The re-entrainment of dust from the roadway and fringe surfaces of urban freeways in desert areas such as Greater Phoenix, Arizona can make measurable contributions to both PM10 and PM2.5. For airsheds like Phoenix that are in non-attainment with regard to PM10, these contributions take on further significance. We have conducted are series of experiments using a variety of aerosol techniques, including TEOMs, sector-controlled filter sampling for single-particle analysis using automated SEM, and portable optical aerosol monitors along Phoenix-area freeways. Emphasis has been on periods of very low wind speed when commonly used line-source models have difficulty. Very low wind speeds are common in both the winter in summer in complex terrain areas of the Southwest US, especially in morning and evening transition periods.

When wind speeds fall below a threshold (which is apparently partly dependent upon freeway structure) aerosols generated from the roadway and adjacent fringes by vehicular turbulence are trapped. Within the freeway turbulence zone PM10 levels can build up to 10 or more times the concentration present immediately before wind speed went below the threshold. Values up to 250 micrograms per cubic meter have been measured during these stagnant conditions, superimposed on an ambient background of 20 to 30 micrograms per cubic meter. Recovery to the “normal” regime is rapid once the threshold is again exceeded. Most of the aerosol volume generated is soil dust particles, plus cement particles and rubber fragments, but most of this volume is in particles smaller than 3 microns. The mineral dust therefore strongly contributes to PM2.5 as well. The contributions to PM10 and PM2.5 during periods of low wind when the wind speed exceeds the threshold also have been measured; commonly the freeway-generated PM10 is in the range of 10-20 micrograms per cubic meter.