Effect of Soil Type and Momentum on Unpaved Road Particulate Matter Emissions from Wheeled and Tracked Vehicles

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 Results from both previously published and unpublished field studies are combined to link emission factors to three related variables: soil type, vehicle momentum, and tred type (i.e., tire or track).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.

We would like to acknowledge the logistical and financial support provided by the National Guard MATES facilities at Yakima Training Center, Yakima, Washington and Ft. Carson, Colorado. Special thanks to the MATES personnel who operated the vehicles for us as well as their flexibility in accommodating our demands. We would also like to thank Yakima Training Center and Ft. Carson for hosting our field measurement campaigns and the SERDP Sustainable Infrastructure program (projects CP-1191 and SI-1399) for their continued support of this research.

[Supplementary materials are available for this article. Go to the publisher's online edition of Aerosol Science and Technology to view the free supplementary files.]

Notes

∗Gravimetric PM₁₀ to DustTrak PM₁₀ ratios from Ft. Bliss were measured on a flux tower downwind of a road. DustTrak PM₁₀ to PM_2.5 ratios were measured from dust sampled onboard the TRAKER vehicle (Kuhns et al., 2005).