The U.S. Environmental Protection Agency (EPA) has designated Spokane, Washington a non-attainment area for particulate matter with a diameter equal to or less than 10 micrometers (PM10). Observed 24-hr PM10 standard exceedances, neglecting windblown dust contributions from surrounding agricultural areas, stem primarily from residential wood combustion and unpaved and paved roads.
Particulate matter emission rates from roadway related activities, that are used to develop emission inventories, are currently based on highly uncertain EPA approved empirical correlations. In this work, stationary and mobile point source tracer release techniques were used to determine PM10 emission rates from 4-lane commercial/residential paved roads under sanded and unsanded conditions, and from unpaved roads relative to site specific vehicular and ambient parameters. Measured street (4+ lanes; >/=10,000 vehicles/day) emission factors for unsanded and sanded roads were respectively 40% and 10% lower than the EPA approved reference value. Notably the sanded condition was not four times greater than the unsanded case, as current EPA guidance suggests. Preliminary results indicate a consistent relationship between PM10 and relative humidity under unsanded conditions. Evidence suggests that street sweeping has a negligible effect on PM10 emission reduction. Considerable uncertainties were observed with the empirical emission factor equation. In fact, experimental observations were on average 90% lower than the empirical predictions.
Within the constraints imposed by the variable experimental conditions, the deduced emission factors for unpaved roads agreed reasonably well with the unpaved road empirical formula. Limited correlations were observed with ambient meteorological parameters.
The capability of the "upwind-downwind" concentration method to provide accurate emission factor predictions is also presented. Within a factor of two uncertainty limit, the SIMFLUX Gaussian dispersion model predictions agreed well with the experimentally determined emission factors. Lateral and vertical dispersion were seen to dominate roadway diffusion characteristics during periods of high and low wind speed, respectively.