Tropospheric Aerosols from Some Major Dust Storms of the Southwestern United States

Abstract

Tropospheric aerosols from major dust storms (visibility <11 km) originating in cultivated areas of the High Plains in west central Texas and adjacent areas of New Mexico, Oklahoma and Colorado, were sampled by ground-based airturbine samplers with stacks 1 to 6 m high, by membrane filters, by airplane-borne dust samplers and by a static ground-level sampler. The particle size distributions of the aerosol dust obtained by airplane sampling fell mainly between 1 and 30 ?m diameter. A bimodal size distribution occurred for the dust from ground samplers, with large concentrations in the 40 to 80 ?m range as well as in the 1 to 30 ?m range. The concentration of dust 2 to 5 km above the ground, measured by both the filtering and impactor methods, ranged from 0.1 to 0.4 mg m?3 for four intense dust storms in Texas during April of 1972 and 1973. The vertical flux for dust storms over the four-year period ranged from 0.25 ? 10?7 to 2.2 ? 10?8 g cm?2 s?1. Oxygen isotopic ratio values of 1 to 10 ?m quartz isolated from 17 dusts collected by ground-based samplers ranged from 16.4 to 19.5? (mean, 18.35 ± 0.77?); three dusts from the airplane samplers averaged 18.2 ± 1.1?. The Texas dusts arose largely from 13 wind-eroding soil mapping units and erodibility classes of sandy to clayey texture in the four states; the δ18O values of the 1 to 10 ?m quartz of these soils averaged 19.55 ± 0.28? (reported elsewhere). Abrasion by wind-induced inter-particle impact may have introduced a small amount of coarser quartz into the 1 to 10 ?m aerosol fraction. Quartz from the coarser fractions of the dusts had δ18O values ranging from 16.9 to 13.9? with the lower values applying to the preponderantly sand sizes (>53 ?m). The fine silt from eroding sandy soils, derived not only from weathering but also possibly from eolian deposition, serves as a reservoir for long-range aerosol minerals, in addition to that from shales.