A two-parameter Weibull function to describe airborne dust particle size distributions

A number of mathematical distributions have been proposed for the descripti on of the particle size distribution of unconsolidated sediments. However, few studies have mathematically described aeolan dust particle size distrib utions. Recent work has shown mathematically how the sequential fragmentati on of materials leads to a Weibull distribution. Since the breakage of aggr egates by saltating grains is a primary mode of aeolian dust production, we test the hypothesis that the Weibull distribution may be used to describe airborne soil grains. Surface samples were collected from 52 sites includin g soils, dirt roads and roadside ditches in the Southern High Plains of wes t Texas. The samples were tumbled in the Lubbock dust generation, sampling and analysis system to generate a dust cloud. The particle size distributio n of the dust was measured in situ by laser diffraction and PM10 concentrat ion was determined gravimetrically (PM10 = airborne particulate matter with diameter less than or equal to 10 mu m). This study demonstrated that the Weibull cumulative distribution function (c.d.f.) is an excellent choice to describe the particle size distribution of dust suspended from mineral sed iment. A Weibull c.d.f. used to describe the dust cloud size distribution, accounted for 94 per cent of the variation in estimates of particles less t han or equal to 50 mu m diameter. The fraction of dust particles less than or equal to 10 mu m diameter, as estimated using the Weibull, was not corre lated to suspended PM10 concentration. However, the fraction of particles l ess than or equal to 10 mu m was correlated with properties of the sediment from which the airborne dust was derived. As clay content increased, the t otal amount of suspended dust increased and the fraction of suspended parti cles less than or equal to 10 mu m in the dust cloud decreased. Analyses of variance showed no significant differences (P < 0.05) among sampling locat ions (roads vs ditches vs soils) for cumulative fraction values for less th an or equal to 2.5, less than or equal to 10 less than or equal to 25, less than or equal to 30 and less than or equal to 50 mu m diameter particles. However, the PM10 concentration values were significantly different among d ust generated from these locations. The road samples produced about twice t he amount of PM10 (490 mg m(-3)) as the soil or ditch samples (235 mg m(-3) ). Published in 1999 by John Wiley & Sons, Ltd.