Particle Size of Windblown Loess within the Columbia Plateau, USA.

Agricultural soils of the Columbia Plateau were largely formed from loess that was deposited about 20,000 years ago. Wind erosion is still an active process in this semi-arid region and currently contributes to communities in the Inland Pacific Northwest exceeding the US National Ambient Air Quality Standard for PM10 (particulate matter ≤10µm in diameter). Knowledge of the size distribution of eroded sediment can aid the identification of mechanisms by which agricultural soils erode during high winds and thus the development of management strategies to control wind erosion and improve air quality. Windblown sediment was trapped at various heights above eroding agricultural fields over seven years and separated into various particle size categories. More than 95% of eroded sediment was ≤100 µm in diameter (suspension-size material) and about 40 and 10% of the sediment was 10-32 µm in diameter and PM10, respectively. The size distribution of eroded sediment trapped nearer the soil surface more closely resembled the size distribution of the parent soil. These observations suggest that suspension is the main process by which loessial soils erode by wind within the Columbia Plateau. Land management strategies that promote aggregation and reduce wind speed and turbulence at the soil surface are likely to suppress suspension of sediment and improve air quality within the Columbia Plateau.