Modeling the size distribution of a soil aerosol produced by sandblasting

Abstract

In order to develop a model providing the mass size distribution of the dust raised from the ground by the sandblasting process, mechanical characteristics of 240 μm saltating sand grains meant to be used as projectiles in wind tunnel sandblasting experiments were carefully determined. It was found that for values of the measured friction velocity less than about 55 cm/s, the constraint of the relatively small dimensions of the wind tunnel test section did not prevent saltation from developing freely. The kinetic energy of the sand grains was also determined. Aerosols were then produced in the wind tunnel by bombarding a clay target with the saltating 240 μm quartz grains. The size distributions of these aerosols were measured for three wind speeds with an optical particle analyzer. For the lowest wind speed the size distribution of the aerosol was similar to that of the 8.6 μm aggregates originally constituting the agglomerates of clay, but disaggregation into smaller particles became more important when wind speed increased. A theory of sandblasting was then developed that gave theoretical results agreeing with the experimental ones. A consequence of this theory was that submicron particles could be released from aggregates for high wind speeds. Experiments meant to check this implication were carried out and confirmed it. Cohesion energies of kaolin particles of three different sizes, 8.6, 2.8, and approximately 0.5 μm, were calculated and found to be a decreasing exponential function of the particle size. This explains (1) why the soil‐derived aerosol size distributions present a common mode in the 1–10 μm size range, these particles being readily released even in not particularly energetic conditions, and (2) why observation of a submicron mode in natural aerosols requires a higher friction velocity than for the common 1–10 μm mode.