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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 163 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 direct 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 resolution should be encouraged. Potential Tool for Study of Planetary-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 radioactive tracers, thoron (54-sec 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 conductivity, they are readily measured. Thus, convection-current density is one of the few turbulent fluxes the profile 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 coefficient for space charge through the boundary layer from individual aircraft soundings (Markson et al., 1981). To realize this potential, it will be necessary to develop 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 meteorological structure of the boundary layer. Further theoretical modeling will then be required to integrate these data into a coherent understanding of the processes 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 eventually lead to the use of atmospheric-electrical measurements 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 balance between ionization of the neutral gas and recombination 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 understood. 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 dependence of ion chemistry on trace gases, (3) measurement 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 identification of exact ion chemistry is required by physiologists before they can evaluate claims of physiological effects 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 absolute values of attachment coefficients have ever been attempted. Usually only ratios of attachment coefficients are measured and compared with theoretically predicted values of the ratios. Better measurement of the absolute values of the coefficients are necessary to predict 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 conductivity is sensitive to the aerosol concentration, and measurements 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 measurements for deducing aerosol burden is complicated by their sensitivity to the ionization rate. This latter sensi