Détermination des coefficients de diffusion verticale entre 0 ET 100 M a l'aide du radon et du ThB

Résumé

Le radon (Rn₈₆ ²²²) et les descendants du thoron (ThB ou Pb₈₂ ²¹²) sont utilisés comme traceurs pour l'étude des échanges verticaux de matière. Diverses méthodes permettant le calcul des coefficients de diffusion sont présentées soit dans l'hypothèse simplificatrice du régime stationnaire, soit dans le cas plus général du régime dépendant du temps. L'application de ces méthodes au calcul des coefficients de diffusion sur des données fournies à partir d'un modèle de simulation numérique permet de préciser le degré d'approximation représenté par les diverses hypothèses simplificatrices. Les méthodes de calcul présentées sont appliquées aux mesures expérimentales du radon et du ThB. Ces mesures ont été réalisées de manière continue durant deux ans en trois points de mesures, entre 0 et 100 m de hauteur. Une statistique est présentée sur la variation diurne des échanges et la variation saisonnière de cette variation diurne, ainsi qu'un exemple particulier du traitement des données en régime dépendant du temps.

Abstract

Continuous measurements of the concentrations of radon and thorium B have been carried out at 3 levels (1.5, 30 and 100 m) during a period of 2 yr (from October 1966 to October 1968) at the site of the future airport of Paris, at Roissy-en-France; at the same time, standard meteorological elements have been measured at the station by the ‘Météorologie Nationale’ and the vertical gradients (wind and temperature profiles in the first 100 m) by J. Saissac. The work has been undertaken in order to determine the vertical exchange of matter near the ground.

The computation of the exchange coefficients has been carried out by using the standard diffusion equation (K theory). The results in the literature as well as statistical analysis of our data lead us to believe that the flux of Rn and ThB at the ground can be considered as horizontally homogeneous in the neighbourhood of the measurement site (the soil is geologically homogeneous over a large area). The differential equation is thus simplified, containing only one space parameter (z), and timet.

In this paper we describe the principal computation methods used for determining the exchange coefficients from the measurement of the vertical gradients of tracers. In order to illustrate the approximation errors and fields of applicability of different methods, we have applied them to the data provided from a numerical integration of the diffusion equation, in which one can introduce any profile of diffusivity.

We show that during stable periods, the value of the flux of Rn or ThB can be determined forz = 0.

Generally, during daylight hours when the motion near the ground is strongly turbulent, the vertical gradient of concentration is not measurable with sufficient accuracy.

The daytime diffusion coefficient is therefore not computed by the general methods presented above. This deficiency can be overcome by assuming that the ratio between the mean concentration observed during these periods and the value of the flux at ground level are characteristic of the whole mixing layer. The values of the exchange coefficients obtained by this method are included and range from 5 to 30 m²s^-1.

The application of the proposed methods to the experimental results has been made for two atmospheric states. Firstly, we have used a simplified method (stationary state) which is applied to the whole experimental data set. We present the diurnal variation, the most probable value of the diffusion coefficients and the seasonal evolution. The values obtained from Rn and ThB measurements have been compared.