Measurements of Aerosol Size Distributions and Vertical Fluxes of Aerosols on Land Subject to Wind Erosion

Abstract

To assess wind erosion as a source of atmospheric soil particles, vertical aerosol fluxes near the ground in an eroding field were computed by assuming a vertical transport mechanism similar to that for momentum. Aerosol gradients were measured by jet impactors located 1.5 and 6 m above the ground, and wind velocity gradients were measured by totalizing-three-cup anemometers located 1.5, 3 and 6 m above the ground. Information on the aerosol size distributions and quantity in the size range 0.3 ≤ r ≤ 6 μm was obtained for a variety of erosive conditions in a field in rural Nebraska. In general, the size distributions in this range suggest the power law, dN/d(log r) ∝r−2 for 1 ≤ r ≤ 6 μm, and a flatter curve for 0.3 ≤ r ≤ 6 μm. The relation of the aerosol size distribution in the range 0.3 ≤ r ≤ 6 μm to the size distribution of soil was determined. Averaged soil size distributions characteristic of the sampling field, the area within a six-mile radius of the sampling field, and soil flowing (creeping) at ground level at the sampling site were found to be nearly identical. In addition, the size distribution curves for all three samples showed a general form in the ranges 0.3 ≤ r ≤ 1 μm and 1 ≤ r ≤ 6 μm which could be basically characterized by the same power laws as those suggested by the aerosol size distributions. Horizontal soil particle fluxes were measured and compared to the vertical fluxes. In addition, the measured erosion was compared to erosion predicted by an empirical formulation based on the parameters of soil moisture, field length, vegetative residue, mechanical stability, ridge roughness, and vertical momentum flux. These predictions agreed qualitatively with observed horizontal soil movement and with the observed vertical flux measurements. Measurements of aerosol size distributions from ground level to 11.9 km on two occasions when soil was very moist and no vertical flux of aerosol was observed showed the aerosol size distribution to be constant in shape and in concentration from the ground to the top of the mixing layer. Above the mixing layer, concentration decreased for all particle sizes above the mixing layer up to the base of the tropopause where an increase in concentration for the smallest particles and a decrease in concentration for the largest particles were observed.