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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 151 220Rn and 222Rn gases and their daughter products in the atmosphere. The area between the two curves illustrates the variability expected depending on atmospheric and soil conditions. The high ionization rate below 1 m indicated by Q 220Rn in the lowest meter and occurs only under strong surface-temperature inversions max is due to an accumulation of and low winds. During convective (daytime) periods with significant wind shear the ionization profile below 10 m would be expected to follow Q min values closely. The data used to construct Figure 11.1 were taken largely from Ikebe (1970), Ikebe and Shimo (1972), Crozier and Biles (1966), and Moses et al. (1960). There is a limited amount of data on the vertical distribution of 220Rn (thoron), and therefore Q max is the expected maximum ionization based on measurements at only three sites. Geographical areas with high ground radioactivity may exhibit even larger ionization rates than shown in Figure 11.1. Figure 11.1 Ionization profiles showing range of values that might be expected over land. Qmax would represent high values; Qmin low values. Ionization within the first few centimeters of the Earth's surface due to Î± particles has never been adequately investigated. There is the possibility that Î±emitting daughter products of radon that attach to aerosol particles are deposited on the Earth's surface, enhancing surface activity. This activity is usually neglected in the study of environmental radiation because it represents a small fraction of the total environmental activity. Yet this source of ionization is likely to be important in determining the ion concentration at the surface. Surface ion concentration is an important boundary condition influencing the atmospheric-electrical structure in the interior of the PBL. Our intention here is not to review the field of environmental radioactivity but rather to point out that over land the ionization in the PBL depends primarily on ground radiation and radioactive gases and to emphasize the necessity of studying the geographical variation of ground radioactivity and the dynamics of the dispersion of radioactive gases if we are to understand atmospheric electricity in the PBL. Over the oceans far from land the ionization rate is determined solely by cosmic rays. The 3.8-day half-life of 222Rn permits it to advect over oceans for hundreds of kilometers before its ionization is negligible compared with that of the cosmic-ray background. Compared with the ionization due to natural sources, ionization from nuclear power plants and weapons is negligible on a global scale. This was true even during the active period of nuclear weapons testing in the 1950s and 1960s (IsraÃ«l, 1973). There can, of course, be locally significant effects (Huzita, 1969). In addition to ionizing radiation, electrical discharges can also form ions in the atmosphere. This requires high electric fields that generally occur only in disturbed weather near thunderstorms and in regions of blowing dust or snow. The field is greatly augmented at points on electrically grounded, elevated objects such as vegetation and antennas. As the electric field increases, the field in very small regions near such points reachs breakdown values and a small ionic current is discharged into the atmosphere. A large number of unipolar ions is injected locally into the atmosphere, and the ionic space charge thus formed tends to reduce the high electric field. Ions can also be produced by the bursting of water films. In nature this occurs in waterfalls, falling rain, and breaking waves. Ions generated by this mechanism are not formed in pairs, and a net charge is introduced into the atmosphere. In most cases the residue remaining after evaporation of the water is much larger than a small ion and is more appropriately identified as a charged aerosol particle. Properties of Ions The radioactive ionization process separates an electron from a molecule of nitrogen or oxygen. The electron attaches rapidly to a neutral molecule to form a negative ion. During the next few milliseconds both positive and negative ions undergo a series of chemical,