Vehicle-Based Road Dust Emissions Measurements

TRAKER is a vehicle-based method for measuring road dust emissions. Particulate matter is sampled in front and behind a vehicle’s tire and the difference in PM concentration (TRAKER signal) is used to infer the airborne flux of particles from the roadway. Two independent tests indicated that the TRAKER signal increases as the cube of the speed for a given road dust loading. Simultaneous measurement of PM10 dust emitted behind the tires by TRAKER with PM10 flux measured using upwind/downwind towers suggested that the emissions factor for road dust was proportional to the cube root of the TRAKER signal. The results also showed a linear relationship between distance based unpaved road dust PM10 emission factors and vehicle speed. Once calibrated with the flux tower measurements, the system was used to investigate temporal changes in emissions from paved roads in both the winter and summer in the Treasure Valley in Southwest Idaho. Measurement of road dust emissions potential after road sanding on dry roads indicated a 75% increase in PM10 emissions after 2.5 hours. This effect was short lived and emissions returned to their pre sanding levels within 8 hours of the sand application. Street sweeping with mechanical and vacuum sweepers was found to offer no immediate measurable reduction in PM10 emissions potential however long term effects of street sweeping on road dust emissions were not evaluated as part of this study. The TRAKER signal was also associated with individual links (section of road) in the Traffic Demand Model network for the Treasure Valley, ID; each link was in turn associated with a number of characteristics including posted speed limit, vehicle kilometers traveled (vkt), road class (local/residential, collector, arterial, and interstate), county, and land use (urban vs. rural). The relationship between these characteristics and road dust emissions potential was assessed. The analysis suggested that while high speed roads are much cleaner (factor of 5.4 in summer), on a vehicle kilometer traveled basis, emissions from high and low speed roads are on the same order. INTRODUCTION Fugitive dust constitutes nearly two-thirds of the primary PM10 emissions according to the US National Emissions Trends inventory for 1997. Particles suspended by vehicular movement on paved and unpaved roads are a major contributor to fugitive dust emissions. Yet, traditional methods for quantifying road dust emissions have been a subject of controversy in recent years. The most common method involves measuring the silt loading or silt content from road surfaces by vacuuming (paved roads) or sweeping (unpaved roads). Laboratory analysis of samples requires dry sieving the bulk material through sequentially smaller sieves. The material that passes through a 200 mesh sieve is the silt fraction of the sample and corresponds approximately to particles with diameter less than 75 μm. The total amount of silt on a section of road (paved roads) or the fractional silt content of the bulk sample (unpaved roads) is then used to estimate the PM10 emissions. A limitation of the silt loading technology is that measurements are expensive and time consuming to acquire. This limits the number of data points used to represent road dust emission potential over an air shed. Moreover, silt loading measurements are usually collected over a brief (< 1 month) period and are unlikely to reflect seasonal changes in emission potential. In recent years new method for estimating road dust emissions has been being developed. The TRAKER (Testing Re-entrained Aerosol Kinetic Emissions from Roads) allows for measurement of PM10 and PM2.5 emissions from a large number of roads with great economy. The concentration of airborne particles in a specific size range is monitored with particle sensors that are mounted behind the front tires of a vehicle. These particle sensors are influenced by the road dust generated from the interaction of the vehicle tire and the road. A background measurement of particle concentrations is obtained simultaneously at a location on the vehicle away from and in front of the tires. The difference in the signals between the influence monitors and the background monitor is related to the amount of road dust generated. This paper summarizes recent developments of the TRAKER technology and describes some of the findings resulting from these measurements.