Contribution to the study of dry deposition velocity of submicronic aerosols on a grassland
Congress title :CFA 2007/2008 - 23ème Congrès Français sur les Aérosols
Congress town :Paris
Congress date :16/01/2008
In Order to know the impact of the aerosol pollution (aerocontamination) on the frame and the ecosystems, first we have to study the transfer function of particles on the buildings or onto vegetated canopies. In function of the meteorological settings, the aerosol deposition can be considered as wet or dry. The dry deposition is defined by a deposition velocity, which is the quotient of particles surface flux and the atmospheric aerosol concentration nearby the surface. This deposition velocity depends on lot of parameters. For example the topography of the ground, the substrate, the micrometeorological conditions (turbulence), the aerosol characteristics (size, electric charge), the extern field (gravity, electric). The study of the aerosols dry deposition velocity stays a subject of research still open, and involves an experimental approach in situ, in order to consider the local particularities (substrate, turbulence, vegetated canopies, etc...) Nowadays, there are several models of aerosol dry deposition which consider the effects of the turbulence, the particles size for a large range of diameter (some nm to 100 µm). In the case of particle bigger than 1 µm, the results of these models can be compared with experimental results. Nevertheless, this comparison is far to be always easy, because of the dispersion of current results (in the model as in the experimental field) on several order and because of the lack of information about certain experimental data. In the case of particles with a diameter less than 1 µm, there is a little of reliable experimental data which allow a comparison with the dry deposition models in the background. The main aim of this work is to find experimentally the dry deposition velocity for ultra-fine and fine aerosols in grassland. To reach this aim two methods will be used; the Eddy correlation and the plotting of aerosols by fluorescein. For the Eddy correlation method, the measuring of atmospheric aerosol concentration is done at high frequency (10 Hz) by an ELPI (Electrical Low Pressure Impactor) or with two CPC (Condensation Particles Counter) coupled to ultrasonic anemometers which measure the vertical wind velocity. The covariance between the vertical wind velocity and the aerosol concentration gives the deposition flux for a size of aerosols until 0.2 µm. for issues of sensitivity over this size (0,2) the plotting method is used. Fluoroscein mono-scattered aerosols are produced in the air, and the part of this, which has been deposed on the grass, is measured by fluorescence. The deposition flux is so the amount deposed by unity of surface and time. For the both methods, the dry deposition velocity is found by dividing the deposition flux by the aerosol concentration in the air. This work presents the used methods, their uncertainty and the first results in the experimental field.