected from the bulk Hamaker constants for silver and copper. Within the measured size range no significant difference in the activation energies of silver and copper agglomerates was found. A maximum in the activation energy was found for particles about 18 nm in diameter. Larger particles showed decreasing activation energy with increasing diameter. The addition of oxygen (50%) before the agglomeration of the primary particles increased the activation energy by a factor of two and shifted the maximum in the activation energy to 15 nm. The prefactor in the rate coefficient for restructuring increased by four orders of magnitude when oxygen was added

New Observations of Subarcsecond Photospheric Bright Points

We have used an interference filter centered at 4305 A within the bandhead of the CH radical (the 'G band') and real-time image selection at the Swedish Vacuum Solar Telescope on La Palma to produce very high contrast images of subarcsecond photospheric bright points at all locations on the solar disk. During the 6 day period of 1993 September 15-20 we observed active region NOAA 7581 from its appearance on the East limb to a near-disk-center position on September 20. A total of 1804 bright points were selected for analysis from the disk center image using feature extraction image processing techniques. The measured Full Width at Half Maximum (FWHM) distribution of the bright points in the image is lognormal with a modal value of 220 km (0 sec .30) and an average value of 250 km (0 sec .35). The smallest measured bright point diameter is 120 km (0 sec .17) and the largest is 600 km (O sec .69). Approximately 60% of the measured bright points are circular (eccentricity approx. 1.0), the average eccentricity is 1.5, and the maximum eccentricity corresponding to filigree in the image is 6.5. The peak contrast of the measured bright points is normally distributed. The contrast distribution variance is much greater than the measurement accuracy, indicating a large spread in intrinsic bright-point contrast. When referenced to an averaged 'quiet-Sun' area in the image, the modal contrast is 29% and the maximum value is 75%; when referenced to an average intergranular lane brightness in the image, the distribution has a modal value of 61% and a maximum of 119%. The bin-averaged contrast of G-band bright points is constant across the entire measured size range. The measured area of the bright points, corrected for pixelation and selection effects, covers about 1.8% of the total image area. Large pores and micropores occupy an additional 2% of the image area, implying a total area fraction of magnetic proxy features in the image of 3.8%. We discuss the implications of this area fraction measurement in the context of previously published measurements which show that typical active region plage has a magnetic filling factor on the order of 10% or greater. The results suggest that in the active region analyzed here, less than 50% of the small-scale magnetic flux tubes are demarcated by visible proxies such as bright points or pores.

Fractional Aerosol Filtration Efficiency of In-Duct Ventilation Air Cleaners

The filtration efficiency of ventilation air cleaners is highly particle-size dependent over the 0.01 to 3 μm diameter size range. Current standardized test methods, which determine only overall efficiencies for ambient aerosol or other test aerosols, provide data of limited utility. Because particles in this range are respirable and can remain airborne for prolonged time periods, measurement of air cleaner fractional efficiency is required for application to indoor air quality issues. The objectives of this work have been to 1) develop a test apparatus and procedure to quantify the fractional filtration efficiency of air cleaners over the 0.01 to 3 μm diameter size range and 2) quantify the fractional efficiency of several induct air cleaners typical of those used in residential and office ventilation systems. Results show that efficiency is highly dependent on particle size, flow rate, and dust load present on the air cleaner. A minimum in efficiency was often observed in the 0.1 to 0.5 μm diameter size range. The presence of a dust load frequently increased an air cleaner's efficiency; however, some air cleaners showed little change or a decrease in efficiency with dust loading. The common furnace filter had fractional efficiency values of less than 10% over much of the measurement size range.

An empirical estimate of the diffusion rate of oxygen in diopside

ABSTRACTThe intracrystalline diffusion rate of oxygen in diopside was constrained based on natural isotopic variations from a granulite facies marble from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, are nearly constant (20.9 ± 0.3‰ vs. SMOW) over the entire measured size range (0.3–3.2 mm diameter). The δ18O values of the cores of calcite grains are 23.0‰. Temperature estimates based on the Δ18O(calcite‐diopside) are 800d̀C, in agreement with the highest previous thermometric estimates for these rocks.The lack of isotopic variation in the diopsides as a function of grain size requires that the oxygen intracrystalline diffusion rate in diopside from the Adirondack samples was very slow. The maximum diffusion rates (D800d̀C parallel to the c‐axis) were calculated with an infinite reservoir model (IRM) and a finite reservoir model (FRM) that incorporates mineral modal abundances and initial isotopic variations. For an assumed activation energy (Q) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 times 10‐20cm2/s is two orders of magnitude faster than from the FRM; at Q=500kJ/mol, the D800d̀C estimate for both methods is c. 5.6 times 10‐20 cm2/s. The present results require that a hydrothermal fluid significantly enhances the diffusion rate of oxygen in diopside if previous data are correct.The δ18O(SMOW) and δ13C(PDB) values of the calcite, measured in situ with a CO2 laser, are 22.9 ± 0.3, 0.1±0.3‰ in the grain cores, 22.1 ±0.3, 0.2 ±0.1‰ at the grain boundaries and 21.7 ±0.4, ‐0.6±0.1‰ abutting diopside grains. The δ18O and δ13δC values measured conventionally are: crystal cores, 22.96, ‐0.95‰; abutting diopside grains, 22.38, ‐0.93‰; bulk, 22.79, ‐0.95%. Use of the bulk δ18O(calcite) values for thermometry yields unreasonably high temperatures. The lower δ18O values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to be a result of a late retrograde fluid infiltration.

Populations of faults and fault displacements and their effects on estimates of fault-related regional extension

Measured populations of fault displacements, derived from regional seismic, oilfield seismic, coalmine plans and outcrop data show a power law distribution with exponents (− S) of −0.45 to −0.95 for singleline samples across an array of faults. The more negative values indicate relatively larger numbers of smaller faults. An expression for a population of active faults, derived from the Gutenberg-Richter magnitude-frequency relation for earthquakes, is log N = a − b D log D, where D = maximum displacement of a fault, N = number of faults of maximum displacement D or greater, and b D ≊ 1.0 and has the same value as b for the corresponding earthquake population. Populations of ‘dead’ faults existing at the end of a tectonic episode have been numerically derived, using a fault growth model, and satisfy the relation log N = a − E log D where E has a value between 1.6 and 2.0. Numerically derived populations of fault displacements in a dead fault population have slopes of − S where S = E − 1. The contribution of an individual fault to the regional strain varies with the lifetime seismic moment of the fault and is proportional to D 2. Estimation of fault-related extension by summing heaves on faults of a limited size range is valid only if the measured size range of faults accommodates most of the extension. Correction can be made if the S value of the fault displacement population is known.

Fiber-optic particle size monitor based on white-light scattering

A new fiber-optic particle size monitor for in situ measurement based on white-light scattering is presented. The particle size is determined from the ratio of two scattered light intensities taken at two wavelength bands. By using white-light scattering, the proper wavelength bands in the broad spectral region can be selected in accordance with the particle size that is to be measured. Then the measurable particle size range can be expanded and the particle size can be measured precisely. The performance of this particle size monitor is theoretically discussed, and experimental data from polystyrene latex suspensions are shown to confirm its operation.

Crystal growth phenomena of paraxylene crystals

A study on the morphology of paraxylene crystals growing from the melt and from different solvents revealed that two distinct morphologies develop under all circumstances: (i) crystals with {001} as the predominant faces and (ii) crystals with {010} as the predominant faces. The occurence of type (i) crystals can be explained on the basis of the crystal structure, but the existence of type (ii) can only be ascribed to the actual supersaturation allowing for the exposure of the {010} face or to the action of an unidentified impurity allowing for the growth of the {010} faces. Experiments on crystal growth kinetics from the melt showed that paraxylene crystals show stable growth up to an undercooling of 2°C. The order of the surface integration step suggests that growth proceeds by the propagation of cooperating growth spirals. At higher supersaturations, the formation of dendrites and needles occurs. Growth rate measurements from solutions containing 70% paraxylene revealed that (i) the growth rate is length-independent in the measured size range, and (ii) growth and dissolution kinetics do not depend on the nature of the solvent within the accuracy of our experiments. Measurements from 30% solutions showed much lower growth rates. Comparison between the kinetic parameters assessed under the different conditions suggests that under the conditions prevailing in the process industry (-45°C), rather low growth rates are to be expected because volume diffusion becomes more rate-determining at lower concentrations.

Photochemical formation of organic aerosols: Growth laws and mechanisms

Smog chamber experiments were conducted to study aerosol formation by chemical reactions involving organics ( o-cresol, nitrocresol, l-heptene, and methyl sulfide), oxides of nitrogen and ammonia in air. Aerosol size distributions were measured as a function of time with an electrical aerosol analyzer (0.01 μm < particle diameter < 0.56 μm) and with a single particle optical counter (0.48 μm < particle diameter < 3 μm). The aerosol data were analyzed with the ‘growth law’ technique to make inferences about chemical mechanisms of gas-to-particle conversion in these model chemical systems. Particle diameter growth rate measurements were made after the production of new panicles had stopped (i.e. while number concentrations were decreasing), but while aerosol mass concentrations were steadily increasing. The growth rates of ‘small’ particles (particle diameter < 0.02–0.1 μm) were typically negative, suggesting that these particles were evaporating. Growth rates of larger panicles tended to increase with particle size within the measured size range. This behavior is consistent with condensation, provided that the dependence of particle curvature on vapor pressure (the Kelvin effect) is taken into account. It is concluded that gas phase chemical reactions (rather than reactions on or within aerosol particles) are responsible for the formation of the aerosol. Saturation vapor concentrations for the condensable species that were formed in these systems were estimated to range from 0.04 to 2 ppb, depending on the chemical system.

Effect of Low and High Relative Humidity on Metered-dose Bronchodilator Solution and Powder Aerosols

Physical properties of two metered-dose bronchodilator aerosols packaged as solutions and two aerosols packaged as finely ground powders were measured at low and high relative humidity. The aerodynamic size distribution and particle concentration were measured in real time using the single-particle aerodynamic relaxation time analyzer, which can measure the aerodynamic diameter of single suspended particles in the respirable size range. The count median aerodynamic diameter, the mass median aerodynamic diameter, the total particles per dose, and the total aerodynamic mass per dose were calculated. Significant increases were noted in the count median aerodynamic diameter for three aerosols and in the mass median aerodynamic diameter for two aerosols. The number of particles in the measured size range increased 3.6- and 4.1-fold for the droplet aerosols and 1.4-fold for the powder preparations. The aerodynamic mass per dose in the measured size range increased 5.7- and 11.4-fold for the droplet aerosols and 3.1- and 1.6-fold for the powder aerosols. These data indicate that all aerosols tested increased in size at high humidity and that aerosols dispensed as droplets may be more unstable than those dispensed as powders.