Avula Sreenath

A Novel Optical Instrument for Estimating Size Segregated Aerosol Mass Concentration in Real Time

A novel optical instrument has been developed that estimates size segregated aerosol mass concentration (i.e., PM 10 , PM 4 , PM 2.5 , and PM 1 ) over a wide concentration range (0.001–150 mg/m 3 ) in real time. This instrument combines photometric measurement of the particle cloud and optical sizing of single particles in a single optical system. The photometric signal is calibrated to approximate the PM 2.5 fraction of the particulate mass, the size range over which the photometric signal is most sensitive. The electrical pulse heights generated by light scattering from particles larger than 1 micron are calibrated to approximate the aerodynamic diameter of an aerosol of given physical properties, from which the aerosol mass distribution can be inferred. By combining the photometric and optical pulse measurements, this instrument can estimate aerosol mass concentrations higher than typical single particle counting instruments while providing size information and more accurate mass concentration information than traditional photometers. Experiments have shown that this instrument can be calibrated to measure aerosols with very different properties and yet achieve reasonable accuracy.

Experimental investigations into the nature of airflows near bluff bodies with aspiration, with implications to aerosol sampling

The research described in this paper was stimulated by the need to understand better the nature of air flow around aerosol samplers of the type widely used in environmental and industrial hygiene. It deals with the application of visualisation techniques to determine the location of stagnation points for air flow about simple two- and three-dimensional bluff bodies (cylinder and sphere, respectively) for the case where there is aspiration of air (i.e. suction) from a point on the body surface. The effect of orientation of the sampling orifice (or sink) with respect to the free stream on the location of stagnation points was compared with theoretical predictions using potential flow models. Good agreement was obtained, even for large angles with respect to the wind. For the two-dimensional cylindrical body, we also experimentally investigated the frequency of the vortex shedding in its near wake and how that is influenced by the aspiration. As represented by the dimensionless Strouhal number, this was found to be strongly dependent on the aspiration flow rate and the slot orientation. The results may be explained qualitatively in terms of the effect of aspiration on the development of the boundary layer over the cylinder surface.

New experimental studies of the basic performance characteristics of aerosol samplers.

The study of the basic physical performance characteristics of aerosol samplers, like those used in the occupational hygiene setting, will provide insights to enable the improved development of new instruments and cost-effective testing procedures. These will be required as the new particle size-selective sampling criteria become the basis of new occupational exposure standards. A new body of work is being conducted, in which the factors influencing sampler performance are being investigated using idealized samplers of spherical shape in small wind tunnels. By the experimental methods described, a large amount of performance data can be acquired in a very short time. The results for wide ranges of particle size, wind speed, sampling flow rate, and sampler orientation conditions show that there are strong trends as functions of these variables. Those trends are very complicated. But it is encouraging that they are broadly consistent with recent semi-empirical models, suggesting that extensions of that type of modelling approach--supported by the large amount of new experimental data now being generated by such experiments--might provide new models of aerosol sampler performance accessible to researchers and occupational hygienists.

A Modified Protocol for Quantitative Fit Testing Using the PortaCount

A modified quantitative fit testing method has been developed for testing half masks using the TSI PortaCount respirator fit tester. This approach focuses on shortening the time for each exercise during fit testing; however, the shortened protocol is applied only to the very good-fitting masks. For marginal-fitting masks, the testing is carried out according to the full Occupational Safety and Health Administration (OSHA) respiratory protection standard (29CFR1910.134).(1) The shortened protocol (currently not approved by OSHA) still uses all the exercises required by the OSHA standard but for a shorter time (30 seconds [s] for each exercise instead of the usual 60 s). How good the fit has to be to qualify for a shortened exercise is determined by the statistical analysis of a large data set containing pass and fail fit-test data. The statistical analysis involves calculating the sensitivity and specificity of the pass and failed fit tests on half masks. From this analysis, a multiplication factor (K) to the OSHA pass/fail criterion was developed. For a respirator to undergo the shortened protocol, the fit factor obtained during any exercise must be K times the OSHA pass/fail criterion of 100 for half masks. Hence, this approach is more conservative than fit testing protocols that involve shortened exercises regardless of the fit. Nevertheless, this approach still saves time without compromising the accuracy of the fit test expressed in terms of sensitivity and specificity. For the existing data, 85 percent of the fit tests would have been performed according to the faster test protocol while only 15 percent of the tests would have been tested according to the full-length OSHA test protocol.

Experimental Study of Particle Losses Close to the Entry of Thin-Walled Sampling Probes at Varying Angles to the Wind

This article summarizes the results of an extensive experimental study of sampling losses in thin-walled probes at various values of velocity ratio R and the probe orientation with respect to the freestream. The purpose of this study was to gain insights into the complex interaction of various parameters that influence sampling losses and the consequent effect on the overall sampling efficiency. A 0.635 cm diameter sharp-edged tube was mounted in a small wind tunnel where the freestream velocity could be varied over a wide range of values. Polydispersed spherical glass beads were used as the test aerosol. The number concentration and the particle size distribution were measured using the aerodynamic particle sizer (APS 3310). The sampling efficiency was determined as a function of orientation for a range of particle sizes (or Stokes number). By using an existing model to predict the aspiration efficiency for thin-walled probes, the sampling losses could be isolated from the sampling efficiency. In this manner a new empirical model was developed to predict the losses as a complex function of Stokes number, sampler orientation, and velocity ratio. The losses appear to be influenced by particle inertia, impaction, gravitational settling in the boundary layer developing inside the thin-walled probe, and vena contracta or flow recirculation loss near the entry. It was evident from the results that these losses are strongly influenced by the Stokes number and sampler orientation. The losses also increased strongly with increasing value of velocity ratio for all orientations.

Aspiration characteristics of idealized blunt aerosol samplers at large angles to the wind

Abstract The research described in this paper extends the body of work contained in earlier publications in which aerosol sampling aspiration efficiency was studied in a small wind tunnel for simple, idealized sampler geometries. These studies are aimed at providing generic knowledge about the physical and functional behaviors of such systems that can then be generalized to sampling systems of more practical interest. In this paper, attention is focused on the little-researched case of a blunt sampler facing away from the prevailing wind velocity. The results show, that for angles greater than 90 ∘ with respect to the wind, aspiration efficiency and particles losses inside the sampler close to the entry are constant as a function of α . This in itself may have some practical usefulness since it enables the definition of a regime of aerosol performance that is relatively “well-behaved”. These results are in marked contrast to what has been learned from the same body of work for sampler behavior when facing forward with respect to the wind. Here not only does aspiration efficiency vary sharply with orientation, but so too does the particle entry loss. From this it is concluded that no general model of aspiration efficiency can be available so long as experimental results embody unknown entry loss biases. With this in mind, caution is recommended in field sampling or in laboratory experiments where the measurement does not explicitly include such entry losses.