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1 1 INTRODUCTION Atmospheric Electricity in the Planetary Boundary Layer WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT Naval Research Laboratory The planetary boundary layer (PBL) is that region of the lower atmosphere in which the influences of the Earth's surface are directly felt. The primary influences of the surface are drag, heating (or cooling), and evapo- ration (or condensation). These processes cause vertical fluxes of momentum, sensible heat, and moisture, which penetrate into the lower atmosphere to a finite height. These fluxes, in turn, generate turbulence, ulti- mately controlling the mean profiles of wind speed, temperature, and water vapor in the PBL. Since the height of penetration depends on the direction, magni- tude, and persistence of the surface fluxes and on the large-scale meteorological conditions, the PBL can range in thickness from tens of meters to a few kilome- ters. Because of its position next to the Earth's surface, the PBL has been the site of the vast majority of atmo- spheric-electrical measurements. If the history of atmo- spheric electricity begins with Franklin, Lemonnier, and Coulomb, as is customary, there have been more than two centuries of effort in this region. It was ob- served early that measured quantities such as electric field respond strongly to meteorological processes. This led Lord Kelvin to suggest that the day would come 149 when weather forecasting would be done with an elec- trometer (Dolezalek, 1978~. Unfortunately, this opti- mistic prediction has not materialized, largely because of the complexity of the dependence of electrical vari- ables on a bewildering variety of other phenomena. Most atmospheric processes are interrelated and can- not be studied in isolation, but it is usually possible to identify one or two dominant influences. In the case of atmospheric electricity in the PBL, however, separating the various causes and their effects can be extremely dif- ficult. In fact, this field may be unique with respect to its sensitivity to many disparate phenomena spanning a tremendous range of scales in both space and time. For example, locally produced turbulent fluctuations in space-charge density have an effect roughly comparable in magnitude to that of changes in global thunderstorm activity on electric-field variations within the PBL. Over the years this responsiveness of atmospheric electricity has led to its exploitation for many different purposes. Electrical measurements have been made in the PBL to observe large-scale processes such as the global circuit, to study local phenomena like boundary- layer turbulence, or simply to examine unusual electri- cal signatures in their own right. In each type of investi- gation it has been found necessary to minimize the effects of unwanted processes on the data. This filtering

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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 environmen- tal radioactivity, air pollution, boundary-layer turbu- lence, 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 be- tween atmospheric electricity and PBL processes. Such a re-examination should ultimately lead to a better un- derstanding not only of atmospheric-electrical phenom- ena 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 variabil- ity of the important electrical parameters. Then the most important aspects of modeling and theory are sum- marized 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 in- vestigations of the sources of atmospheric ions led to the discovery of cosmic rays. Cosmic radiation is the pri- mary source of ions over the oceans and above a couple of kilometers over land. In the PBL the cosmic-ray con- tribution 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) oes, Us, and lays radiating directly from the Earth's surface and (ii) radiation from radioactive gases and their radio- active daughter products exhaled from the ground. The WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT gases originate in both the uranium and the thorium de- cay 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 es- capes depends on the amounts of 226Ra and 232Th in the ground; the type of ground cover; and porosity, damp- ness, and temperature of the soil. The height distribu- tion 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 varia- tions in ground radioactivity. Ground radiation inten- sity also decreases with height; ax ionization is confined to the first few centimeters, ~ to the first few meters, and to the first few hundred meters. The amount of ioniza- tion in the first few centimeters resulting from offs is largely unknown. Values of ionization due to As in the first few meters are typically in the range of 0.1 to 10, and those due to days in the lowest hundred meters are in the range of 1 to 6 ion pairs cm ~ 3 see ~ i. Ionization due to radioactive gases in the air is even more variable and depends not only on the amount ex- haled from ground but also on atmospheric dispersion. Direct measurements of ionization due to radioactive gases in the atmosphere are difficult and have not gener- ally been satisfactory. Estimation of the ionization rate is therefore based on measurements of radioactive prod- ucts in the air. The height distribution of Rn in the at- mosphere 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 con- vective periods, the gases can be dispersed over an alti- tude of several kilometers. Ionization at 1 to 2 m due to radioactive gases and their short-lived daughter prod- ucts is typically in the range of 1 to 20 ion pairs cm~3 sec~ ~ and is predominantly caused by or particles. Figure 11.1 illustrates the vertical variation of ioniza- tion in the PBL. Ionization due to cosmic rays is nearly constant in the first kilometers. The ionization from ground ~ and By radiation will vary geographically de- pending on the abundance of radioactivity in the local soil. The curves shown are typical, with ~ radiation be- ing predominant below 1 m and the effect of By radiation extending to several hundred meters. The curves labeled Qmax and Qmin represent the sum of ionization due to cosmic rays, lays and As from the ground, and the decay of

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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 100 1n 1.0 0.1 TOTAL 'me FROM EARTH \ POW///: \ ~ TYPICAL RANGE ~ \ ~ DUE TO RADIOACTIVE ~- \ ~ EMANATIONS FROM EARTH .01 1.0 10 IONIZATION RATE (pairs/cm~s) 100 FIGURE 11.1 Ionization profiles showing range of values that might be expected over land. Qmax would represent high values; Qmin low values. 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 indi- cated by Qmax is due to an accumulation of 220Rn in the lowest meter and occurs only under strong surface-tem- perature inversions and low winds. During convective (daytime) periods with significant wind shear the ion- ization profile below 10 m would be expected to follow Qmin values closely. The data used to construct Figure 11.1 were taken largely from Ikebe (1970), Ikebe and Shimo (1972), Crozier and Bites (1966), and Moses et al. (1960~. There is a limited amount of data on the vertical distribution of 220Rn (thoron), and therefore Qmax 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. Ionization within the first few centimeters of the Earth's surface due to ax particles has never been ade- quately investigated. There is the possibility that o`- emitting daughter products of radon that attach to aero- sol particles are deposited on the Earth's surface, enhancing surface activity. This activity is usually ne- glected in the study of environmental radiation because it represents a small fraction of the total environmental 151 activity. Yet this source of ionization is likely to be im- portant in determining the ion concentration at the sur- face. Surface ion concentration is an important bound- ary condition influencing the atmospheric-electrical structure in the interior of the PBL. Our intention here is not to review the field of envi- ronmental radioactivity but rather to point out that over land the ionization in the PBL depends primarily on ground radiation and radioactive gases and to empha- size the necessity of studying the geographical variation of ground radioactivity and the dynamics of the disper- sion of radioactive gases if we are to understand atmo- spheric 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 (Israel, 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 vegeta- tion and antennas. As the electric field increases, the field in very small regions near such points reachs break- down values and a small ionic current is discharged into the atmosphere. A large number of unipolar ions is in- jected 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 remain- ing 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 elec- tron from a molecule of nitrogen or oxygen. The elec- tron attaches rapidly to a neutral molecule to form a negative ion. During the next few milliseconds both pos- itive and negative ions undergo a series of chemical,

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152 charge-exchange, and clustering reactions with the mo- lecular species present in the air. Much progress has re- cently been achieved in understanding this chain of ionic reactions in well-defined laboratory gases (Huer- tas et al., 1978) and in the upper atmosphere (Arnold and Ferguson, 1983~. In the troposphere, where trace gases are numerous and variable, the ion chemistry is complicated, and the composition of the terminal ion must be regarded as uncertain at best. If mass spectro- scopic measurements can be extended to identify ambi- ent ions, it is possible that this identification can be used as a measure of certain elusive trace gases in the tropos- phere. Fortunately, the ion nature enters the equations that govern atmospheric electricity in the PBL only as it af- fects the ionic mobility and recombination coefficient. The mobility is defined as the mean drift velocity of the ion per unit electric field. The mobilities of ambient ions are more easily determined than their mass or chemical composition. (There is no unique relationship between mobility and mass, and a rather large uncertainty in mass translates into a much smaller uncertainty in mo- bility.) Values for aged positive and negative ion mobili- ties at STP are about 1.15 and 1.25 cm2 V ~ ~ sec~ i, re- spectively (Mohnen, 1977), are inversely proportional to air density, and are independent of electric-field strength. If ions are formed in pairs by ionizing radiation and annihilated in pairs by ion-ion recombination, then the positive and negative ion concentrations are equal and, in the absence of particulates, are given by n+ = (q/cx)~/2 (11.1) where q is the ionization rate and is the recombination coefficient. If there are no aerosol particles in the atmo- sphere the ion lifetime is given by T = n+Iq = (llqor) ~/2 (11.2) Recombination of ions in the troposphere is a three- body process. The present theoretical treatment of three-body recombination is inadequate for calculation of absolute values of the recombination coefficient. This is due in part to our inability to identify the ion chemis- try. Measurements of the recombination coefficient yield a value of about 1.4 X 10 - 6 cm3/sec for aged atmo- spheric ions at STP (Nolan, 1943~. The movement of ions in an electric field gives rise to the electrical conductivity of the atmosphere. The con- ductivity is defined as A= e~n+k+ + n k ), (11.3) where k+ is the mobility and e is the elementary charge. WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT The conductivity is non-ohmic (field dependent) if the ion densities depend on electric field, as is the case near a boundary (electrode effect). Equations (11.1~-~11.3) predict concentrations of about 3000 ions/cm3, lifetimes of about 5 min. and a conductivity of about 1 X 10- i3 mho/m for an ioniza- tion rate of 10 ion pair cm ~ 3 see ~ i. In reality the values of these variables are considerably smaller because ions are destroyed not only by recombination but also by at- tachment to aerosol particles, as described in the next section. Attachment of ions to Aerosol Particles Suspended in the atmosphere are many small parti- cles mostly in the size range between 0.01 to 0.5 ,um ra- dius. The concentration of aerosol particles varies from a few hundred per cubic centimeter over remote regions of the oceans to a hundred thousand per cubic centi- meter in polluted urban environments. Ions diffuse to these particles and, on contact, transfer their charge to them; thus the particles act as centers of recombination. In most continental areas the loss of ions by attachment to aerosol particles is greater than the loss by ion-ion recombination. The attachment process also establishes a size-dependent statistical charge distribution on the aerosol particles. (Some are negatively, some positively, and some neutrally charged.) To treat the problem of ion-aerosol attachment in its totality requires the solution of a system of balance equations for ions and particles with various numbers of elementary charges. Since we are interested here only in the effect of aerosols on ion depletion, we can write a simplified ion balance equation as <3' = q - orn2 - hnZ, (11.4) where Z is the total particle concentration irrespective of the charge state of the individual particles and ,6 is the effective ion-aerosol attachment coefficient. For steady- state conditions and q = 10 ion pairs cm~3 sec~i, the dependence of ion density on the particle concentration is shown in Figure 11.2 for an effective attachment coef- ficient of ,B = 2 X 10 - 6 cm3 sec~ i. It is readily seen that when the particle concentration is greater than about 1000 cm ~ 3, the concentration of ions is more dependent on the aerosol attachment than on ion-ion recombina- tion. Only in remote oceanic and Arctic regions can the effect of particles on the ion density sometimes be ig- nored. Atmospheric aerosols are hydroscopic, and their equi- librium radius increases as the relative humidity in

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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 10,000 - o - o 1,000 An He o o _ ' . ~ - 100 1 ,'.,lel1 , 11111111 1 ',,,,'1 ,W,B,, 10 100 1,000 10,000 100,000 PARTICLE CONCENTRATION (NO. PER cc) FIGURE 11.2 Ion depletion as a function of increasing aerosol con- centration for an ionization rate of 10 ion pairs cm ~ 3 sec~ ~ and a typi- cal value of ion-aerosol attachment coefficient. creases. This growth is especially pronounced at relative humidities above about 90 percent. An increase in the size of aerosol particles makes them more efficient scav- engers of ions, resulting in a lower conductivity. When the relative humidity exceeds 100 percent, some of the particles are activated and grow rapidly (by about 2 or- ders of magnitude) to radii larger than 1 ,um, forming fog or cloud droplets. These droplets are effective scav- engers of ions and are responsible for the low conductivi- ties found in fogs and clouds. The atmospheric-electric fog effect (see section below on Phenomenology of At- mospheric Electricity in the Planetary Boundary Layer), where conductivity decreases and the electric field increases before the formation of fog, is probably related to aerosol growth with increasing humidity. Recent advances in aerosol measurements have re- sulted in accurate aerosol size distributions that could now be used to evaluate more thoroughly the ion-aero- sol equilibrium. An accurate treatment would include not only a realistic aerosol size distribution but also the statistics of diffusional charging of particles by ions. Effect of the Global Circuit Electric fields and currents in the PBL arise primarily from the voltage impressed across it by the global circuit discussed in Chapter 15 (this volume). The weak con- ductivity of the PBL causes it to act as a resistive element in the global circuit, conducting a fair-weather current density of about 1 to 3 pA/m2 to ground where the fair 153 weather electric field is about 100-200 V/m. Because the PBL is the region of greatest electrical resistance, it largely controls the discharge rate of the global circuit. If the conductivity of the atmosphere were uniform, it would be a passive ohmic medium with no accumula- tion of space charge to alter the electric field. Space charge is generated internally in the unperturbed atmo- sphere in two ways: (1) by conduction down a conduc- tivity gradient and (2) by the imbalance of ion flow near a boundary. The first mechanism can be understood in terms of Ohm's law, 7 = XE, where..T is the conduction- current density. For steady-state conditions and in the absence of any convective charge transport, .! is uni- form, and, therefore, the electric field is inversely pro- portional to the conductivity. Since conductivity changes with altitude, there must be an inverse altitude dependence of the electric field. Poisson's equation re- quires this change in field to be accompanied by a space charge given by ~ = rev ~ = 6 v ~ - 6> E. v\, ~ - flu ~ - ~ (11.5) where en is the dielectric permeability of air. By the steady-state assumption, the first term on the right- hand side vanishes and the space charge is proportional to the electric field and conductivity gradient and in- versely proportional to the conductivity. This process can be thought of as a pileup of space charge due to con- duction down a conductivity gradient. The second mechanism for producing space charge operates only near a boundary. Across any horizontal area in the atmosphere stressed by a vertical electric field, positive and negative ions will flow in opposite directions. However, at a boundary, ions of one sign can flow to that boundary, but there will be no compensat- ing flow of the opposite sign away from it. This im- balance in ion flow gives rise to a space charge in the vicinity of the boundary. This second mechanism for generating space charge is aptly referred to as the elec- trode effect. For the case of uniform volume ionization in laminar airflow with no aerosols and bounded on the bottom by a conducting surface, the ion-balance equa- tions can be solved together with Poisson's equation to give the solution shown in Figure 11.3. The ionization rate was taken to be 10 ion pairs cm-3 sec~i, and the solution illustrates that the effect of the electrode, in this idealized case, would extend to about 3 m (Hopper, 1967). In the turbulent atmosphere, the electrode effect extends to much higher altitudes and the space charge formed by these two mechanisms is dispersed by turbu- lent mixing, causing a convective flux of charge in addi- tion to the conduction current.

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54 4.0: 3.5 _ 3.0 _ an ~ 2.5 _ - 2.0 1.5 1.0 _ 0.5 O 0 20 5.0( ) Oj4 4.5 _ no x log (IONS PER M3) 0.8 1.2 1.6 2.0 2.4 1 1 l l l l WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT Cal integral scale comparable to the height above the . . surface. Both heating and evaporation from the surface I I _ tend to produce an unstable density gradient. The struc ture of the turbulence produced by buoyant instability can be quite different from that of shear-generated tur bulence because warm, moist parcels starting near the surface accelerate as they rise through an unstable envi ronment. The resulting eddies tend to be elongated ver tically and to have a size scale determined by the thick ness of the entire PBL. The turbulent kinetic energy injected into the flow by these two mechanisms is not dissipated at the scales / where it is produced. Instead, energy cascades down to n / I ever smaller eddy sizes and is dissipated primarily at t-F ~In+ scales smaller than a centimeter and results in a wide l range of eddy sizes. These eddies efficiently mix passive contaminants like ions, aerosols, space charge, and ra dioactive gases. The turbulent transport of space charge is equivalent to an electrical convection current through the atmosphere. It is evident from Figure 11.3 that tur bulent diffusion would disperse the space charge result I ing from the imbalance of positive and negative ions in the lower layers, producing a net upward flow of charge (opposing the downward conduction current). Electric field fluctuations are also caused by turbu lent movement of this space charge. To specify the aver age electric field, the instantaneous field must therefore be averaged over an interval of time longer than the pe riod of the largest eddy. Figure 11.4 shows an electric field profile over the ocean in the tradewinds off the coast of Barbados and illustrates the increase in vertical extent caused by turbulence. Each circle represents an average of 50 to 300 measurements of 10-see duration, and the bars are the standard deviations. The large devi ations illustrate the variability in the instantaneous field and the necessity of averaging to obtain a meaningful profile in the PBL. The variations are greatest where the gradient is steepest and are much less at a height of 160 m above the surface. The solid and dotted lines are the result of numerical modeling where x is a parameter re lated to the strength of turbulent mixing. It is obvious from this discussion that any treatment of atmospheric electricity in the PBL must include the effects of turbu lence. 40 60 80 100 120 -E (VOLTS PER M) FIGURE 11.3 Simple electrode effect in nonturbulent air with con- stant volume ionization rate of 10 ion pair cm~3 sec~i over a plain surface (from Hoppel, 1967~. Turbulent Transport of Electrical Properties Most aspects of the structure of the PBL are domi- nated by the effects of turbulence (Haugen, 1973; Wyn- gaard, 1980), and electrical processes are no exception. Turbulent mixing prevents the buildup of radioactive emanations in shallow layers near the ground except un- der very stable conditions, disperses aerosols over a greater depth increasing the columnar resistance, and redistributes space charge, producing convection cur- rents. The almost continual state of turbulent motion in the atmosphere is caused by the combined influences of drag, heating, and evaporation from the underlying surface. It is only in cases of extremely low wind speed and strong surface cooling that laminar flow may be found, and even then only for short periods and over limited areas. Drag generates turbulence through shear instability, which transfers kinetic energy from the mean flow to the turbulence. The energy goes into eddies on the scale of the mean velocity gradient, which is strongest near the surface, and tends to produce turbulence with a lo This variability with time and position at low alti- tudes demonstrates the danger inherent in balloon soundings used to obtain integrated ionospheric poten- tial. Neither spatial nor temporal averaging is possible during the rapid ascent and only a coarse vertical resolu- tion is available where fields are largest and most vari- able.

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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 180 _ 160 ~ , 140 `~: 120 ~100 L 80 ~ . I 60 40 _ O 11 1 ~1 1 0.3 0.40.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 NORMALI ZED ELECTRIC F I E LO - NUMERICAL SOLUTION ~ BARBADOS MEASUREMENTS - - --NUMERICAL SOLUTION WITH x=6 Z= 500 cm~3 X=6\\ \X=12 ' \\\' '' \~= ' FIGURE 11.4 Variation of the electric field with height (electrode effect) found over the tropical ocean illustrating the observed large fluctuations of the instantaneous field from the average field caused by atmospheric turbulence. The curves are numerical solutions for the governing equations using gradient diffusion model for ion transport (from Hoppel and Gathman, 1972~. PHENOMENOLOGY OF ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER Hi Although typical average values are often cited for atmospheric-electrical observables, the greatest interest and significance by far has been attached to variations with time over a broad range of time scales. There have ~ been studies of atmospheric-electrical variations with 2 annual and even 11-yr periodicities; short-period fluc tuations have also received some attention in the past, and they are currently a subject of renewed interest, as will be discussed later. By far the greatest effort has been focused on diurnal variations with respect to either uni versal or local time and, to a lesser extent, on seasonal changes in diurnal patterns. One of the first, and certainly the most famous, dem onstrations of a reproducible variation pattern with uni versal time is the hourly average potential gradient curves obtained over the world ocean on the Carnegie cruises. These curves have long served as he facto stan dards with which to evaluate the viability of attempts to measure universal variations. The result of the l928 1929 Carnegie cruise (Torreson et al., 1946) is plotted in Figure 11.5 along with several of the more successful subsequent attempts to measure this average variation. 155 These are integrated (and extrapolated) ionospheric po- tential from 125 balloon ascents (Muhleisen, 1977), 20 aircraft profiles in the Bahamas (Markson, 1977), and 6 aircraft profiles at a selected Arctic location (Clark, 1958~. Also shown are quasi-continuous measurements of current density made above the PBL during the Arc- tic winter, which, when allowance is made for the small (6 percent) measured change in columnar resistance during the measurement period, should accurately mir- ror variations in ionospheric potential (Anderson, 1967~. Consideration of these curves strongly indicates that there is indeed a well-defined average global diurnal variation but that there are equally important real local deviations with time. This is demonstrated in Figure 11.6 (Israel, 1973) in which the effect of turbulent pro- cesses on warmer (March to October) afternoons is seen to dominate the average diurnal curve. For this reason a better knowledge of ionospheric potential variations would play a key role in the identification of effects at- tributable to PBL processes. However, subtraction of - 2t o _ 1 N N A - IN Cal \ ~ ~ . en, O _.' o o O '/ _ >~- .; ,~ W.' ,.r ~, `` ,_, a - Current Density (Anderson) Potential Gradient (Carnegie) --- Total Potential (Muhleisen) - Total Potential (Markson) 0 0 Total Potential (Clark) 0 12 TIME - GMT . . o 400 J 200~ C) It ~2 FIGURE 11.5 Universal diurnal variations in atmospheric electric- ity from different measurement techniques.

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156 FIGURE 11.6 Average diurnal variations in potential gradient showing effect of after- noon turbulence mixing (from Israel, 1958~. % 120 278 V/m= 100 ~0 t20 2tO K/m = 100 80 120 166///m= 100 ~0 o the average diurnal variation does not necessarily isolate a particular local phenomenon. Electric field] and vertical current density are driven by the global circuit and, as shown in Figure 11.5, can exhibit the characteristic variation pattern even within the PBL in certain cases if suitably averaged. There are other observables such as the ionic conductivity, which are only weakly influenced by global processes while de- pencling strongly on local effects. Conductivity has been seen to be a result of ionization, ionic mobility, recombi- nation, and attachment of ions to particulates, all of 14.0 12.0 to x LL, 1 0.(3 cr LU ~ 8.0 WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT Pled am J an. Feb. ~ l hiarch _ Am, . _ _ l ~ 6 12 18 24 Local time % 120 ADO =270 ~0 Ah 120 15 7 K/m-100 ~0 120 t69~/m- 100 BE \ April 120 _ in 100 = 1861//m slay 7: 80 120 June / \ 120 80 100 = 151/m 726V//n= 100 a _ ~ , ~ , ~ , ~ Porsd am | July | _ ~ 6 12 10 Local time F'f . i. OWL 1 ~ l 51%0 ~100 - 154///tn V Sept. ~1 120 1 ~I -~^ a^^ .,, 1120 :100 = t90 Jam Nov. I .~\1 SO T Dec. | Em' 1 120 100 s276K/m ~0 ~4 lIr which can be influenced by local conditions and vari- able trace constituents. It is reasonable therefore to ex- - pect that large variations in conductivity will be primar- ily determined within the PBL and have a marked dependence on local time. The columnar resistance is a parameter usually defined as the resistance of a vertical column of unit cross section from ground level to the base of the ionosphere. It is commonly derived from a measured vertical profile of atmospheric conductivity, and it is observed that most of this resistance lies within the PBL. The validity of this is seen in Figure 11.7, ~ : A: 1 1 1 1 1 1 ~K ~: 4 8 12 16 20 24 4 8 EDT HOURS FIGURE 11.7 Average diurnal variation in columnar resistance as a function of local time (from Sagalyn and Faucher, 1956~.

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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER a 150 '= I30 J - 110 o J o 90 ~,/ it_ N UNPOLLUTED (2 STATIONS) - POLLUTED (3STATIONS) 501 l I t , , , , , , 0 2 4 6 8 10 12 14 16 18 20 22 24 GMT which shows the resistance R of a 1-cm2 column from the surface to 4.57 km over land as a function of local time. The strong effect of the midday upward dispersion of aerosols on R is readily apparent, and the shape of the curve and the 40 percent variation are in reasonable ac cord with our knowledge of the daily variation of turbu lent activity. The strong dependence of the columnar resistance on PBL conditions provides a mechanism for the modifica tion of electrical observables with local time. Such mechanisms can embrace the entire PBL as does fully developed turbulent convection, or they can be con fined to a shallow region near the surface. One example of a shallow effect is seen in the observed response to the typical urban morning rush hour (Anderson and Trent, 1969~. In the morning there is still little turbulent activ ity, since solar heating of the surface has just begun, and there is an abrupt injection of combustion products into the stratified atmosphere. Conductivity is reduced by particulates, but this reduction is confined to a shallow layer; so the total columnar resistance and thus the ver tical current are largely unaffected. Consequently, the local surface field increases as required by Ohm's law. This increase is seen in Figure 11.8 between 1100 and 1600 GMT (0600 and 1100 EST) at three sites located in urban areas. Because of these local diurnal variations, single-station potential gradient recordings, even when heavily averaged, rarely exhibit a classical Carnegie type diurnal variation pattern (Israel, 1961~. These diurnal variations with both universal and lo cal time are not the only fluctuations observed in atmo spheric-electricity recordings. Shorter-period variations are always observed and usually dismissed as noise. Ob served fluctuations on atmospheric-electricity record ings made within the PBL are comparable in magnitude to the mean values of those recordings (Takagi and To riyama, 1978~. A typical recording is seen in Figure 11.9. Much of the fluctuation content in the range from 157 FIGURE 11.8 Normalized potential gradi- ent showing rush-hour effect at polluted ur- ban sites (from Anderson and Trent, 1969). roughly 0.1 see to tens of minutes is produced by local turbulence. There are two obvious consequences of this coupling. First, all the methodology of turbulence analysis, such as eddy-correlation, profile, and dissipation analyses, can be applied to the electrical observations. The second consequence is the possibility of utilizing electrical ob- servations as a tool in the study of turbulent processes. The relationship between atmospheric electricity and turbulence will be considered in more detail in the sec- tion below on Modeling and Theory. There is also a sug- gestion that fluctuations in the total Maxwell current density in the range of 10 to 1000 see can correlate at intercontinental distances (Ruhnke et al., 1983~. It is clear that short-period fluctuations in atmospheric-elec- trical recordings strongly influence attempts to observe global scale phenomena, are relatively underexploited, and constitute a fertile area for possible applications. Although horizontal gradients of atmospheric-elec- trical variables within the PBL are much smaller than vertical ones (largely a consequence of the geometric scales involved), significant horizontal variability is ob- served. Significant instances include the effect of organ tL FIGURE 11.9 Records of the potential gradient (F), air-earth cur- rent density (1), and brightness (H) at Aachen on July 31, 1954 (from Israel, 1958).

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158 ized convection activity on a large scale as seen in Figure 11.10 (Markson, 1975), terrain effects produced by mountains and coastlines, and the sunrise effect caused by differences in turbulent mixing between the heated and dark regions. To a first approximation such effects are seen to be the result of imposition of a local perturba- tion on an otherwise uniform situation and are, hence, essentially comparable with local phenomena such as the rush-hour effect previously described. The spatial variation on which attention has been fo- cused is in the vertical dimension. The interest in global- scale phenomena has led to the use of a vertical profile as a convenient observational unit. Measurements are made of one or more atmospheric-electrical variables, typically field, conductivity, and/or current density, at a variety of altitudes in a relatively short time span (from a few to tens of minutes). The sensors are carried aloft with aircraft, balloons, or rockets, and data are presented both as profiles and as numerically integrated totals. Profiles have been made over land because of convenience and to study specific terrain effects and over water in attempts to eliminate land effects. Profile data have been responsible for the detection of convec- tion currents in the PBL comparable in magnitude with the total current, the classical electrode effect over wa- ter under stable conditions, the response of columnar resistance to pollutant buildup, and the classical diurnal variation in ionospheric potential. Typical vertical profiles of atmospheric potential through the PBL are seen in Figure 11.11. The Green- land profiles are characterized by extremely low levels of particulate contamination, and the vertical variation of conductivity closely approximates that predicted the- oretically for an aerosol-free atmosphere. The addition of particulate burdens, whether in a shallow layer as in curve C or in a thick layer as in D, markedly affects the observations within the PBL. It is apparent that, in the presence of atmospheric contaminants, the voltage drop across the PBL is significantly greater than in the Green- land observations. 70/ / ~ / / ~ / / ~\ i\ ~ o 0 10 20 30 40 50 60 DISTANCE (km) FIGURE 11.10 Horizontal variations in potential gradient showing effect of organized convection (from Markson, 1975). WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT 7.S O 100 200 300 400 ATMOSPHERIC POTENTIAL ( kVl A- GREENLAND 2200 GMT 30 AUG 1955 TYPE B-GREENLAND 1700 GMT 28 AUG 1955 o TYPE C 6 0 _ CALIFORNIA COAST . 1900 GMT 8 DEC 1955 x TYPE D VIRGINIA 1945 6MT -23 M" 1955 . 4.S _ E 30 - 1_ 1.5 o A // ~ // ~J / . _ FIGURE 11.11 Typical vertical distributions of atmospheric poten- tial (from Clark, 1958~. The conduction-current density, defined as the prod- uct of electric field and conductivity, can easily be com- puted from airborne measurements. Two typical pro- files of conduction-current density are shown in Figure 11.12. Above the PBL the current density is seen to be essentially constant with altitude. This is a direct result of the small space-charge density above the PBL and the greatly reduced turbulent mixing found there. This ver- tical constancy led to the aforementioned use of current- density measurements above the PBL to follow univer- sal variations. The increases seen at low altitudes were the first unambiguous evidence of the existence and sig- nificant magnitude of convective charge transport within the PBL, as discussed in the next section. In addition to variations that depend on time or height are variations that are associated with a specific phenomenon. The most well known such case is the at- mospheric-electric fog effect. It has been observed that the conductivity decreases markedly in fog and that the start of the decrease may precede the actual fog onset.

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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 7.5 1 1 1 1 ~ CALIFORNIA COAST j 2330 GMT ~ as_ ! AVE RAG E VA LUE I ABOVE E X C MANGE \! LAYER 1 1.85 t 0.19 . 1 . 1 1 1 1 | TOP OF EXCHANGE f LAYER ~ 1 VIRGINIA COAST 1530 GMT MAY 17, 1956 AVERAGE VALUE I ABOVE EXCHANGE LAYER 1.48 t 0.13 1 TOP O F EXCHANGE LAYER U . , . W~ 0 2 4 0 2 4 6 CONDUCTION CURRENT DENSITY MAGNITUDE (10~~2omp/m2) FIGURE 11.12 Two profiles of convection current density com- puted from aircraft measurements of field and conductivity (from Kraakevik, 1958~. Analogously, the increase in conductivity at the termi- nation of the fog event may also precede actual dissipa- tion. A typical data recording through a fog event is seen in Figure 11.13. This phenomenon has been reported by many observers (Dolezalek, 1963), but it has not as yet received an adequate physical analysis. Similarly, there are effects associated with surf and waterfalls wherein some charge separation is produced 400 z 300 '-ad E Z C ~ 200 ~ ~ - o ~ 1 00 O > _ 5 o 4 3 o ~ c) o 2 ~ o ~1 1 o ~ in 4 , an LOCAL TIME 159 by mechanical breakup of the water surfaces (Blan- chard, 1963~. For example, the positive space charge produced by breaking surf can lead to an appreciably larger electric field on shore than outside the surf zone during onshore winds. In contrast to the positive charge produced along ocean coasts, sufficient negative charge has been observed along the shore of a freshwater body (Lake Superior) during heavy surf to reverse the fair- weather electric field (Gathman and Hoppel, 1970~. Negative charge is also observed from waterfalls. In some cases there are also strong local effects associated with smoke plumes and with volcanic eruptions. Again there is a separation of charge, which then diffuses away from the source. MODELING AND THEORY The complicated dependencies of the local electrical variables on the ionization profile, aerosol concentra- tions, turbulent structure of the PBL, and temporal var- iations in the global electrical circuit make it dangerous to trust intuitive notions when interpreting measure- ments made in the PBL. Increased insight into the meaning of the observations is obtained by modeling various physical mechanisms mathematically. Some of the more important results of these theoretical efforts follow. The electric field tends to be nearly vertical in fair weather, and the meteorological structure of the PBL usually changes slowly in comparison to the electrical relaxation time. This has led naturally to the assumption of a quasi-steady, horizontally homogeneous mean state and to one-dimensional, time-independent models of the mean electrical structure. These assumptions imply that the conduction- and convection-current densities FIGURE 11.13 Atmospheric-electric fog effect-a typical example of successful fore- casts of both on set and dissipation (from Serbu and Trent, 1958~.

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160 are vertically directed and that their sum, the total cur- rent density, is height independent, leading to great simplifications from a modeling point of view. About this mean state, of course, are fluctuations caused by turbulent eddies. (A parallel may be drawn here to the prevalence of one-dimensional models of the mean me- teorological structure of the PBL, in which small-scale phenomena such as turbulence enter only in terms of their horizontally averaged effects.) The primary goal of electrical modeling of the bound- ary layer to date has been to understand the mean pro- files of the electrical variables resulting from currents driven by the global circuit. Thus, theoretical develop- ments have focused on the sources of charge within the PBL and the phenomena produced by the vertical tur- bulent transport of that charge. After an introduction to the modeling of turbulent mixing, we shall examine the electrode layer and the convection currents resulting from electrode-effect space charge. Turbulence Modeling The simplest and most pervasive mathematical de- scription of turbulent mixing is the gradient-diffusion model. Applied to the vertical transport of space-charge density by vertical velocity fluctuations w ', it states that the mean turbulent flux (or convection current) is pro- portional to the local mean gradient: he'd'= -K(z) aaP. (11.6) Here K(z) is the eddy-diffusion coefficient, usually taken to be proportional to height above the surface. Values at 1 m generally lie between 0.01 and 1.0 m2/sec. This model is a form of first-order closure of the con- servation equation for space charge. It is generally be- lieved to provide an acceptable description of mixing near the surface, where shear production predominates over buoyant production of turbulent kinetic energy. Gradient diffusion is often applied throughout the PBL, sometimes with a stability-dependent coefficient and a decrease at the top to represent an inversion, although it is known to provide poor results in the interior of an unstable mixed layer. In atmospheric electricity its use is best restricted to modeling of the electrode effect. There are many more sophisticated (and more com- plex) approaches to turbulent transport modeling. A popular one is second-order closure, in which conserva- tion equations are derived for evaluating the second mo- ments, such as w 'p ' and p '8 v (A is the virtual potential temperature), in terms of each other and of mean vari- ables like reply. Such models allow the mean charge flux WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT at a given height to depend on the dynamics of the entire PBL, not just on local properties. This capability is es- sential for the correct modeling of unstable mixed lay- ers, where local flux-gradient relationships are known to break down. Before leaving this discussion of turbulent transport modeling, some cautionary mention should be made of secondary flows. There are many circumstances when the largest scale of motion in the boundary layer is or- ganized into a nonrandom structure. The most familiar example is the stationary roll vortices, which sometimes form over the tropical ocean. Such organized motions can carry a large fraction of the vertical transports, and since they cannot be adequately represented by turbu- lence models of the types discussed above, these trans- ports must be described explicitly with a dynamical model of two or more dimensions. The Electrode Effect The electrode effect has already been defined, and its simplest manifestation has been described for laminar flow and uniform ionization in aerosol-free air (Figure 11.3~. In this case the electrode layer has a thickness of only a few meters, over which the electric-field magni- tude decreases by about a factor of 2. The importance of the phenomenon lies in its ability to separate substantial amounts of charge near the Earth's surface. In condi- tions of low turbulence this leads to high space-charge densities in shallow layers, which can produce high elec- trical noise levels as intermittent eddies move this charge around. In strongly unstable conditions, on the other hand, it provides a source of charge to be carried deep into the interior of the PBL by convection currents. The theory of the nonturbulent electrode effect is fully developed and has been verified over water and, at least qualitatively, over land. The simplest case, the so- called "classical" electrode effect, is illustrated schemat- ically in the first frame of Figure 11.14. The thickness of the layer is determined by the lifetime of the small ions and their drift velocity in the ambient electric field. Since aerosol particles act as recombination sites for small ions, they reduce the ion lifetimes and, hence, the thickness of the nonturbulent electrode layer, as illus- trated in the second frame of Figure 11.14. A shallow layer of enhanced ionization, which can arise from sur- face radioactivity or trapping of radioactive emanations from the soil, can cause a reversed electrode effect, as illustrated in the third frame of the figure. Here a stra- tum of negative space charge is developed owing to the sweeping of negative ions upward out of the highly io- nized layer, causing the electric-field magnitude to in

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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER ELECTRIC FIELD PROFILES IN THE ELECTRODE LAYER: EFFECT OF AEROSOL, TRAPPED RADIOACTIVITY, TURBULENCE - ~ 2 - E~ :e C) 3 1 o l \ . \ CLASSICAL AEROSOL / / .~ 1 1 RADIOAC TIVITY TURBULENC E crease with height. Unfortunately, these relatively sim- ple cases rarely occur in the real atmosphere because of pervasive turbulent mixing. The most obvious effect of turbulence on the elec- trode layer is to increase its thickness by mixing the space charge upward. The impact of nonuniform ionization is reduced as the turbulence intensity increases, both be- cause trapping of radioactive emanations is eliminated and because the thicker layer appears to be less sensitive to surface radioactivity. The presence of aerosol parti- cles thickens the layer further, in contrast to their effect in the nonturbulent case, by increasing the electrical re- laxation time. These processes may increase the height scale of the electrode layer so much as to make it virtu- ally undetectable with surface-based measurements, as illustrated in the fourth frame of Figure 11.14. For this reason turbulence blurs the distinction between the electrode effect proper and convective currents in the interior of the PBL. Turbulence can also cause signifi- cant loss of ions and space charge by diffusion to the surface. The theory of the turbulent electrode effect is not so fully developed as that of the nonturbulent case, owing primarily to the difficulty of parameterizing the lower boundary conditions at an aerodynamically rough sur- face (Willett, 1983~. Hoppel and Gathman (1972) ob- tained reasonable agreement between experimental ob- servations and a numerical model of the turbulent electrode layer in clean maritime air over the tropical ocean (see Figure 11.4) . At present, however, there is no satisfactorily verified model that applies over land. In view of the importance of the electrode effect as a charge source for convection currents throughout the PBL, the development and testing of such a model should be a high priority for future research. 161 FIGURE ~ I. 14 Heuristic profiles of electric field in the electrode layer illustrate the effects of various physical processes. From left to right are the "classical" electrode effect in clean, nonturbulent air with uniform ioniza- tion, as in Figure 11.3; the effect of aerosol attachment on the nonturbulent case; the ef- fect of a shallow layer of high ionization rate on the nonturbulent case; and the effect of strong turbulence. Convection Currents in the Planetary Boundary Layer The downward conduction-current density is often observed to vary with altitude in the PBL. Based on the reasonable assumption of a steady, horizontally homo- geneous, mean charge-density distribution, Kraakevik (1958) concluded that these deviations from vertical uniformity imply the existence of a height-dependent "convection-current density" such that the total current density is constant with altitude. He speculated that this convection current is produced by the upward turbulent transport of space charge produced near the surface. Convection currents can be modeled relatively easily in many circumstances using only the mean charge-con- servation equation _ - 6\ p - E ap = at co LIZ ,`8 (w'p') (11.7) and Poisson's equation. This assumes that the mean con- ductivity profile is not influenced by the electric field, which appears to be the case under conditions of strong turbulent mixing. The first term on the right-hand side of Eq. (11.7) represents local electrical relaxation due to the mean conductivity. The effect of mean conduction down the conductivity gradient causes "piling up" of space charge and is represented by the second term. The third term is usually negligible compared with one of the first two. The convergence of convection current is, of course, represented by the final term. Recent modeling of convection currents has shown that they only become important in unstable mixed lay

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162 ers, where the turbulent transport time across the entire PBL can be comparable with the electrical relaxation time. The controlling meteorological variables are the surface fluxes of momentum and buoyancy and the mean conductivity and thickness of the mixed layer. It appears that the convection of electrode-effect space charge can have a major impact on the electrical struc- ture of the boundary layer. It can even reduce the mag- nitude of the total downward current density locally on the order of 50 percent owing to a mechanically gener- ated electromotive force (EMF) of more than 100 kV in extreme cases. Although these theoretical predictions are not inconsistent with existing data, they remain to be tested thoroughly in the field. Effect of the Planetary Boundary Layer on the Fair-Weather Electrical Circuit Two boundary-layer processes can have a substantial impact on the fields and currents appearing throughout the entire atmospheric column from the Earth to the ionosphere. These are variations in the columnar resis- tance and convection currents. To appreciate the im- portance of these effects, consider a steady convection- current density JC(Z) below an inversion at height H when a steady ionospheric potential VOO is applied from above. Since the total current density JO must be inde- pendent of height, it is easy to show that VOO = - Jt;Roo + H Jc dz (11.8) o where Roo is the total columnar resistance. The second term on the right may be considered an EMF generated by boundary-layer convection. This steady-state analy- sis is valid as long as X, Jc, and VOO change slowly com- pared to the electrical relaxation time near the ground. If we assume VOO to be constant, Eq. (11.8) shows that the magnitude of J' is inversely proportional to Roo and decreases linearly as the boundary-layer EMF increases. An aerosol-related increase in columnar resistance of 40 percent can therefore produce a similar decrease in the total current density. A simultaneous 100-kV increase in the PBL EMF can cause a further 30 percent decrease, for a total reduction in JO of 52 percent. This makes J' alone a relatively poor indicator of global processes. NEEDED RESEARCH AND POTENTIAL APPLICATIONS Measurement of Global-Scale Phenomena As discussed in detail in other chapters of this volume, there are ample reasons for interest in global-scale atmo WILLIAM A. HOPPEL, R. V. ANDERSON, and JOHN C. WILLETT spheric-electrical phenomena. For example, valuable information about the distribution and temporal vari- ability of horizontal potential differences in the iono- sphere could be provided by monitoring the ionospheric potential simultaneously in different locations. Further- more, the widely accepted relationship between global thunderstorm activity and ionospheric potential has yet to be verified on any but the crudest statistical basis. From the present perspective, finally, a detailed knowl- edge of the forcing from the global circuit would be use- ful in evaluating the electrical response of the PBL. Unfortunately, the measurement of global-circuit pa- rameters is complicated by the action of boundary-layer processes. Although local PBL structure cannot appre- ciably affect the total current in the global circuit, or even the local ionospheric potential, it can cause a redis- tribution of that current and alter the vertical profile of electric field. Therefore, the proper interpretation of lo- cal measurements in terms of global parameters requires a thorough understanding of noise sources in the PBL. The electrostatic potential of the upper atmosphere with respect to Earth is the single parameter most indic- ative of the electrical state of the global circuit. Yet the temporal variability of this ionospheric potential is largely unknown outside of its average diurnal varia- tion. Methods of measuring the ionospheric potential (such as aircraft and balloon soundings) have for the most part systematically excluded the detection of any shorter-term variations. Fluctuations in electric-field and current-density measurements in the PBL with pe- riods shorter than a few hours are usually attributed en- tirely to local sources, primarily turbulence and pollu- tion. One way to separate local and global sources is to cor- relate measurements at widely separated stations or to make instantaneous measurements averaged over large horizontal areas. A preliminary attempt (Ruhnke et al., 1983) to detect global variations with periods of seconds to minutes in the total Maxwell current (conduction, convection, and displacement) measured simultane- ously in the United States and the Soviet Union revealed an apparent correlation that is difficult to attribute solely to chance. This approach deserves further atten- tion as a relatively simple prospective method of moni- toring short-period variations in the global circuit. If it can be demonstrated that short-period and day- to-day global variations do indeed exist, then not only is the source of these variations of importance but also the usual interpretation of local variations in terms of tur bulence must be re-evaluated. In light of the importance of the ionospheric potential as an indicator of the electri- cal state of the global circuit and the need to separate its variation from local fluctuations in the PBL, the iono

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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER spheric potential should be measured continuously and simultaneously at two or more locations for a period of days and with a time resolution of seconds, preferably in conjunction with observations of global thunderstorm activity and of upper-atmospheric disturbances. The di- rect measurement of potential in the lower atmosphere using a tethered balloon has been attempted (Willett and Rust, 1981; Holzworth et al., 1981~. Extension of these techniques to higher altitudes and faster time reso- lution should be encouraged. Potential Toolfor Study of PZanetary-Boundary Layer Turbulence We have seen how strongly the electrical structure of the PBL is influenced by turbulent mixing. Space charge is unique among natural scalar contaminants in having a lifetime (the electrical relaxation time) comparable in magnitude with the time scales of the largest eddies in the PBL. Charge density, used in conjunction with a conservative tracer like water vapor, therefore offers the possibility of useful information about the structure of these energy-containing motions, which the natural ra- dioactive tracers, thoron (54-see half-life) and radon (3.8-day half-life), cannot provide. If the sources of space charge and moisture are understood, comparison of their relative distribution through the boundary layer might be useful in determining the Lagrangian time scale of the transport process. Another important consequence of the finite lifetime of charge density is that the convergence of its turbulent flux can be deduced from its mean distribution and the convergence of conduction-current density under steady-state conditions. Since these functions depend only on the mean profiles of electric field and conduc- tivity, they are readily measured. Thus, convection- current density is one of the few turbulent fluxes the pro- file of which can be observed without recourse to the complex and technology-intensive eddy-correlation method. Because the lifetime of space charge is comparable with that of the largest eddies, it resists becoming well mixed in an unstable PBL, where conservative scalars tend to be uniformly distributed. This fact and the ease of measuring the turbulent transport have recently been exploited to obtain profiles of the eddy-diffusion coeffi- cient for space charge through the boundary layer from individual aircraft soundings (Markson et al., 1981~. To realize this potential, it will be necessary to de- velop a more thorough understanding of the sources and turbulent transport of electrical charge within the PBL. The most urgent need is for a field program to gather data on the dependence of these phenomena on the me 163 teorological structure of the boundary layer. Further theoretical modeling will then be required to integrate these data into a coherent understanding of the pro- cesses involved. Areas of particular ignorance at present are the ionization rate within the plant canopy and the disposition of space charge accumulating at an inversion because of the discontinuity of conductivity usually found there. Further research into these areas may even- tually lead to the use of atmospheric-electrical measure- ments to observe, perhaps remotely, the meteorological structure of the PBL. Ion Physics and Balance in the Planetary Boundary Layer Small atmospheric ions, existing by virtue of a bal- ance between ionization of the neutral gas and recombi- nation and attachment to aerosol particles, cause the conductivity of the air. Yet, many facets of the nature and behavior of these particles are still poorly under- stood. More research is needed in the area of ion physics, especially (1) identification of the terminal positive and negative species in the PBL, (2) determination of the de- pendence of ion chemistry on trace gases, (3) measure- ment of the attachment coefficients of ions to charged and uncharged aerosol particles of various sizes, and (4) evaluation of the resulting charge distribution on the aerosols. (1) and (2) show promise of becoming sensitive methods for detecting certain trace gases. Further iden- tification of exact ion chemistry is required by physiolo- gists before they can evaluate claims of physiological ef- fects of air ions (MEQB, 1982~. Values of ion-aerosol attachment coefficients as a function of particle radius and charge are necessary to determine accurately the loss of ions (or conductivity) as a function of aerosol load. Few measurements of abso- lute values of attachment coefficients have ever been at- tempted. Usually only ratios of attachment coefficients are measured and compared with theoretically pre- dicted values of the ratios. Better measurement of the absolute values of the coefficients are necessary to pre- dict ion loss and validate theory. The use of conductivity or columnar resistance as a pollution monitor depends on the inverse relationship between conductivity and aerosol burden. The conduc- tivity is sensitive to the aerosol concentration, and mea- surements spanning several decades have been used to evaluate changes in global particulate pollution (Cobb and Wells, 1970~. However, the quantitative reliability of these indicators of particulate burden should be more fully investigated. The use of conductivity measure- ments for deducing aerosol burden is complicated by their sensitivity to the ionization rate. This latter sensi

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164 tivity has even led to the suggestion that conductivity be used as a monitor of nuclear operations and accidents. The electrical state of the atmosphere depends criti- cally on the ionization profile. Over the ocean ioniza- tion is due only to cosmic rays, and oceanic measure- ment of the electrode effect are in satisfactory agreement with numerical solutions of the governing equations. Over land, the ionization profile is compli- cated owing to ionization from ground radioactivity and radioactive gases as discussed earlier. Simultaneous measurement of all contributions to the ionization pro- file has never been accomplished. It is imperative that future studies of atmospheric-electrical profiles in the PBL over land include such measurements. A theoretical explanation is needed for the atmo- spheric-electric fog effect. This is most likely to be found in the dependence of the conductivity on changes in the aerosol size distribution with changes in relative humid- ity. Measurements of all pertinent parameters are needed during a fog event to formulate a physical theory that adequately accounts for the observations. The pos- sible usefulness of the observed precursor phenomenon could then be evaluated. Finally it should be mentioned that there are some experimental techniques that do not yet exist in satisfac- tory form for proper studies of PBL electrical processes. Briefly, these include a continuous measurement of ionospheric potential with fine time resolution, ion- sampling techniques with spatial resolutions suitable for profile determinations in the lowest few centimeters of the PBL (and within the plant canopy), and an ade- quately instrumented platform for making accurate at- mospheric-electrical and micrometeorological profiles throughout the PBL. REFERENCES Anderson, R. V. (1967~. Measurement of worldwide diurnal atmo- spheric electricity variations, Mon. Weather Rev. 95, 899. Anderson, R. V., and E. M. Trent (1969~. 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