Mechanistic study of aerosols dry deposition on vegetated canopies
Résumé
Aerosols dry deposition onto agricultural and forest areas is investigated. A special attention is given to highly rough surfaces and sub-micronic aerosols, for which the estimation of the deposition is still uncertain. In fact the models used in radiological risk or air quality assessment studies are highly empirical and fail to reproduce the results of the most recent measurement campaigns. Therefore a theoretical framework, based on a mechanistic description, has been developed. The proposed approach consists in two steps. First, the interaction between aerosols and foliar surface is formulated by using a set of parameters, which are defined on the local scale of one foliar element. In the second step, the collective effect of the foliage is taken into account through statistical distribution of these parameters. The model integrates the three main aspects of aerosol dry deposition. These are the local aerodynamic characteristics of the flow within the canopy, the aerosol mechanisms governing the deposition, and the structural and morphological properties of the canopy. The physical processes considered in the study are inertial impaction, gravitational settling, brownian and turbulent diffusion, interception and turbulent impaction. The canopy characteristics considered are the spatial distribution, orientation and micro-structure of the foliar surfaces. The applicability of this framework is demonstrated in a realistic situation : for a given canopy and aerodynamic conditions, the spatial distribution of the aerosol captation by the foliage can be quantified. It enables to simulate numerically the distribution of aerosol concentration within the canopy and the overall deposition flux.