ELECTRICAL AND CHEMICAL CONSEQUENCES OF POINT DISCHARGES IN A FOREST DURING A MIST AND A THUNDERSTORM

The first part of this paper is devoted to the electrical characterist ics of the air below the canopy of a pine and spruce forest. In fair w eather conditions, the site influence, i.e. the filtering effect of th e trees on air conductivity and electric field, is evidenced. Under di sturbed weather conditions, the meteorological influence is depicted t o show: (i) that electric fields sufficiently high to produce 'point d ischarges' occur not only during thunderstorms but also during mist; ( ii) that, by taking into account the gaseous ions produced by the poin t discharges, it becomes possible to understand on the one hand the fi eld divergence with height observed in the mist, and on the other hand the detection of alternatively positive and negative gaseous clouds o f charges during the thunderstorm. The second part presents the result s of chemical analysis performed below and just above the canopy on th e same site and for the same period. Increased ozone and hydrogen pero xide concentrations were measured during the thunderstorm. It is shown that these chemical species were not only locally produced by photoch emistry and/or transported from different (industrial and traffic) sou rces, but also arose from transient electrical point discharges in the forest under high electric field conditions. Their local concentratio n is shown to be influenced by the electrical discharge current densit y on the one hand, and by the local conditions of atmospheric stabilit y and of water content, determining the evolution of the chemical prod ucts, on the other hand. Actually, after their production, the initial ly gaseous chemical products were shown to be involved in the local dr oplet chemistry. Under specific weather conditions, ionic densities as well as chemical by-products of a forest therefore depend both on the electrodynamical characteristics of the lower atmosphere and on the l ocal environmental conditions (liquid water content of the air and atm ospheric stability) associated with the two different situations inves tigated, a mist and a thunderstorm.