Abstract
Excluding windblown dust, unpaved road dust PM(10) emissions in the US EPA's 2002 National Emission Inventory account for more than half of all PM(10) emissions in the arid states of the western U. S. (i.e., CA, AZ, NV, NM, and TX). Despite the large size of the source, substantial uncertainty is associated with both the vehicle activity (i.e., number of kilometers traveled at a particular speed) and the emission factors (i.e., grams of PM(10) per kilometer traveled). In this study, emission factors were measured using the flux tower method for both tracked and wheeled military vehicles at three military bases in the Western U. S. Test vehicle weights ranged from 2400 kg to 60,000 kg. Results from both previously published and unpublished field studies are combined to link emission factors to three related variables: soil type, vehicle momentum, and tred type (i.e., tire or track). Current emission factor models in US EPA's AP-42 Emission Factor Compendium do not factor both speed and weight into unpaved road emission factor calculations. Tracked vehicle emission factors from Ft. Carson, CO, and Ft. Bliss, TX were related to vehicle momentum (speed * mass) with ratios ranging from 0.004-0.006 (g-PM vkt(-1))/(kg m s(-1)). For similar vehicle momentum, wheeled vehicles emitted approximately 2 to 4 times more PM(10) than tracked vehicles. At Yakima, WA, tracked vehicle PM(10) emission factors were substantially higher (0.38 (g-PM vkt(-1))/(kg m s(-1)) due to the unique volcanic ash soil characteristics (48% silt). Results from PI-SWERL, a portable wind tunnel surrogate, are presented to assess its utility to predict unpaved road dust emissions without the deployment of flux tower systems. PI-SWERL showed only a factor of 6 variation between sites in comparison with the 60-fold variation as measured by the flux towers.