Billy W. Loo

Development of High Efficiency Virtual Impactors

The authors briefly review the development of the virtual impactor which, as an inertial particle separator according to aerodynamic sizes, has played a unique role in particle sampling, concentration, classification, and generation. Its performance characteristics in size separation are predictable by theoretical model calculations. However, its behavior in terms of internal wall losses has thus far defied quantitative analysis, and its ultimate control has eluded most practitioners in virtual impactor design. Through experimentation, the authors identify the relevant parameters in a virtual impactor and indicate their relative sensitivity and acceptable ranges of variability. With the detailed illustration of specific high-efficiency virtual impactor design, which has a cutpoint of 2.5 μm and wall losses of under 1%, it is demonstrated the underlying principles cited are crucial to minimizing losses and may be generally applicable to future developments.

A .beta.-gage method applied to aerosol samples.

An instrument for the routine measurement of aerosol mass using the beta-particle attenuation method is described. Factors affecting the precision and accuracy of the measurement are discussed in detail. Results of intercomparison studies between the beta gauge method and conventional gravimetric are presented. The design of the present instrument is particularly well suited for the automatic analysis of membrane filter obtained from modern dichotomous samplers.

Large-scale measurement of airborne particulate sulfur☆

Abstract We describe an aerosol sampling and analysis system which represents an integral approach to large-scale monitoring of airborne particulate matter. During our two-year participation in the St. Louis, Missouri, Regional Air Pollution Study (RAPS), 34,000 size-fractionated samples were collected by automated dichotomous samplers characterized by a particle size cutpoint of 2.4 μm. The total mass of the particulate matter was measured by beta-particle attenuation and the elemental composition, including sulfur, was determined by photon-excited X-ray fluorescence. The long-term performance of the system will be reported. Potential systematic effects related to the sampling and analysis of sulfur particles are treated here in detail. Both the accuracy and precision of sulfur measurement are estimated to be 2%. While the X-ray attenuation correction required is typically only a few per cent, a larger correction is required for a small fraction of the samples due to the migration of the sulfur into the filter. This correction is derived from the ratio of sulfur determinations made on the front and back surfaces of the membrane filter. Laboratory and field experiments have shown insignificant gaseous SO2 conversion on the type of filters employed in the study. Preliminary data on the composition and the temporal and spatial distribution of the St. Louis aerosol are presented. About 90% of the sulfur was found in the fine-particle fraction. Sulfur variations were significantly slower than those of the trace elements. Sulfates usually constitute about 35% of the total fine-particle mass, but may rise to 41% during an ‘episode’. The long-term (4 month average) sulfur data indicate that the background air masses arriving at St. Louis from the west and north were about 30% lower in particulate sulfur than those from the east and south. Also, an urban station may experience local increases in sulfur level up to a factor of two greater than the general background. Short-term (6 h average) data indicate that the effects of stationary SO2 sources extend for long distances, (at least 40 km) and are highly directional in character.

A New Compton Densitometer for Measuring Pulmonary Edema

Pulmonary edema (P.E.) is the pathological increase of extravascular lung water found most often in patients with congestive heart failure and other critically ill patients who suffer from intravenous fluid overload. The chest x-ray, the standard method for validating the presence of P.E., is neither quantitative nor sensitive. A non-invasive lung density monitor that is accurate, easily portable, safe and inexpensive is needed for clinical use. To deal with the problem of attenuation along the beam paths, previous gamma-ray techniques require simultaneous measurement of transmitted and scattered beams. Since multiple scattering is a strong function of the density of the scattering medium and the mass distribution within the detection geometry, there will be inherent uncertainties in the system calibration unless it is performed on a body structure closely matched to that of each individual patient. Other researchers who have employed Compton scattering techniques generally used systems of extended size and detectors with poor energy resolution. This has resulted in significant systematic biases from multiply-scattered photons and larger errors in counting statistics at a given radiation dose to the patient. We are proposing a patented approach in which only backscattered photons are measured with a high-resolution HPGe detector in a compact system geometry. By proper design and a unique data extraction scheme, effects of the variable chest wall on lung density measurements are minimized.

Automatic particulate sulfur measurements with a dichotomous sampler and on-line x-ray fluorescence analysis.

An instrument is described which employs a dichotomous sampler to acquire fine particulate samples on a continuous tape filter, Analyses for elemental sulfur are performed immediately following acquisition using a sensitive X-ray fluorescence spectrometer. Sample sequencing, data acquisition, and calculation of concentrations are performed continuously using an on-line calculator, Precision and accuracy of the method are discussed and results obtained from ambient measurements are presented. The increased emission of sulfur-containing compounds into the atmosphere has become a major consideration in the design of air pollution monitoring and control strategies. This has stimulated considerable interest in the development of monitoring equipment capable of analyzing the atmospheric aerosol for sulfur-containing compounds. We describe a rapid, sensitive and accurate X-ray fluorescence method for the analysis of elemental sulfur collected from ambient aerosol samples. The instrument includes a dichotomous sampler, air filter transport system and high-sensitivity wavelength dispersive X-ray fluorescence analyzer. An associated calculator/controller enables one to obtain real-time measurements of ambient particulate sulfur concentrations over short time intervals.

A LASER-BASED MONODISPERSE CARBON FIBER GENERATOR

Abstract A generator for delivering a controlled number of carbon fibers of prescribed uniform lengths for the purpose of on-line detector calibration is described. Results showed that it is feasible to generate carbon fibers from about 5 mm long down to 30 μm or less with an accuracy of approximately 5 μm.

Advances in the Measurement of Ambient Carbon Fibers

The authors briefly summarize various methodologies that have been used in a number of passive collectors and active detectors for measuring ambient carbon fibers. The need for improved performance in terms of specificity, sensitivity, detection efficiency, accuracy, precision, speed, stability, and serviceability are identified. The design principles for a new detector based on the technique of measuring the induced charge acquired by a conducting fiber in an electric field are discussed in detail. A prototype of such a detector has been tested to cover the measurement of fiber length distributions between 0.08 and 9 mm and fiber concentrations between 1 and 10/sub 6/ fibers/m/sup 3/ of air. The precision of this instrument has been demonstrated to be about 2% for fibers in the 1-9-mm range and to approach one fiber diameter (or 10..mu..m) for the submillimeter fibers. 15 references, 2 figures, 1 table.

Elemental sulfur analysis using x-ray techniques and. beta. -gauge mass measurements

The activities funded under the present EPA/ERDA interagency agreement have been concentrated mainly in two areas. An elemental sulfur monitor for the measurement of fine particulates has been designed, constructed, and is currently being tested. A prototype of the device will be delivered in the near future. A second program has involved the development of the ..beta..-attenuation technique for the large-scale measurement of the masses of particulate deposits. The emphasis has been primarily concerned with the fabrication of improved hardware to perform the measurement conveniently and accurately. Other activities included consultation with EPA on questions concerning air sampling techniques and X-ray fluorescence analysis of aerosol samples. An additional long-term interest has been the development of X-ray techniques which are capable of determining the chemical form of the sulfur atoms in an environmental sample. Since these latter activities are in a more speculative period of development, the present progress report emphasizes the activities in the area of elemental sulfur monitoring and ..beta..-gauge development.