Andrew R. McFarland

Flattening coefficient for DOP and oleic acid droplets deposited on treated glass slides

The flattening coefficient, F, which is the ratio of apparent droplet diameter as measured on a microscope slide to the true diameter of a spherical drop, has been determined for particles of oleic acid and DOP aerosols collected on treated slides. The slides were made oilphobic by immersion in a widely used fluorocarbon surfactant (NYEBAR Type CT or 3 M Co. Chemical FC-721). The mean value of F for oleic acid was found to be 1.34, and for DOP, 1.35. There is no apparent variation of F with particle diameter for aerosols in the 2.7−29.1 µm range. For DOP, a single technique was used to measure F1 and for oleic acid three techniques were employed. The different techniques gave essential the same result for oleic acid. The addition of a commonly used fluorescent analytical tracer, sodium fluorescein, to oleic acid in amounts of 5–20 percent (m/v) causes a slight shift in the flattening coefficient. With 10 percent uranine, the value of F is 1.32.

A circumferential slot virtual impactor

A virtual impactor aerosol concentrator has been developed that uses circumferential slots for acceleration of aerosol particles and for collection of the coarse fraction. This allows for accurate and economical machining of small slot widths, which leads to low-pressure losses for the separation process. One important application of the device is in the concentration of bioaerosols, especially for military field applications where minimization of power consumption is necessary. A prototype configuration of the circumferential slot virtual impactor (CSVI), which was designed using numerical methods, was constructed and tested. The device has a curvilinear slit nozzle with a diameter of 150.3 mm (5.918 in), which provides a total slot length of 472 mm. Its slot width was 0.499 mm (0.0197 in). According to Loo and Cork, for circular-jet virtual impactors the misalignment between the axis of the acceleration jet and the receiver nozzle will cause an increase in wall losses of about 1.6% for each 1% of misalignment. Measurements were made of the nozzle dimensions in the critical region of the CSVI that showed 1.8% relative misalignment. When this prototype was operated at a flowrate of 122 l/min and a flow fraction (minor air flowrate/total air-flowrate) of 10%, the cutpoint was 2.2 μm aerodynamic diameter and the corresponding cutpoint Stokes number was 0.58. The collection efficiency was greater than 72% for particle sizes larger than twice the cutpoint, up to the largest particle size tested (10 μm aerodynamic diameter). The peak collection efficiency was greater than 95%. For virtual impactors, a critical performance parameter is the loss of particulate matter to the inner walls of the system. For the prototype system, where numerical methods had been used to generate designs that reduced wall losses, the losses at the cutpoint size of 2.2 μm aerodynamic diameter, are approximately 3%. For an operational condition of a total flowrate of 122 l/min and a coarse particle flow fraction of 10%, the pressure drop across the major flow stream (fine particle stream) was 63 Pa (0.25 in of water), with an ideal power consumption of 0.14 watts.

Wind tunnel evaluation of a modified andersen impactor and an all weather sampler inlet

Abstract Modifications were made to a one-cfm (4.72 × 10 −4 m 3 / s ) Andersen impactor to permit the collection of larger particles than previously possible. An All Weather Sampler Inlet was developed for the unit which enables acquisition of unbiased aerosol samples for varying meteorological parameters. When tested with a uranine-tagged oleic acid aerosol in a wind tunnel operated at a speed of 4.6 m/s (15 ft/sec), the standard Andersen design precludes collection of more than 50 percent of particles larger than approximately 7 μm diameter whereas the corresponding particle size for the modified system is 14 μm. Wind tunnel tests with the modified unit show the performance to be relatively unaffected by wind speeds over the range of 5–15 ft/sec or by turbulence with intensities up to eight percent. The geometrical configuration precludes wind directional effects.

Turbulent Deposition of Aerosol Particles in Large Transport Tubes

ABSTRACT Particle deposition from turbulent flows in large tubes (up to 102 mm) was measured experimentally and a correlation developed for the results. The correlation allows prediction of the nondimensional deposition velocity as a function of the nondimensional relaxation time and the flow Reynolds number. The correlation shows good agreement with previous correlations for deposition in small diameter tubes as well for the full range of tube sizes (13–102 mm diameter) and Reynolds numbers (up to 55,000) upon which the correlation is based. As an example of the improvement in predictive abilities rendered by this new model, the penetration of 20-mm-aerodynamic-diameter aerosol particles in a 102-mm-diame-ter tube at a flow rate of 2260 L/min was measured to be 59%. The present model predicts a penetration of 62%, while two previously reported models that do not include Reynolds number effects predict 80% and 82% penetration. This new model should be of benefit in the design and evaluation of large-sized...

Wetted Wall Cyclones for Bioaerosol Sampling

A wetted wall bioaerosol sampling cyclone with an aerosol sampling flow rate of 1250 L/min and a continuous liquid outflow rate of about 1 mL/min was developed by upgrading an existing system. The aerosol-to-hydrosol collection efficiency curve for the upgraded device was shown to have a cutpoint of 1.2 μ m aerodynamic diameter (AD) and an average collection efficiency of 90% over the size range of 2 to 10.2 μ m AD. Tests with near-monodisperse cells and clusters of Bacillus atrophaeus (aka BG) spores showed an average aerosol-to-hydrosol collection efficiency of 98% over the size range from 1.7 to 9.8 μ m AD. Pressure drop across the cyclone, which is also the ideal specific power, was 5.5 kPa (22 inches H2O). Stokes scaling was used to design geometrically similar cyclones with nominal air sampling flow rates of 100 and 300 L/min. Extensive tests were performed with the 100 L/min unit and check tests with the 300 L/min. Results with the scaled units showed similar, although somewhat lower collection efficiencies than the 1250 L/min device, but with lower consumption of liquid and lower pressure losses. For the 100 L/min cyclone, the cutpoint of the aerosol-to-hydrosol efficiency curve was 1.2 μ m AD, and the average collection efficiency for single cells and clusters of BG spores was 86% over a size range of 1.2 to 8.3 μ m AD. Also, for the 100 L/min cyclone, typical output liquid flow rates were 100 μ L/min, and the pressure loss was 1.6 kPa (6.4 inches H2O).

Continuum field-diffusion theory for bipolar charging of aerosols

Abstract Continuum regime field-diffusion theory for the acquisition of charge by particles exposed to bipolar ions is developed. An approximate numerical solution to the governing equations is described which, in the limit as the external field approaches zero, yields results within 1% of exact solutions. The numerical method is used to calculate particle charge as a function of time, external electric field strength, ratio of positive to negative ion conductivity, particle radius and particle dielectric constant. With increasing time, charge is predicted to approach a steady state value asymptotically. For a given conductivity ratio, steady state charge is predicted to increase almost linearly with increasing external electric field. For a given charging time and field strength, charge asymptotically approaches unipolar levels with increasing conductivity ratio. Predictions based on theory generally show good agreement with experiment, typically being within ± 10–15%. The exception to this is for intermediate conductivity ratios, where theory exceeds experiment by as much as 30°.

Sampling and Retention Efficiencies of Batch-Type Liquid-Based Bioaerosol Samplers

Four commercially available batch-type bioaerosol samplers, which collect time-integrated samples in liquids, were evaluated. Sampling efficiency was characterized as a function of particle size using near-monodisperse polystyrene spheres (sizes of 1–5 μ m) and oleic acid droplets (3–10 μ m). Results show the sampling efficiency of AGI-30 impingers range from 4–67% for particle sizes of 1 to 5.1 μ m with significant variations between units; those of SKC BioSampler impingers range from 34–105% for particle sizes from 1 to 9 μ m; those of a batch-type wetted wall cyclone with compensation for evaporation (BWWC-EC) range from 5 to 65% for particle sizes 1 to 10 μ m; and, those of a batch-type wetted wall cyclone with no evaporation compensation (BWWC-NC) range of 55 to 88% for particle sizes of 1–8 μ m. Retention efficiency was measured for 1 and 10 μ m polystyrene spheres. For the AGI-30 and BWWC-EC, the retention efficiency of 1 μ m particles after 1 h was less than 30%, while that of the SKC BioSampler was 59%. Due to liquid evaporation, the BWWC-NC could not be operated for 1 h. Retention efficiencies for Bacillus atrophaeus spores and Pantoea agglomerans vegetative cells were measured for the AGI-30 and the SKC BioSampler. Results for the spores were about the same as those for 1 μ m non-viable polystyrene particles; however, the vegetative bacteria lose culturability and consequently show lower retention efficiencies. For the impingers, significant performance differences were observed in units delivered by vendors at different times.

Design of Stairmand-Type Sampling Cyclones

An empirical, nondimensional correlation of cut-point Stokes number (Stk0.5) and flow Reynolds number (Re) has been established for small Stairmand-type sampling cyclones. Four cyclones with body diameters of 38, 57, 89, and 140 mm were constructed and tested with monodisperse aerosols over a range of flow rates. The flow rates were chosen to provide preselected increments of particle Froude numbers. These flow rates for the four cyclones spanned the range of 9.4 to 1080 L/min and provided Froude numbers of 1.5, 2.0, 2.5, and 6.0. The resulting Reynolds numbers (based upon cyclone body diameter and inlet flow rate) covered the range of 2.1 x 10(3) to 6.4 x 10(4). Sizes of monodisperse aerosols used in this study were from 3.0- to 17.4-microns aerodynamic diameter. The graphical correlation between cut-point Stokes number and Reynolds number showed there to be no effect of Froude number (for the range of Froude numbers tested). The data have been fit by a least squares procedure to a quadratic logarithmic function. In addition to development of the empirical correlation, the results of this study also provide data pertinent to the regional deposition of liquid particles within the cyclone and to the transmission of solid particles through the cyclone. The carryover of solid, 19-microns diameter particles is only 0.5% greater than that of liquid particles of the same size.

Deposition of Aerosol Particles in Contraction Fittings

ABSTRACT Particle deposition was measured in contraction fittings with half-angles of 12°, 45°, and 90°. It was found that for a given contraction half angle, the losses in a contraction fitting correlate well with the parameter Stk(1-Ao Ai ) where: Stk is a Pich-type Stokes number based on inlet velocity and outlet diameter, Ao is the outlet area, and Ai is the inlet area. A correlation developed from the experimental results allows prediction of the particle losses in contraction fittings as a function of Stokes number, area ratio, and contraction half-angle. Test conditions on which the correlation was based include a range of 0.001 ≤ Stk(1-Ao/Ai ) ≤ 100. Area ratios Ao/Ai used in the tests were 0.062, 0.215, and 0.571, and the test conditions encompassed Reynolds numbers based on downstream port diameter of 1120 to 58,500. The aerosol particle deposition in the contraction fitting was also modeled numerically and the numerical results show good agreement with the experimental data. Flow turbulence was...

Bioaerosol Sampling with a Wetted Wall Cyclone: Cell Culturability and DNA Integrity of Escherichia coli Bacteria

Contemporary near-real-time bioaerosol identifiers that read labeled DNA require a minimum DNA length of about 500,000 base pairs; and for critical applications, instrumental identification results must be verified through the use of classical microbiological culturing techniques. A 300 L/min Wetted Wall Cyclone (WWC) and an 800 L/min inertial impactor were used in a comparative study to collect aerosolized single cells of Escherichia coli (E. coli) at temperatures of 24°C and 46°C. Classical microbiological plating techniques showed that the culturability of E. coli collected with a WWC is a factor of about 100 higher than that of the impactor when the sampled aerosol is at room temperature (RT) and a factor of about 4000 higher when the sampled aerosol is at 46°C. DNA integrity was qualitatively evaluated with pulsed field gel electrophoresis (PFGE) and photographic evidence shows a significant difference in the amount of high molecular weight DNA (molecules larger than 500,000 base pairs) collected with the WWC compared with the impactor. Extracted DNA was also digested by the NotI enzyme, and the qualitative results of the restriction analysis showed there to be high integrity of the WWC-collected DNA, whereas the impactor-collected DNA showed considerable fragmentation. Real-Time polymerase chain reaction (RT-PCR) showed samples required for E. coli identification need to be about 100 times more concentrated if they are collected with the impactor rather than that of the WWC. Also, it appears that only the intact genomic DNA of the culturable cells provides adequate templates for traditional and RT-PCR amplification. Copyright 2012 American Association for Aerosol Research