Phil A. Lawless

The design and field implementation of the Detroit Exposure and Aerosol Research Study

The US Environmental Protection Agency recently conducted the Detroit Exposure and Aerosol Research Study (DEARS). The study began in 2004 and involved community, residential, and personal-based measurements of air pollutants targeting 120 participants and their residences. The primary goal of the study was to evaluate and describe the relationship between air toxics, particulate matter (PM), PM constituents, and PM from specific sources measured at a central site monitor with those from the residential and personal locations. The impact of regional, local (point and mobile), and personal sources on pollutant concentrations and the role of physical and human factors that might influence these concentrations were investigated. A combination of active and passive sampling methodologies were employed in the collection of PM mass, criteria gases, semivolatile organics, and volatile organic compound air pollutants among others. Monitoring was conducted in six selected neighborhoods along with one community site using a repeated measure design. Households from each of the selected communities were monitored for 5 consecutive days in the winter and again in the summer. Household, participant and a variety of other surveys were utilized to better understand human and household factors that might affect the impact of ambient-based pollution sources upon personal and residential locations. A randomized recruitment strategy was successful in enrolling nearly 140 participants over the course of the study. Over 36,000 daily-based environmental data points or records were ultimately collected. This paper fully describes the design of the DEARS and the approach used to implement this field monitoring study and reports select preliminary findings.

Particle charging bounds, symmetry relations, and an analytic charging rate model for the continuum regime

For many years, ionic particle charging has been described in terms of field or diffusion charging because of their simplicity as models. Important advances in charging theories have come with publication of numerical models of the process. Still, numerical models exact a computational penalty that has led to investigations of approximations for them. In this paper, we present new analytical upper and lower charging bounds for continuum charging, useful for checking the accuracy of charging models. The best of the numerical models predicts charging within these bounds over much of its range, but exceeds the upper bound at moderate fields and long charging times. The standard models and the charging bounds exhibit a symmetry between positive and negative charges which may provide another test for numerical theories. We formulate an analytic charging rate model that remains within the bounds and gives good agreement with unipolar and bipolar charging data. The charging rate symmetry makes bipolar charging calculations quite simple. The adequacy of existing data for testing particle charging models is also discussed.

Ozone generation in DC-energized electrostatic precipitators

Ozone emissions from a short wire-plate precipitator and three commercial electronic air cleaners were measured. Ozone generation was most strongly affected by the corona current and the polarity of the discharge electrode. To a lesser extent, the type of corona (i.e. whether tuft or glow) was also important. Water vapor seems to reduce the ozone emission rate for negative corona, but ozone emissions from positive coronas are relatively unaffected. Ozone emission rate was also affected by the radius of the discharge electrode, but the measured values did not fit Castles model (see G.S.P. Castle et al. Trans. IEEE, vol.5, no.4, p.489-96 (1969), and M.B. Awad and G.S.P. Castle, J. APCA, vol.25, no.4, p.369-74 (1975)).<<ETX>>

A mathematical model for calculating effects of back corona in wire‐duct electrostatic precipitators

A new mathematical model for the electrical conditions in wire‐duct electrostatic precipitators is described. The model is based on the solution of Maxwell’s equations for the electric field components and offers several advantages over numerical solutions of Poisson’s equation. The model is used to calculate the effects of back corona on (1) the voltage‐current characteristic, (2) the current‐density distribution at the collector plate, and (3) particle collection for five simple mechanisms describing the generation of back corona. Each mechanism is capable of simulating the V‐I curves characteristic of precipitators operating in back corona, but two in particular are identified as potential back corona mechanisms on the basis of generating hysteresis in the V‐I curves or characteristic current density variations. These mechanisms are consistent with limited experimental data. This method of calculation is useful and can be extended to more complex mechanisms for back corona generation.

Negative corona in wire-plate electrostatic precipitators. Part I: Characteristics of individual tuft-corona discharges

Abstract The characteristics of individual fuft discharge in negative corona are invvestigated by modelling them with a micropoint projecting from the corona wire. The characteristics of these micropoints are shown to be similar to naturally produced tufts. Their V-I relationships and current density distributions are based on the better understood needleplane geometry characteristics but modified by the presence of the equipotential surface of the wire electrode. The maximum current density is calculated from the saturated current density equation. The onset voltage for the tuft corona on a wire is a function of the efficiency of production of electrons by positive ions. A fractional efficiecy of 1.0 corresponds to an onset voltage about 0.6 times the Peek onset voltage for a clean wire.

Maximizing data quality in the gravimetric analysis of personal exposure sample filters

The weighing of particle sampler filters has always been plagued by problems in the weighing environment: humidity, temperature, drafts, vibration, and electrostatic charges on the filters. These are particularly critical for samples with small mass collections, such as those encountered in personal exposure monitoring for PM25. While modern electronic balances offer substantial reductions in the effects of temperature and vibration, these balances are still sensitive to factors such as thermal drafts and zero shifts from tilting. Drafts may be controlled through room ventilation modifications, and zero drift can be eliminated by computer-assisted data collection algorithms. A less well-understood influence is static charge, which is often controlled with a simple radioactive neutralizer. Although radioactive neutralizes are effective, their effectiveness decays rapidly with time, and their use may be objectionable for nontechnical reasons. We have analyzed a number of environmental factors influencing gravimetric microbalance operations and have developed methods to minimize or eliminate them.

Comparison of PM2.5 and PM10 monitors.

An extensive PM monitoring study was conducted during the 1998 Baltimore PM Epidemiology-Exposure Study of the Elderly. One goal was to investigate the mass concentration comparability between various monitoring instrumentation located across residential indoor, residential outdoor, and ambient sites. Filter-based (24-h integrated) samplers included Federal Reference Method Monitors (PM2.5-FRMs), Personal Environmental Monitors (PEMs), Versatile Air Pollution Samplers (VAPS), and cyclone-based instruments. Tapered element oscillating microbalances (TEOMs) collected real-time data. Measurements were collected on a near-daily basis over a 28-day period during July–August, 1998. The selected monitors had individual sampling completeness percentages ranging from 64% to 100%. Quantitation limits varied from 0.2 to 5.0 µg/m3. Results from matched days indicated that mean individual PM10 and PM2.5 mass concentrations differed by less than 3 µg/m3 across the instrumentation and within each respective size fraction. PM10 and PM2.5 mass concentration regression coefficients of determination between the monitors often exceeded 0.90 with coarse (PM10–2.5) comparisons revealing coefficients typically well below 0.40. Only one of the outdoor collocated PM2.5 monitors (PEM) provided mass concentration data that were statistically different from that produced by a protoype PM2.5 FRM sampler. The PEM had a positive mass concentration bias ranging up to 18% relative to the FRM prototype.

Narrow-gap point-to-plane corona with high velocity flows

Corona discharge has been used in the detoxification of chemical agents or their simulants, for which the degree of destruction depends on the strength of the electric field or electron energy. To help clarify this process, a mathematical model describing the narrow-gap point-to-plane corona system has been developed. Narrow-gap electrodes are characterized by extremely high electron activity and the presence of positive and negative ions. The three-dimensional spatial distributions in the electron density and the electric field are of primary interest. The results indicate the potential problems of the corona device and may prove useful for obtaining an optimum design. >

Negative corona in wire-plate electrostatic precipitators. Part II: Calculation of electrical characteristics of contaminated discharge electrodes

Abstract A new analytical method has been developed which enables the main electrical characteristics to be calculated for a wire—plate geometry in which the wire is contaminated with dust and excited with a negative voltage. The characteristics of a single negative corona discharge tuft and how it interacts with another tuft when forced to do so, were investigated. The results were used to develop a complete analytical solution. The analysis identifies a significant region of corona at voltages well below the peek critical coltage, Vc, beginning at the tuft onset voltage, Vt = 0.6 Vc. The method also calculates the maximum current density under each tuft discharge and makes a reasonable estimate of the current distribution pattern on the plate and the number of tuft discharges per unit length of wire.

Triangle-shaped DC corona discharge device for molecular decomposition

An earlier point-to-plane geometry corona device with a rectangular cross section demonstrated low decomposition efficiencies. The numerical simulation of this device suggested that three were zones of low electron density and electric field significant decrease in resulting in electrical sneakage which caused a significant decrease in the kinetic rate coefficients in the chemical reaction process. A triangle-shaped DC corona discharge device was developed to improve the electrical sneakage problem, and a semiempirical mathematical model was developed to describe the detailed electrical characteristics and to refine estimates on the optimum shape of the device. The preliminary experimental results indicated that the decomposition efficiency was much improved. >