High electric fields created by thunderclouds at ground level are enhanced by the irregularities of the surface. This cause corona discharges to occur. The generated ions, when they are not blown away, for a space charge layer which reduces the magnitude of the electric field are ground level. During the LANDES 79 experiment (south western France), electric field soundings have been performed. The sensor is a balloon-borne double field-mill device designed for measuring the vertical and horizontal components of the external electric field. At several occasions, a strong vertical field increase with height was detected in the lower part of the sounding. This reveals the existence of important space charge layers near the ground. Space charge density profiles are deduced from electric field soundings. They display density values running from 2 to 7 nC m-3, distributed over shallow layers. A numerical model taking into account ion capture by the neutral aerosol particles leads to theoretical charge density profiles which display features close to those of the experimental profiles. Computation shows that the initial number of neutral aerosol particles has a drastic effect on both density maximum values and layer depths. According to aerosol density, charge density values run from 0.6 nC m-3 (in clean air) up to 3.1 nC m-3 (high aerosol density).