Variability of Natural Dust Erosion from a Coal Pile
A study of fugitive dust emissions from a pile of crushed Powder River Basin (PRB) coal in Tennessee revealed that the pile contributes dust to the atmosphere during periods without human activity on the pile. Hourly downwind measurements of fine+coarse (i.e., smaller than 10 µm, or PM10) particle mass concentrations at two sites revealed that excess dust was present in the air even when wind speeds were below the erosion threshold as determined from nearby wind speed measurements and Environmental Protection Agency guidance on coal pile aerodynamic characteristics. Excess PM10 levels in the absence of human pile activity were investigated to determine the factors most strongly associated with “natural” airborne dust. During periods with higher wind speeds, downwind concentrations were strongly associated with µ2--the squared excess of 1-min maximum wind speed above the erosion threshold--consistent with previous work on wind erosion potential. However, most hours experienced lower winds for which wind speed was not a good predictor of dust levels. Evidence was found for natural low-wind PM10 concentrations to vary significantly with relative humidity, air temperature and measures of turbulence (σu and σw). Smoke from coal dust combustion was ruled out as a significant factor in PM10 levels, but statistical evidence along with visual observations suggested that turbulence over the pile--including dust devils--was a significant source of PM10 mass during periods of low wind speed. The localized behavior of turbulence and the associated dust emission points are problematic for developing statistical models of downwind concentrations based on off-pile meteorological measurements. A methodology is presented that captures the most variability in natural PM10 concentrations for subsequent analysis of emission factors due to human activity.