Influence des panaches ionisés sur le champ électrique terrestre

Abstract

Several months of measurements of atmospheric electrical parameters, in particular of the earth's electric-field, using a mobile laboratory, have made possible an extension of previously obtained results (Laurent and Peyrous, 1979) to the industrial area of Pardies-Nogueres. This study confirms the presence of a negatively charged aerosol in the plume emitted by factory chimneys. Down-wind, under the chimney plume, the earth's electric field is inverted and takes on large negative values and, if fall-out occurs, the space charge density due to large ions becomes negative. In the first theoretical part of our paper, we reduce the chimney plume to an equivalent charged line whose charge varies with distance as a result of recombination and diffusion. Atmospheric turbulence is neglected, and only the vertical component of the electric field is measured. By mapping charts (Fig. 5) which give cross-sections of the electric field measured along-lines perpendicular to the wind direction one measures the distance (OP) between the inversion maximum (O) and the beginning of the inversion (P) and one can thus calculate the height of the chimney plume (equations (11), (12) and (13)). Then, using equation (6), and knowing the wind speed and the distance from the source one can calculate the charge. Furthermore, if the width of the chimney plume is assumed to be the same as that of the inversion indicated on the chart, and knowing the original diameter of the plume, one can calculate the mobility k of the negative particles by using the relationship indicated by Jones and Hutchinson (1976) (equation (16)). The experimental results were obtained by using a Field Mill installed on the roof of the laboratory van with the recording synchronized to the vehicle speed (Fig. 6, 7 and 8). Two sites were studied: the Lacq gas treatment plant and a plant producing aluminium by electrolysis from molten alumina at Nogueres. The distance over which the chimney plume can be localized depends on the plume charge and the wind speed. From the measurements made, it was estimated that the recombination coefficient of positive and negative charges was 6.8 m −3 s −1 and the mobility of negative ions in the chimney plume was 5 · 10 −5 m 2 v −1 s −1. Our calculations of the height of the chimney plume correspond well with the increase in height of the plume above the chimney outlet given by the HOLLAND formula. The measurements made on the fixed site of Lacq (Fig. 14) show the effect of wind and the relationship between the chimney plume and the electric charge. The measurements of the earth's vertical electric field to detect chimney plumes permits the localization of charged aerosols and the calculation of the charge and its altitude. It also allows one to follow the plume and, with space charge measurements, to determine where fall-out occurs. A study of the mechanism of charge separation at the source together with that of diffusion and recombination can be envisaged. A comparison between calculations using diffusion theory and results obtained from our measurements of the height of the plume is underway.