Dust Generated by Vehicular Traffic on Unpaved Roadways: Sizes and Infrared Extinction Characteristics

Abstract

Measurements of the size distribution, mass concentration, morphology, and infrared extinction coefficient of dust particles generated by vehicular traffic on unpaved roadways in the arid southwestern U.S. are reported. Measurements were made within a few meters of the point of closest approach of the various tracked and rubber-tired vehicles used in the experiment. For sandy soil, all vehicular dust size distributions reveal a bimodal character with mode mass mean radii about 4 μm and 45 μm, regardless of vehicle type and vehicle speed (for speeds ranging from 5 to 12 ms⁻¹). The larger mode of particles appears to be characteristic of the parent soil, but not the smaller mode. The smaller mode likely derives from large soil grains through some process involving disaggregation. A limited number of measurements made in silty soil also display bimodal size distributions, although with slightly larger mean size and broader distribution for the smaller mode. The shapes of the dust particles generated from sandy soil can be modeled by a bimodal shape distribution of spheroids, with spheroids ranging from 2:1 prolates to spheres to 2:1 oblates most commonly occurring. The dust particles are composed of quartz, the clay minerals montmorillonite, kaolinite, and illite, and calcite and aragonite. Many consist of aggregates of fine particles. Measured mass extinction coefficients (the ratio of extinction coefficient to mass concentration) at the 9.504 μm (CO₂ laser) wavelength are in rough agreement with a theoretical estimate of 0.11 m²g⁻¹, obtained by modeling the dust as a polydis-persion of particles having a shape distribution of both prolate and oblate spheroids. The sizes and composition of vehicular dust are similar to those of wind-blown dust.