Abstract
The first part of this paper is devoted to the electrical characteristics of the air below the canopy of a pine and spruce forest. In fair weather conditions, the site influence, i.e. the filtering effect of the trees on air conductivity and electric field, is evidenced. Under disturbed weather conditions, the meteorological influence is depicted to show: (i) that electric fields sufficiently high to produce `point discharges' occur not only during thunderstorms but also during mist; (ii) that, by taking into account the gaseous ions produced by the point discharges, it becomes possible to understand on the one hand the field divergence with height observed in the mist, and on the other hand the detection of alternatively positive and negative gaseous clouds of charges during the thunderstorm.
The second part presents the results of chemical analysis performed below and just above the canopy on the same site and for the same period. Increased ozone and hydrogen peroxide concentrations were measured during the thunderstorm. It is shown that these chemical species were not only locally produced by photochemistry and/or transported from different (industrial and traffic) sources, but also arose from transient electrical point discharges in the forest under high electric field conditions. Their local concentration is shown to be influenced by the electrical discharge current density on the one hand, and by the local conditions of atmospheric stability and of water content, determining the evolution of the chemical products, on the other hand. Actually, after their production, the initially gaseous chemical products were shown to be involved in the local droplet 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 local environmental conditions (liquid water content of the air and atmospheric stability) associated with the two different situations investigated, a mist and a thunderstorm.
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