Richard G. Sextro

Control of Respirable Particles in Indoor Air with Portable Air Cleaners

Abstract Eleven portable air cleaning devices have been evaluated for control of indoor concentrations of respirable particles using in situ chamber decay tests. Following injection of cigarette smoke in a room-size chamber, decay rates for particle concentrations were obtained for total number concentration and for number concentration by particle size with and without air cleaner operation. The size distribution of the tobacco smoke particles was log normal with a count median diameter of 0.15 μm and a geometric standard deviation of 2.0. Without air cleaner operation, the natural mass-averaged surface deposition rate of particles was observed to be 0.1 h −1 . Air cleaning rates for particles were found to be negligible for several small panel-filter devices, a residential-sized ion-generator, and a pair of mixing fans. Electrostatic precipitators and extended surface filters removed particles at substantial rates, and a HEPA-type filter was most efficient air cleaner studied.

Determining size-specific emission factors for environmental tobacco smoke particles

Because size is a major controlling factor for indoor airborne particle behavior, human particle exposure assessments will benefit from improved knowledge of size-specific particle emissions. We report a method of inferring size-specific mass emission factors for indoor sources that makes use of an indoor aerosol dynamics model, measured particle concentration time series data, and an optimization routine. This approach provides--in addition to estimates of the emissions size distribution and integrated emission factors--estimates of deposition rate, an enhanced understanding of particle dynamics, and information about model performance. We applied the method to size-specific environmental tobacco smoke (ETS) particle concentrations measured every minute with an 8-channel optical particle counter (PMS-LASAIR; 0.1 m 2+ w m diameters) and every 10 or 30 min with a 34-channel differential mobility particle sizer (TSI-DMPS; 0.01 m 1+ w m diameters) after a single cigarette or cigar was machine-smoked inside a low air-exchange-rate 20 m 3 chamber. The aerosol dynamics model provided good fits to observed concentrations when using optimized values of mass emission rate and deposition rate for each particle size range as input. Small discrepancies observed in the first 1-2 h after smoking are likely due to the effect of particle evaporation, a process neglected by the model. Size-specific ETS particle emission factors were fit with log-normal distributions, yielding an average mass median diameter of 0.2 w m and an average geometric standard deviation of 2.3 with no systematic differences between cigars and cigarettes. The equivalent total particle emission rate, obtained by integrating each size distribution, was 0.2-0.7 mg/min for cigars and 0.7-0.9 mg/min for cigarettes.

A Concentration Rebound Method for Measuring Particle Penetration and Deposition in the Indoor Environment

Continuous, size resolved particle measurements were performed in two houses in order to determine size-dependent particle penetration into and deposition in the indoor environment. The experiments consisted of three parts: (1) measurement of the particle loss rate following artificial elevation of indoor particle concentrations, (2) rapid reduction in particle concentration through induced ventilation by pressurization of the houses with HEPA-filtered air, and (3) measurement of the particle concentration rebound after house pressurization stopped. During the particle concentration decay period, when indoor concentrations are very high, losses due to deposition are large compared to gains due to particle infiltration. During the concentration rebound period, the opposite is true. The large variation in indoor concentration allows the effects of penetration and deposition losses to be separated by the transient, two-parameter model we employed to analyze the data. For the two houses studied, we found that as particles increased in diameter from 0.1 to 10 w m, penetration factors ranged from ∼1 to 0.3 and deposition loss rates ranged from 0.1 and 5 h m 1 . The decline in penetration factor with increasing particle size was less pronounced in the house with the larger normalized leakage area.

A time-zero detector utilizing isochronous transport of secondary electrons

Abstract A time-zero detector has been developed for use in reaction product mass identification which has as its novel feature a 180° isochronous transport of secondary electrons in a magnetic field. The secondary electrons produced when particles pass through a thin carbon foil are accelerated to approximately 2 keV by a parallel-wire harp of 99% transmission. The accelerated electrons are then transported 180° in a uniform magnetic field of 80 G containing a collimator placed at the 90° position. A background suppression grid is placed just in front of the electron detector which is comprised of two microchannel plates in series acting as an electron multiplier. The device allows placement of the thin foil perpendicular to the fragment flight path and permits shielding of the electron detector from the beam and reaction products while using only modest accelerating voltages. The time-of-flight resolutions measured between this timing detector and a 120 μm silicon detector when using 104 MeV 16O ions and 8.78 MeV alpha particles were 90 and 150 ps, respectively (full widths at half maxima).

Potable water as a source of airborne 222Rn in U. S. dwellings: a review and assessment

Using a long-term-average, single-cell model and available data for U.S. housing, the concentration of 222Rn in indoor air due to the use of potable water is assessed. The ratio of the airborne 222Rn concentration to the concentration in water is represented by a lognormal distribution with geometric mean and geometric standard deviation of 0.65 X 10(-4) and 2.88, respectively, in fair agreement with the previously reported results of direct measurements of the ratio in 13 houses. By combining this result with data on 222Rn concentrations in U.S. water supplies, potable water is estimated to contribute an average of 24, 1.3, and 0.1 Bq m-3 to the airborne 222Rn concentration in residences served by private wells, public ground water, and surface water supplies, respectively.

Soil-Gas Contamination and Entry of Volatile Organic Compounds into a House Near a Landfill

Toxic volatile organic compounds (VOC) are commonly found in landfills, including those accepting only municipal waste. These VOC can migrate away from the site through the soil and result in contaminated off-site soil gas. This contaminated soil gas can enter houses built near landfills and is a potential source of human exposure to VOC. This study investigated soil-gas contamination and the mechanisms of entry of VOC into a house with a basement sited adjacent to a municipal landfill. The VOC were identified and quantified in the soil gas and in indoor and outdoor air. Pressure coupling between the basement and the surrounding soil was measured. Using soil-gas tracers, the pressure-driven advective entry of soil gas was quantified as a function of basement depressurization. From the measurements, estimates were made for the diffusive and advective entry rates of VOC into the house. A comparison of the chlorinated hydrocarbons found in soil gas at the site and in the landfill suggests that the landfill i...

Confirmed proton radioactivity of 53Com

Abstract Proton-induced reactions on 54Fe produce a proton activity [1.57±0.03 MeV; 242±15 ms] with a threshold of 26.3±0.4 MeV which can only arise from 53Com. Failure to detect positron-proton coincidences in the decay of this isomer establishes its direct proton radioactivity.

High-resolution measurements of beta-delayed protons from 37Ca and 41Ti

Abstract Protons with energies from 870 keV to 5.4 MeV have been observed following bombardment of a 40 Ca target with 3 He at beam energies of 29.5, 36.5 and 60 MeV, as well as from 40 MeV 3 He bombardment of an 36 Ar gas target. These data, and those resulting from 20 MeV proton bombardment of 40 Ca, permit accurate identification of proton unbound levels in 41 Sc, 37 K, and 40 Ca fed by allowed beta decay from 41 Ti, 37 Ca or 40 Sc, respectively. Absolute ft values have been determined for beta decay to these levels. The half-life of 41 Ti has been remeasured to be 80±2 ms and the log ft for its superallowed decay branch has been found to be 3.35±0.02, indicating an isospin purity for the lowest T = 3 2 state in 41 Sc of ≈ 91 % . The ft values for 37 Ca β + decay are compared to shell-model calculations for beta-decay in A = 37 nuclei.

An algorithm for real-time tomography of gas concentrations, using prior information about spatial derivatives

We present a new computed tomography method, the low third derivative (LTD) method, that is particularly suited for reconstructing the spatial distribution of gas concentrations from path-integral data for a small number of optical paths. The method finds a spatial distribution of gas concentrations that (1) has path integrals that agree with measured path integrals, and (2) has a low third spatial derivative in each direction, at every point. The trade-off between (1) and (2) is controlled by an adjustable parameter, which can be set based on analysis of the path-integral data. The method produces a set of linear equations, which can be solved with a single matrix multiplication if the constraint that all concentrations must be positive is ignored; the method is therefore extremely rapid. Analysis of experimental data from thousands of concentration distributions shows that the method works nearly as well as smooth basis function minimization (the best method previously available), yet is about 100 times faster.

Soil-gas entry into houses driven by atmospheric pressure fluctuations—The influence of soil properties

Abstract Atmospheric pressure fluctuations can draw soil gas into houses without the indoor-outdoor pressure differences commonly associated with the advective entry of radon and other soil-gas contaminants. To study this phenomenon, we employ a transient finite-element model based on Darcys law to simulate the soil-gas flow around a prototypical basement caused by changes in atmospheric pressure. The characteristic response time and the capacitance of the soil are used to characterize how changes in permeability, air-filled porosity, and water-table depth affect this soil-gas flow. The shorter the characteristic response time and the larger the capacitance of the soil, the larger the soil-gas flow rate into a basement caused by a given fluctuation in atmospheric pressure. Such a soil must have a high permeability and a large air-filled porosity. The addition of a high permeability subslab gravel layer increases the soil-gas flow rate into the basement by a factor of ∼ 3. Relative to entry driven by steady indoor-outdoor pressure differences, contaminant entry induced by atmospheric pressure fluctuations will likely be most important in houses situated in a soil of low permeability ( −12 m 2 ) and large air-filled porosity.