Experimental evidence of dust flux size distribution variation along two consecutive erosion seasons

The determinants controlling the particle size distribution (PSD) of emitted dust in the atmosphere during erosion events are still poorly understood despite the significant impact of mineral dust on meteorology and air quality. Here, we report dust emission flux PSD from a plot in Tunisia during two consecutive erosion seasons, using the same measurement set-up and method to estimate size-resolved dust fluxes. The first year, the plot was a bare soil while the second year the plot was sparsely vegetated, the vegetation covering less than 2% of the plot. Surprisingly, the emitted dust flux PSD exhibited significant variation along the second-year erosive season, with overall a larger proportion of submicron particles, differing from the more constant PSD during the first year erosive season. We show that this PSD variation of the dust flux during the second year is not explained by the presence of the vegetation nor by the atmosphere wind-dynamic and thermodynamic conditions. The emission transfer velocity of dust particles appears independent of the particle size and constant during and between both erosive seasons. We rather suggest that this PSD variation can only be explained by modifications of the soil surface conditions depending on surface tillage and soil humidity during the erosion season, both impacting the available soil aggregates and inter-particle cohesion. This result highlights the crucial role played by the soil surface conditions on the PSD of emitted dust fluxes.