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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 150 is never entirely successful, however, and investigators must always be aware of the whole range of influences and alert for contamination. While atmospheric-electrical variables respond to many processes, they usually have little influence on the phenomena to which they respond. Thus the electrical state of the PBL is irrelevant to the fields of environmental radioactivity, air pollution, boundary-layer turbulence, and global meteorology, for example. The inverse is not true, however. Atmospheric electricity in the PBL is a truly interdisciplinary study requiring a knowledge of all these areas in addition to ionic conduction in gases, aerosol physics, and electrostatics. Recent advances in the disciplines cited above make it a propitious time to re-examine the relationships between atmospheric electricity and PBL processes. Such a re-examination should ultimately lead to a better understanding not only of atmospheric-electrical phenomena but of the related disciplines as well. With this in mind, the remainder of this chapter presents a brief overview of atmospheric electricity in the PBL. First the primary physical mechanisms influencing the electrical phenomena are discussed. The gross phenomenology is described next, including spatial and temporal variability of the important electrical parameters. Then the most important aspects of modeling and theory are summarized in an effort to relate the physical causes and their electrical effects. Finally, the chapter is closed with a discussion of principal applications and areas of needed research. PHYSICAL MECHANISMS THAT AFFECT ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER Ionization The electrical conductivity of the air is due to ions produced primarily by ionizing radiation. The early investigations of the sources of atmospheric ions led to the discovery of cosmic rays. Cosmic radiation is the primary source of ions over the oceans and above a couple of kilometers over land. In the PBL the cosmic-ray contribution to the ionization rate is about 1 to 2 ion pairs per cubic centimeter per second. It is quite constant in time, and the latitudinal dependence caused by the Earth's magnetic field is well understood. The primary source of ions in the PBL over land is natural radioactivity originating from the ground. This ionization source can be divided into two parts: (i) Î±s, Î²s, and Î³s radiating directly from the Earth's surface and (ii) radiation from radioactive gases and their radioactive daughter products exhaled from the ground. The gases originate in both the uranium and the thorium decay series where one of the daughters is the noble gas, radon. In the uranium decay series, the daughter is 222Rn with a half-life of 3.8 days; and in the thorium series, 220Rn (thoron) with a half-life of 54 sec. During the radon part of the decay the radioactivity can diffuse from the ground into the atmosphere and contribute to the volume ionization. The amount of radon that escapes depends on the amounts of 226Ra and 232Th in the ground; the type of ground cover; and porosity, dampness, and temperature of the soil. The height distribution in the atmosphere depends on atmospheric mixing in the boundary layer and the half-life of the isotope. It is obvious that radiation directly from the ground will vary greatly depending on the geographical variations in ground radioactivity. Ground radiation intensity also decreases with height; Î± ionization is confined to the first few centimeters, Î² to the first few meters, and Î³ to the first few hundred meters. The amount of ionization in the first few centimeters resulting from Î±s is largely unknown. Values of ionization due to Î²s in the first few meters are typically in the range of 0.1 to 10, and those due to Î³s in the lowest hundred meters are in the range of 1 to 6 ion pairs cmâ3 secâ1. Ionization due to radioactive gases in the air is even more variable and depends not only on the amount exhaled from ground but also on atmospheric dispersion. Direct measurements of ionization due to radioactive gases in the atmosphere are difficult and have not generally been satisfactory. Estimation of the ionization rate is therefore based on measurements of radioactive products in the air. The height distribution of Rn in the atmosphere as a function of turbulent diffusion has been the subject of a number of investigations and is often used to determine the turbulent diffusion coefficient. On cool nights with nocturnal temperature inversions the radioactive gases can be trapped in a concentrated layer close to the ground, whereas during unstable convective periods, the gases can be dispersed over an altitude of several kilometers. Ionization at 1 to 2 m due to radioactive gases and their short-lived daughter products is typically in the range of 1 to 20 ion pairs cmâ3 secâ1 and is predominantly caused by Î± particles. Figure 11.1 illustrates the vertical variation of ionization in the PBL. Ionization due to cosmic rays is nearly constant in the first kilometers. The ionization from ground Î² and Î³ radiation will vary geographically depending on the abundance of radioactivity in the local soil. The curves shown are typical, with Î² radiation being predominant below 1 m and the effect of Î³ radiation extending to several hundred meters. The curves labeled Qmax and Qmin represent the sum of ionization due to cosmic rays, Î³s and Î²s from the ground, and the decay of