Dry deposition velocities of submicron aerosols on water surfaces: Laboratory experimental data and modelling approach

This article presents new experimental values of deposition velocities of submicron and hygroscopic aerosols of uranine with the objective to analyze dry deposition mechanisms of submicron aerosols on water surfaces in function of wind, water current and differences between air and water temperatures. A special attention is paid to phoretic effects which have been poorly considered by previous experimental studies. Experiments were performed in a laboratory wind tunnel for different conditions precisely characterized in function of aerosol size distribution, air turbulence, water current, water surface deformations, air and water temperatures and relative humidity. The experimental results do not allow detecting any effects of water current nor water surface deformation. The deposition velocity dimensionless with the friction velocity varies according to a V curve as a function of the air-water temperature difference. The minimum of this curve (≈ 2.0×10−5) is reached when the air-water temperature difference is equal to −7.5°C. To analyse these experimental results, appropriate models of hygroscopicity growth, diffusiophoresis and thermophoresis are implemented in the Slinn resistance model (Slinn & Slinn, 1980). Comparisons between experimental and modelling results confirm that the dry deposition velocity of submicron aerosol over water surface is mainly influenced by the turbulent diffusion and show that the influence of hygroscopicity growth is negligible. Furthermore, it is shown that adding the diffusiophoresis and thermophoresis velocities in the deposition layer transfer coefficient greatly improves the agreement between the experimental deposition velocity and the model. Particularly, these results reveal the important role of the thermophoresis when the air-water temperature difference becomes lower than -3 °C.