Fugitive dust (FD) emissions arise from the mechanical disturbance of soils, which injects particles into the air. Common sources of FD include vehicles driving on unpaved roads, agricultural tilling, and heavy construction operations. For these sources, the dust-generation process is caused by two basic physical phenomena: pulverization and abrasion of surface materials by application of mechanical force through implements (wheels, blades, tracks, etc.) and entrainment of dust particles by the action of turbulent air currents such as wind erosion of an exposed surface. The impact of an FD source on air pollution depends on the composition and quantity of dust generated and the transport characteristics of the dust particles injected into the atmosphere.
Current efforts to study FD are hampered by the lack of an effective, real-time instrument to measure the broad range of particle sizes that make up FD. To meet this need, the overall objective of this project was to develop and evaluate a novel instrument capable of measuring, in real time, ambient concentrations of particles between approximately 2 and 100 μm. It should be able to classify particles into 20 size bins with mean bin sizes between 4 and 73 μm. The instrument should measure a broader range of particle sizes than with those obtained using other techniques to measure large particles.
This instrument, LPI-FM-100, consisted of a new large particle inlet (LPI) combined with a commercially available particle spectrometer (Droplet Measurement Technology FM-100). LPI-FM-100 performance was characterized in the laboratory, in a wind tunnel, and in a series of intensive field experiments at Fort Drum, a U.S. Army military reservation in Jefferson County, New York. Laboratory measurements consisted of aspirating glass beads of known size directly into the FM-100. Wind-tunnel experiments of the new inlet (LPI) mounted on the FM-100 were conducted by measuring upstream and downstream particle concentrations to measure the sampling efficiency. In the field, the LPI-FM-100 was deployed alongside traditional filter-based particle measurements and other real-time instruments capable of measuring particles up to 20 μm in size. These measurements were compared, and the performance of the new sampler evaluated.
Laboratory experiments with glass beads showed that the FM-100 was able to measure particles into 20 size bins and that the measured sizes were consistent with those measured by another light scattering instrument. Wind-tunnel measurements found that, as predicted using computational fluid dynamics, the LPI was able to sample large particles at speeds as high as 5 m/s, with sampling efficiency decreasing with particle size and wind speed. During the field measurements, there were periods of low particle concentrations corresponding to traffic-free conditions interspersed with high concentration periods that were linked to local traffic. The concentrations quickly returned to background levels after the traffic passed. In the region where size measurements overlap, the LPI-FM-100 size distributions were largely consistent with those measured using another real-time instrument. The LPI-FM-100 detected the presence of particles larger than 30 μm during times of traffic movement. A comparison of the filter-based and LPI-FM-100 PM>10 values found there was a reasonable match between the measurements made by the two techniques. A first estimation of the fugitive number and mass distribution was obtained from the difference of the background and high concentration periods. This FD size distribution is mono-modal with a peak at approximately 8 μm. The FD size distributions suggest a significant mass concentration of these particles in sizes larger than 10 μm.
Overall, the results of this project indicate that military vehicle traffic generates a significant amount of FD that is highly dynamic and that there are significant peak concentrations near the roadway that quickly return to background levels. The LPI-FM-100 was able to make FD concentration measurements in real time even though much of this dust is larger than can be measured using traditional samplers.