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Traffic-generated fugitive dust on gravel roads impairs visibility and deposits on the adjacent environment. Particulate matter smaller than 10 μm in diameter (PM10) is also associated with human health problems. Dust emission strength depends on the composition of granular material, road moisture, relative humidity, local climate (precipitation, wind velocity, etc.), and vehicle characteristics. The objectives of this study were to develop a reliable and rapid mobile methodology to measure dust concentrations on gravel roads, evaluate the precision and repeatability of the methodology and correspondence with the currently used visual assessment technique. Downwind horizontal diffusion was studied to evaluate the risk of exceeding the maximum allowed particulate matter concentration in ambient air near gravel roads according to European Council Directive [European Council Directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air. Official Journal of the European Communities. L163/41.]. A TSI DustTrak Aerosol Monitor was mounted on an estate car travelling along test sections treated with various dust suppressants. Measured PM10 concentrations were compared to visual assessments performed at the same time. Airborne particles were collected in filters mounted behind the vehicle to compare the whole dust fraction with the PM10 concentration. For measuring the horizontal diffusion, DustTraks were placed at various distances downwind of a dusty road section. The mobile methodology was vehicle and speed dependent but not driver dependent with pre-specified driving behaviours. A high linear correlation between PM10 of different vehicles makes relative measurements of dust concentrations possible. The methodology gives continuous data series, mobility, and easy handling and provides fast, reliable and inexpensive measurements for estimating road conditions to make road maintenance more efficient. Good correlations between measured PM10-values, visually assessed dust generation and dust collected in filters were obtained. PM10 seems to be correlated to the whole dust fraction that impairs visibility on gravel roads. A decay in PM10 concentration as a function of distance from the road was observed. Measured particles principally did not travel further than 45 m from the road. The risk of exceeding the PM10 concentration stated in the EC-directive seems small.

Testing Re-Entrained Kinetic Emissions from Roads (TRAKER) is a new technique to infer paved and unpaved road dust PM 10 emission potentials based on particulate matter (PM) measurements made onboard a moving vehicle. Light scattering instruments mounted in front and behind the vehicle's tires measure the differential particle concentration of dust suspended by the vehicle's tire in contact with the road surface. Through empirical regressions relating the differential concentration (i.e. TRAKER signal) with the vehicle speed and the downwind flux of PM 10 particles from the road, an equation is derived to infer the speed independent road dust emission potential from the measured TRAKER signal. Measurements from TRAKER offer a new perspective on the processes that affect road dust emissions. The system was used to investigate temporal changes in emission potentials from paved roads in both the winter and summer in the Treasure Valley in Southwest Idaho. During the 3-week wintertime sampling period, the residential road dust PM 10 emission potential decreased by ∼50%. Summertime PM 10 emission potentials were similar to those observed at the end of the winter sampling and showed no upward or downward trends. Wintertime unpaved road emissions increased consistently with the number of days since the last rainfall. Measurement of road dust emission potentials after road sanding on dry roads indicated a 75% increase in PM 10 emissions after 2.5 h. This effect was short lived and emission potentials returned to their pre-sanding levels within 8 h of the sand application. Street sweeping with mechanical and vacuum sweepers was found to offer no measurable reduction in PM 10 emission potentials. On several roads, the PM 10 emission potentials actually increased immediately after vacuum sweeping. Long term effects of street sweeping on road dust emissions were not evaluated as part of this study and may offer some overall reduction in PM emissions from paved roads.

The testing re-entrained aerosol kinetic emissions from roads (TRAKER) road dust measurement system was used to survey more than 400 km of paved roads in southwestern Idaho during 3-week sampling campaigns in winter and summer, 2001. Each data point, consisting of a 1-s measurement of particle light scattering sampled behind the front tire, was associated with a link (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). Overall, the TRAKER-based emission factors based on location, setting, season, and speed spanned a narrow range from 3.6 to 8.0 g/vkt. Emission factors were higher in winter compared to summer, higher in urban areas compared to rural, and lower for roads with fast travel speeds compared to slower roads. The inherent covariance between traffic volume and traffic speed obscured the assessment of the effect of traffic volume on emission potentials. Distance-based emission factors expressed in grams per kilometer traveled (g/vkt) for roads with low travel speeds (∼11 m/s residential roads) compared to those with high travel speeds (∼25 m/s interstates) were higher (5.2 vs. 3.0 g/vkt in summer and 5.9 vs. 4.9 g/vkt in winter). However, emission potentials which characterize the amount of suspendable material on a road were substantially higher on roads with low travel speeds (0.71 vs. 0.13 g/vkt/(m/s) in summer and 0.78 vs. 0.21 g/vkt/(m/s) in winter). This 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 of the same order. Emission inventories based on the TRAKER method, silt loadings obtained during the field study, and US EPA's AP-42 default values of silt loading were compared. PM 10 paved road dust emission inventories calculated with the TRAKER method were 61% higher in winter and 180% higher in summer than inventories calculated from on-site silt loading measurements. Emissions calculated from silt loading measurements conducted on-site indicated that the AP-42 default values are too low for the Treasure Valley by a factor of 1.5 for summer conditions and by a factor of 3.8 for winter. Both silt loading and TRAKER are techniques that were calibrated against the horizontal flux of dust, which was estimated by the difference in PM 10 concentration between instruments located upwind and downwind of an unpaved road. The upwind/downwind method, and therefore both silt loading and TRAKER, gives a measure of the dust emitted near the source, and not the dust that can be transported on a regional or air shed scale. Correcting the measured dust emissions for deposition and removal near the source is outside the scope of this work, but is a continuing area of research among dispersion modelers.

Testing re-entrained aerosol kinetic emissions from roads (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 is related to emissions. This paper describes the most recent developments of TRAKER. The loss of particles within the inlet lines, the response of the TRAKER signal (differential PM concentration) to vehicle speed, and the relationship between the TRAKER signal and unpaved road dust PM 10 emissions were examined. Losses of particles to the walls in the inlet lines were similar for the left and right inlets and were less than the inter-instrument precision for particles between 0.56 and 7.3 μm in diameter. When summed over the PM 10 size range, losses were less than 20%. Line losses were also characterized when a dilution system was used to sample emissions from unpaved roads. Two independent tests indicated that the TRAKER signal increases as the cube of the speed for a given road dust loading. Simultaneous measurement of PM 10 dust emitted behind the tires by TRAKER with PM 10 flux measured using upwind/downwind towers suggested that the emission factor for road dust was proportional to the cube root of the TRAKER signal. The results also showed a linear relationship between unpaved road dust PM 10 emissions and vehicle speed.