Continuous flow hygroscopicity-resolved relaxed eddy accumulation (Hy-Res REA) method of measuring size-resolved sodium chloride particle fluxes.

The accurate representation of aerosols in climate models requires direct ambient measurement of the size- and composition-dependent particle production fluxes. Here, we present the design, testing, and analysis of data collected through the first instrument capable of measuring hygroscopicity-based, size-resolved particle fluxes using a continuous-flow Hygroscopicity-Resolved Relaxed Eddy Accumulation (Hy-Res REA) technique. The Hy-Res REA system used in this study includes a 3D sonic anemometer, two fast-response solenoid valves, two condensation particle counters, a scanning mobility particle sizer, and a hygroscopicity tandem differential mobility analyzer. The different components of the instrument were tested inside the US Environmental Protection Agency's Aerosol Test Facility for sodium chloride and ammonium sulfate particle fluxes. The new REA system design does not require particle accumulation, and therefore avoids the diffusional wall losses associated with long residence times of particles inside the air collectors of traditional REA devices. A linear relationship was found between the sodium chloride particle fluxes measured by eddy covariance and REA techniques. The particle detection limit of the Hy-Res REA flux system is estimated to be ~3 × 105 m-2 s-1. The estimated sodium chloride particle classification limit, for the mixture of sodium chloride and ammonium sulfate particles of comparable concentrations, is ~6 × 106 m-2 s-1.

Transport of airborne particles within a room.

The objective of this study is to test a technique used to analyze contaminant transport in the wake of a bluff body under controlled experimental conditions for application to aerosol transport in a complex furnished room. Specifically, the hypothesis tested by our work is that the dispersion of contaminants in a room is related to the turbulence kinetic energy and length scale. This turbulence is, in turn, determined by the size and shape of furnishings within the room and by the ventilation characteristics. This approach was tested for indoor dispersion through computational fluid dynamics simulations and laboratory experiments. In each, 3 mum aerosols were released in a furnished room with varied contaminant release locations (at the inlet vent or under a desk). The realizable k approximately epsilon model was employed in the simulations, followed by a Lagrangian particle trajectory simulation used as input for an in-house FORTRAN code to compute aerosol concentration. For the experiments, concentrations were measured simultaneously at seven locations by laser photometry, and air velocity was measured using laser Doppler velocimetry. The results suggest that turbulent diffusion is a significant factor in contaminant residence time in a furnished room. This procedure was then expanded to develop a simplified correlation between contaminant residence time and the number of enclosing surfaces around a point containing the contaminant. Practical Implications The work presented here provides a methodology for relating local aerosol residence time to properties of room ventilation and furniture arrangement. This technique may be used to assess probable locations of high concentration by knowing only the particle release location, furniture configuration, inlet and outlet locations, and air speeds, which are all observable features. Applications of this method include development of 'rules of thumb' for first responders entering a room where an agent has been released and selection of sampler locations to monitor conditions in sensitive areas.

A Flow Visualization Study of Spore Release Using a Wind Tunnel-Mounted Laser Light Sheet.

A phase Doppler anemometry system in combination with a laser light sheet was used in a low-speed recirculating wind tunnel to examine the flow field around an individual leaf. Turbulence similar to that encountered near the surface of the earth in a neutral stability boundary layer was generated using a grid at the upwind end of the wind tunnel test section. Individual healthy and diseased plant leaves were introduced into the tunnel with the leaf tip pointing downwind. The Mie-scattered radiation from the spores departing the diseased leaf was captured on videotape. Image processing software was used to enhance the visual quality of the individual frames from the videotape and to make spore velocity calculations. Three main vortex regions around the leaf were identified. The importance of these regions to the separation of the spores from the leaf surface and their subsequent downwind movement was analyzed.

Experimental considerations for the study of contaminant dispersion near the body.

Experimental considerations are discussed for conducting controlled studies of the dispersion of contaminants released near a mannequin. A 183 cm x 183 cm cross section wind tunnel was modified to study the low velocity range of 10 to 100 cm/sec (20 to 200 ft/min). Installation of a removable biplanar slat grid produced turbulent intensities up to 15%. The results of validation testing for selected experimental components are reported, including (1) a minimum, unambiguous velocity measurement capability of 2.0 cm/sec (4.0 ft/min); (2) a minimum required integration interval for velocity and contaminant measurements of at least 3 min; (3) a determination that smoke streamline plume settling may be a problem at velocities < or = approximately 15 cm/sec (approximately 30 ft/min); (4) a determination that a 14% tunnel blockage by the mannequin was not of consequence for frontal measurements; and (5) a finding that the biplanar grid produced turbulence spectra representative of low velocity indoor settings. A deceleration zone was noted that extended 50 cm upstream from the mannequin, with freestream velocities reduced 50 to 60%, 2.5 cm from the chest. A contaminant tracer released as a point source 60 cm upstream typically dispersed laterally only 10 to 15 cm and diluted by a factor of 10(4) before reaching the chest.

Use of a pilot study for designing a large scale probability study of personal exposure to aerosols.

The major objective of the U.S. Environmental Protection Agency's (EPA's) Particle Total Exposure Assessment Methodology (PTEAM) Study is to estimate the frequency distribution of aerosol exposures of a target population of individuals. This objective requires the use of probability sampling techniques for selecting a representative sample of participants from a prescribed target population. To design such a population exposure study in a cost-effective fashion, a number of issues must be addressed. For instance, when and for how long and for whom should personal samples be obtained? What other samples are needed or desirable? Issues like these must be considered from several perspectives--from the point of view of data collection costs, burden on participants, precision and representativeness of resultant estimates, etc. To help address such design issues for the PTEAM population exposure study, we generated descriptive statistics and performed statistical analyses on data from a preliminary nine-home pilot study conducted in March 1989 in the San Gabriel Valley area of Southern California. The analyses showed large temporal variation, with day versus night being a major component (generally higher daytime concentrations); large systematic time-of-week differences were not found. Large house-to-house and person-to-person variabilities were evident, with high exposure levels noted especially in homes with tobacco smoking. Within many homes, there appeared to be little variability in the particulate concentrations among different rooms. The results of the pilot were used to make decisions regarding the spatial and temporal sampling units, the benefits of stratification, and the overall allocation of resources (e.g., multiple monitors within a home versus more homes and participants) for the subsequent population study.