National Academies Press: OpenBook

The Earth's Electrical Environment (1986)


Suggested Citation:"NO PRODUCTION BY LIGHTNING." National Research Council. 1986. The Earth's Electrical Environment. Washington, DC: The National Academies Press. doi: 10.17226/898.
Page 72

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THE ROLE OF LIGHTNING IN THE CHEMISTRY OF THE ATMOSPHERE 72 NO PRODUCTION BY LIGHTNING Similar to the Z'elovich mechanism for the fixation of nitrogen in explosions (Z'elovich and Raizer, 1966), the production of NO in lightning discharges is believed to be driven by high-temperature chemical reactions within a rapidly cooling parcel of air; the rapid cooling causes NO levels above its thermochemical abundance to be "frozen" into the gas. A simple physical analogy to this chemical production mechanism is that of dropping a bead through a column of rapidly cooling water in a gravitational field. Because the bead wants to minimize its potential energy with respect to the gravitational field, the bead will tend to fall to the bottom of the water column. If, however, the water were to cool so repidly that it froze before the bead reached the bottom of the column, the bead would be frozen in the column at a position of higher potential energy and would be prevented from reaching its energetically preferred position at the bottom of the column. In the case of NO production in lightning, the high temperatures in and surrounding the discharge channel result in a series of chemical reactions that both produce and destroy NO. NO production is initiated by the thermal dissociation of O2. The equilibrium NO concentration, , is the NO level at which NO-producing and NO-destroying reactions are in balance. As illustrated in Figure 6.2, is a strong function of temperature. As the temperature rises above 1000 K the dissociation of N2 and O2 causes an increase in the NO equilibrium level. At about 4000 K, peaks at a value approaching 10 percent. For higher temperature, N and O atoms become increasingly more stable relative to NO (See Figure 6.2) and decreases. Thus if NO were always to maintain thermochemical equilibrium, its concentration would reach a maximum when the temperature in and around the discharge tube was ~ 4000 K and would then decrease to a negligibly small value as the heated air cooled to ambient temperatures. However, similar to the equilibrium NO concentration, the time, τNO, required to establish thermochemical equilibrium for NO also varies with temperature. As illustrated in Figure 6.2, this time becomes increasingly longer as temperature decreases because the reactions acting to establish equilibrium become slower. (In this figure, τNO was calculated by summing the loss frequencies for NO due to Reactions 6.4-6.7.) Whereas only a few microseconds are required for NO to equilibrate at 4000 K, equilibrium requires milliseconds at 2500 K, a second at 2000 K, and approximately 103 years at 1000 K. Hence, as the air cools, a temperature is eventually reached at which the rates of reaction Figure 6.2 The NO equilibrium volume mixing ratio f 0 , represented by the solid curve, and the NO chemical lifetime τNO, represented by the dashed curve, as a function of temperatures in heated tropospheric air. (After Borucki and Chameides, 1984.)

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This latest addition to the Studies in Geophysics series explores in scientific detail the phenomenon of lightning, cloud, and thunderstorm electricity, and global and regional electrical processes. Consisting of 16 papers by outstanding experts in a number of fields, this volume compiles and reviews many recent advances in such research areas as meteorology, chemistry, electrical engineering, and physics and projects how new knowledge could be applied to benefit mankind.

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