Evaluation of Fugitive Dust Deposition Rates Using Lidar

Ambient measurements suggest that source inventories of PM10 from geologic sources are overestimated by 50 percent or more. This discrepancy may be due to inaccurate emission calculations and/or due to the rapid deposition of PM10 after entrainment into the atmosphere. A two-wavelength scanning backscatter lidar was used to investigate PM10 deposition rates from artificially generated fugitive dust. Dust was generated by vehicles on unpaved roads, a tilling operation, and from a blower fan, that dispersed known amounts of finely ground calcium carbonate or native soils. The size and concentration of the resulting dust plumes were monitored for up to a half-hour and a distance of several kilometers. The changes in these dust plumes’ characteristics with time are depicted using a lidar to measure the relationship between backscatter and extinction at two wavelengths. An outdoor test chamber was prepared and used to examine the particulate size distribution and optical scattering properties of several different natural dust types and different preparations of powdered CaCO3 samples under controlled conditions. These same materials were used to generate plumes for open atmosphere tests. Backscatter and extinction values calculated from models, based upon Mie theory for spherical particles, are compared to actual signals. These models show the dependence of optical backscatter and extinction upon the size, number density and refractive index of the particles. Thus, simultaneous measurements of the backscatter and extinction at two different wavelengths permitted the examination of settling rates of dust particles as a function of size. The larger particles, which contain most of the PM mass, settle out of the air fairly quickly, however, the fine particles contribute primarily to the backscatter, and remain suspended much longer. The results suggest that rapid deposition of PM10 particles, and the relatively longer residence time of the optical plume associated with small particles (< 2:m), may have led to overestimates of airborne particle mass in plumes.