Influence of friction velocity and saltation intensity on the particle-size distribution of dust (PM32) emitted from an agricultural soil

Abstract

The size distribution of dust emitted from soils determines particle attributes such as the enrichment in elements that are important for biogeochemical cycles, global climate and human health. There is an ongoing discussion about the size distribution of dust and its relationship with saltation and wind speed. In this work, we present our own results and discuss them in the context of other studies, emphasizing on dust emission from agricultural soils, which is relatively complex and less studied. The objective of this work was to evaluate the particle size distribution of the emitted dust or particulate matter (PM) up to 32 μm in diameter (PM32) from an agricultural sandy soil under two saltation-supply conditions, using a wind tunnel coupled to a dust-suspension chamber. We found strong correlations between friction velocity, saltation flux and dust emission. Additional saltation supply increased the total dust PM32 emission at high wind speeds up to three-fold. Correlation between wind speed and dust concentration was best explained by a power law, with exponents up to 12.04 for the 20 to 32 μm particle-size range under high-saltation conditions, but much lower (up to 4.34) for the 0.25 to 1 μm size range under low saltation. An increase of the wind speed affected significantly the concentration of the coarser dust fraction (PM > 10 μm), but affected less the concentration of fine dust (PM from 10 to 1 μm) and did not affect the concentration of the ultrafine (PM < 1 μm) fraction in the suspension chamber. Results found suggest that dust is mostly produced from particles less than 60 μm in diameter, both by fragmentation and direct wind lift. Results found herein reinforce one of the existent theories, but raise questions about the influence of the soil type, and the place and timing of the sampling arrangements.