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Dry deposition of submicronic atmospheric aerosol over water surfaces in motion

Névénick Calec has defended his thesis on 2nd December in Cadarache.

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PRP-ENV/SERIS/LM2E

Authors > CALEC Névénick

Publication Date > 02/12/2013

Summary

Whether by chronic or accidental releases, the impact of a nuclear installation on the environment mainly depends on atmospheric transfers; and as the accidents at Chernobyl and Fukushima show, affect the contamination of surfaces and impacts in the medium and long-term on the environment and the population. In this context, this work focuses on the characterization and modeling of dry deposition of submicronic aerosols on liquid surfaces in motion such as rivers.

Unlike wet deposition which is conditioned by washout and rainout (rain and clouds), dry deposition is a phenomenon that depends entirely on the characteristics of aerosols, receiving surfaces, and air flow. In practice, the evaluation of dry deposition is based on the estimation of flux modeling as the product of particle concentration and deposition velocity which can vary over several orders of magnitude depending on the receiving surfaces (forest, snow, urban, grassland…). This topic is motivated by the virtual non-existence of studies on the mechanisms of dry deposition on continental water systems such as rivers; and respect for submicronic aerosols. They have the lowest deposition efficiencies and filtration and the longer residence time in the atmosphere. In addition, they are potentially the most dangerous to living beings because they can penetrate deeper into the airway. Due to the lack of data on the dry deposition of submicronic aerosols on a liquid surface in motion, the approach was based on two axes: 1) the acquisition of experimental deposition velocities and 2) the analysis and interpretation of results through modeling.

The experiments were performed with uranine aerosols released into the IOA wind tunnel (Interface Ocean Atmosphere) of the Institute for Research on Non Equilibrium Phenomena which is configured to study the coupling between the air flow and water. These experiments have given many dry deposition velocities for different configurations characterized according to wind conditions (central wind speed: 1 , 2, 4 , 5, 7.5 and 9.5 m/s), current (co - current and counter-current, water flow velocity: 0 , 6 and 12 cm/s ), ambient (temperature and relative humidity of the air and water temperature), the liquid surface deformations (measured significant wave height) and size distribution of aerosols released.

The modeling part was to adapt the model to resistance. Slinn and Slinn (1980). This model is based on the assumption of conservation of vertical flow in the boundary layer. This one is divided into two layers: a very thin deposition layer near the surface which is water-saturated and a transfer layer located above. The transfer layer provides the particle deposition layer by turbulent diffusion and sedimentation. In the deposition layer the particles are deposited under the effect of several mechanisms such as Brownian diffusion, sedimentation, impaction and phoretic mechanisms. The main adjustments made by this work have been to take specific account of the different classes of particle size distribution, the spectrum variation as a function of hygroscopicity, and mechanisms of aggregation. It is integrated mechanisms of diffusiophoresis and thermophoresis, respectively produced by the evaporation of water and the temperature gradient at the air-water interface.

To account for hygroscopic uranine aerosols, the deposition rates are analyzed in terms of humidity and rescaled by the friction velocity. In all cases, the deposition rates rescaled by the friction velocity range from 10-3 to a wind speed of 1 m/s to 10-5 - 10-4 for wind speeds above 5 m/s . It is shown that the changing speed rescaled is inversely proportional to the wind velocity when the water surface is smooth (between 1 and 5 m/s ) and becomes proportional to the deformation when the surface becomes significant (over 5 m/s ) . Although the results do not clearly identify the effect of the current on the deposition velocity, the modeling shows that turbulent diffusion is dominant in the transfer layer and the effect of hygroscopic mechanisms of sedimentation is insignificant. In the deposition layer, the effects are more hygroscopic and negligible deposition of finer particles can be blocked by the phoretic mechanisms when the difference between the temperatures of air and water increases. In all cases, the disparities between the model and experiment can be reconciled under conditions that properly account for the size distribution of aerosols.

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