Relationships between horizontal transport flux and vertical deposition flux during dry deposition of atmospheric dust particles

Wind tunnel experiments were conducted to investigate the relationships between vertically settling dust and the horizontal dust flow from which the particles originate. The study primarily focused on the grain size characteristics and on the horizontal and vertical sediment fluxes. Simultaneous samplings of vertical deposition flux and horizontal transport flux were carried out at an almost identical altimetric level (a very narrow layer immediately above a water surface generating no resuspension). This made it possible to relate deposition flux to the horizontal transport flux and to study how deposition alters the sedimentological characteristics of horizontally moving dust. The use of water as a deposition surface implied that the dust was subjected to some degree of dispersion after the settling. This restricted the analysis to grains >10 μ m, but the trends for grains <10 μ m were easy to reconstruct. The experiments show that deposited dust is generally coarser than the parent dust from which it originates but only for friction velocities below 0.34 m s −1 . For higher wind speeds the median grain diameter of settling dust does not differ from that of the parent dust. Similar observations were made for the horizontal and vertical dust fluxes. At a constant horizontal transport flux ( Fh ) vertical deposition flux ( Fs ) drops with wind speed; however, from the same critical friction velocity of 0.34 m s −1 the ratio Fs/Fh remains constant. It was also found that the relationships between Fh and Fs depend on the grain size of the particles but only for sufficiently fine particles. Particles >50 μ m no longer affect these relationships. Vertical mixing created by the turbulent nature of the flow strongly affects deposition of atmospheric dust. Mixing significantly hampers deposition of the grains, especially the small ones. The wind tunnel experiments allowed the magnitude of the effect mixing exerts on deposition to be calculated. For coarse grains the effect is small, but for fine grains mixing may hamper deposition by several dozen per cent. Theoretical dust deposition models should thus consider vertical mixing or otherwise include it in the numerical value of the velocity of deposition v d .