Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

THE ROLE OF LIGHTNING IN THE CHEMISTRY OF THE ATMOSPHERE 73 become too slow to keep NO in equilibrium. Instead of falling to the thermochemical equilibrium concentration of the ambient temperature, a higher NO level becomes frozen into the gas. This higher concentration, which corresponds to the NO equilibrium level at the temperature at which the NO concentration departs from equilibrium, is called the "freeze-out" temperature. The freeze-out temperature of NO, TF , is approximately determined by the relationship where Ï T is the characteristic cooling time of the heated air. When T > T F , then ÏNO <ÏT and the chemical reactions are sufficiently rapid to keep NO in the thermochemical equilibrium. However, for T <T F, ÏNO> Ï T and chemical reactions are too slow to adjust to the rapidly decreasing T; NO, therefore, freezes out with a mixing ratio (T F ). Although a lower abundance of NO is favored thermodynamically at low T, the kinetics are too slow for readjustment. P, the net yield of NO produced by this process, is then approximated by where M is the number of molecules per meter heated to, or above, TF in the region where NO is being produced for a discharge energy of E 0 (in units of J/m). Thus it is necessary to determine values for T F and M that, when combined with the results of Figure 6.2, will allow an estimate of P(NO) from Eq. (6.2). Once P(NO) is obtained, the global rate of NO production by lightning, Ï(NO), can be estimated from in units of teragrams (tg) (i.e., 10â12g or 106 metric tons) of N per year, where R is the number of joules dissipated globally by lightning per second. Because of the current interest in developing global budgets for the flow of fixed nitrogen and nitrogen oxides through the atmosphere, reasonably accurate estimates for Ï(NO) are desirable. A brief discussion of how the parameters needed to solve Ï(NO) are calculated is presented below. Estimate of P (NO) Following the approach of Borucki and Chameides (1984), we infer values for T F and M that are needed to calculate P(NO) in Eq. (6.2) from the laboratory study of linear discharge channels by Picone et al. (1981). This study indicated that lightning-like discharge channels cooled with a Ï T of about 2.5 Ã 10â3 sec. For this choice of ÏT , TF and (Tf ) can be estimated from Figure 6.2 to be about 2660 K and 0.029, respectively. Furthermore, using the result of Picone et al. (1981) that in spark discharges 1 J of energy is required to heat each 1 cm3 of air to a temperature of 3000 K and assuming that the gas cools from 3000 K to the freeze-out temperature of 2660 K by mixing with the ambient atmosphere, it can be inferred that Substituting the above values for (TF ) and M(TF )/E0 into Eq. (6.2), A comparison of the above-estimated NO yield with those of previous investigators is presented in Table 6.1 and indicates a rather good agreement with a wide variety of theoretical calculations, laboratory spark experiments, and atmospheric measurements. The largest discrepancy appears to be with the NO yield attributed to Drapcho et al. (1983). The yield of Drapcho et al. was based on their observation of a sudden increase in NO and NO2 levels in the vicinity of a cloud-to-ground discharge; given the many assumptions necessary to infer a yield from this observation the disparity between the yield of Drapcho et al. and the others in Table 6.1 is not very surprising. The Global Dissipation Rate, R The rate at which energy is dissipated by lightning globally can be expressed as a function of two other parameters, i.e., where E F is the average number of joules dissipated per lightning flash and F is the number of lightning flashes occurring globally per second. Borucki and Chameides (1984) recently examined the existing data base on lightning flashes to estimate these parameters. Combining optical and electrical measurements of the energy of a single stroke, observations of the number of strokes per flash, as well as measurements of the distribution of energy among the first and subsequent strokes, E F was estimated to be about 4 Ã 108 J/flash with a factor of 2.5 uncertainty. From satelliteborne optical detection sys