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Dale A. Gillette and Irving H. Blifford Jr.

Abstract

A series of aerosol samples was collected using a three-stage impactor at altitudes to 10 km from locations near Scottsbluff, Neb., Death Valley, Calif., and over the Pacific Ocean about 250 km west of Santa Barbara, Calif. A few samples were obtained from other locations viz., the central Pacific, Venezuela, and Chicago, Ill. Chemical analyses were made by means of x-ray techniques. At the locations for which concentrations near the ground were measured, most average mass concentrations of the elements Cl, S, K, Ca and Ti decreased rapidly from the ground to about half or less the ground concentration at 1 km and then maintained more or less constant values up to 10 km. Low Cl concentrations (except near the ocean surface) suggest a small sea salt contribution at the higher altitudes. The soil component of the aerosol believed to Be indicated by uniform average mass ratios of Ti/SI, K/Si and Ca/Si was quite constant with respect to altitude and location. Ratios of these latter elements were found to be distributed virtually log normally. The distribution of the ratios Cl/Si and S/Si was not consistently log normal, presumably because of contributions from pollution sources of these elements. Averaged concentrations at all altitudes for the locations sampled ranged from 0.18–1.52 µg m−3 for Cl, 0.03–0.31 µg m−3 for S, 0.03–0.60 µg m−3 for K, 0.01–1.27 µg m−3 for Na, 0.04–0.76 µg m−3 for Si, 0.02–2.22 µg m−3 for Ca, and 0.004–0.13 µg m−3 for Ti. Mass median diameters ≲0.3 μm were indicated for most of the determined elements at the altitudes sampled.

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Irving H. Blifford Jr. and Lynn D. Ringer

Abstract

A series of 22 aerosol collections made during the year 1966 at altitudes to 10 km in the vicinity of Scottsbluff, Nebr., have been analyzed. Several additional samples were taken at other locations in the centralUnited States when low tropopause conditions permitted collecting in the lower stratosphere. A few additional measurements were made over Limon, Colo. The sampling device was a single stage impactor mountedin a Beech Queen Air aircraft. Particle size and number distributions were determined by direct countingfrom photomicrographs. The collecting and counting techniques are described and the experimental resultspresented. Systematic changes were observed in the aerosol distribution function with altitude in whichboth the large and small particulates diminish with height. Short term variations in the size and numberdistribution, seasonal changes, and correlations with various meteorological parameters were investigated.Characteristic aerosol distribution functions for tropospheric and lower stratospheric air were found onoccasions of low tropopause. Correlations of the total number of large and intermediate size particles withrelative humidity and a more or less constant mixing ratio of intermediate sized particles with altitudesuggest the importance of cloud modification processes in determining the aerosol distribution function.

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Dale A. Gillette, Irving H. Blifford Jr., and Charles R. Fenster

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.

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