CHARACTERISTICS OF AIRBORNE PARTICLES PRODUCED BY WIND EROSION OF SANDY SOIL, HIGH PLAINS OF WEST TEXAS

Abstract

Mass size distributions of airborne particles generated by wind erosion of two soils, one coarse-grained and one fine-grained, show two distinct modes in each. The coarser mode, between 10 and 100 μm equivalent radius, is due to the presence in the parent soil of loose erodible particles which x-ray analyses indicate are almost exclusively quartz grains. The finer mode is between 1 and 10 μm equivalent radius which x-ray analyses show to be clay minerals consisting predominantly of illite and mixed layer clays. The proportion of fine particles to large ones increases relative to wind speed for the finer soil but not for the coarse soil; hence, we ascribed the fineparticle mode as due to sandblasting of the soil, which gives rise to fine particles by disaggregation. Scanning electron microscope (SEM) photographs of airborne fine and large particles give supporting evidence in that the fine particles are similar to clay platelets which adhere to the surfaces of the larger particles and which almost certainly are removed from the grains by the wind erosion sandblasting mechanism. In addition, when coupled with wind velocity data, the SEM photographs indicate that with increased wind speed a higher percentage of the large particles eroded from both soils consists of aggregates of clay. These aggregates are very fragile and they should be readily disaggregated by sandblasting. It is concluded, therefore, that the fine airborne particles (1–10 μm equivalent radius) consist predominantly of clay minerals which are derived from the exposed soils by sandblasting during wind erosion. The clay is removed from the soil as individual platelets, as coatings of platelets on quartz grains, and as coarse aggregates of platelets. Continued sandblasting during wind transport removes the clay platelets from the exposed surfaces of quartz grains and separates the aggregates. As should be expected, the finer-grained soil contains the highest percentage of clay and, upon erosion, yields the highest percentage of fine airborne particles.