This report describes field experiments conducted to determine particulate emission amounts for PM10 along freeways in the greater Phoenix area. A field site at Mile 47 on Loop 101 near the Chaparral Road exit was chosen for experiments conducted in Feb-Mar 2001 and Feb-Mar 2002. Three mobile laboratories with instruments for measuring and sampling aerosol particles and measuring meteorological parameters were deployed, one on each side 3 m from the roadway and, in Mar 2002, one 100 m to the west of the roadway. A 10 m tower was also used to determine the vertical profiles of wind velocity and direction, and aerosol concentration. Soil dust particles dominate the freeway aerosol , but it also has significant concentrations of black carbon (soot) and iron oxide or other metal Many of the iron-rich particles are hypothesized to result from engine wear, although other sources of iron-rich particles are known in the Phoenix area. The measured emission factor for this experiment was 0.1 grams per vehicle kilometer traveled, a value that is similar to an emission factor calculated for roads in southern California through modeling by other investigators. However, emission factors such as this are not constants and can be expected to vary with changes in conditions such as vehicle fleet composition and loading of dust on the road surface. Vertical profiles of wind and aerosol are in disagreement with the assumptions made in current line source models; if the measured vertical profiles are used in the emission factor calculation, the emission factor is reduced to 0.06 grams per vehicle kilometer traveled. Both values are similar to MOBILE6 model results by the Arizona Department of Environmental Quality for freeways in Maricopa County. Low wind conditions are very common in the Phoenix area. When wind speeds are below a critical value (4-5 mph), PM10 aerosol concentrations build to high values. This is because the turbulence from passing vehicles becomes more pronounced than the normal surface winds; under such conditions the aerosols are trapped in an essentially narrow, elongate plume above the freeways. When the low wind conditions break down, wind speed increases, and the plume of aerosol is dispersed down wind. The ratio of PM2.5 to PM10 is dependent on wind speed as well and can be very high when the wind speed is below the critical value. The evidence suggests that new dust is constantly being loaded onto the freeway surface so that the freeway is a perpetual source of PM10. The observations about high PM concentrations during frequent periods of low wind speed have broader implications for urban freeways. The control of noise by building urban freeways below grade, with high sound walls or both can artificially create low wind conditions for longer periods than naturally occur; the consequences of artificially created wind stagnation with regard to PM levels is as yet undetermined.