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THE ROLE OF LIGHTNING IN THE CHEMISTRY OF THE ATMOSPHERE 70 6 The Role of Lightning in the Chemistry of the Atmosphere William L. Chameides Georgia Institute of Technology ABSTRACT The high temperatures in and around the discharge tube of a lightning stroke cause the dissociation of the major atmospheric constituents N2, O2, CO2, and H2 and the formation of trace species such as NO, CO, N2, OH, N, O, and H. As this cylinder of hot air cools, the levels of these trace species drop. However, if the cooling is sufficiently rapid the concentrations of these trace species can be "frozen-in" at levels significantly above their ambient, thermochemical equilibrium abundances, thereby leading to a net source of these gases to the background atmosphere. It is estimated that about 3 tg of N yr-1 as NO are produced in the present-day atmosphere by lightning through this process. Other gases produced by lightning are CO and N2O in the Earth's atmosphere; HCN in the Earth's prebiological atmosphere; CO, NO, and O2 in the cytherian atmosphere; and CO, N2, and a variety of hydrocarbons in the jovian atmosphere. The major uncertainty in quantifying the role of lightning in the chemistry of the terrestrial atmosphere, as well as that of other planetary atmospheres, arises from the lack of accurate statistics on the energy and frequency of lightning. The role of coronal discharges in the chemistry of clouds also needs to be investigated. INTRODUCTION In addition to the spectacular visual and aural effects that accompany a lightning flash, intense chemical reactions occur, which, on a relatively short time scale, can radically alter the chemical composition of the air in and around the discharge tube and, on longer time scales, can ultimately affect the composition of the atmosphere as a whole. The short-term chemical changes associated with lightning have been well documented by spectroscopic studies of lightning strokes (cf., Salanave, 1961; Uman, 1969). For instance, the strong emissions from neutral atomic nitrogen (N I ), singly ionized atomic nitrogen (N II ), neutral atomic oxygen (O I ), and singly ionized atomic oxygen (O II ) typically observed from the hot core of discharges, are indicative of the widespread dissociation of atmospheric N2 and O2 and the subsequent ionization of their atomic daughters. Other prominent spectroscopic features are the emission lines from CN and H, species arising from the dissociation of CO2 and H2O. For the most part the large changes in the chemical composition of the air in and around the discharge tube can be related to the rapid variations in temperature in
THE ROLE OF LIGHTNING IN THE CHEMISTRY OF THE ATMOSPHERE 71 this region. The lightning bolt and associated shock wave produce a cylinder of very hot air within which chemical reactions between the atmospheric gases proceed rapidly to bring the mixture into thermochemical equilibrium. Immediately after the energy deposition, the temperature in the discharge tube approaches 30,000 K and the gas is a completely ionized plasma. As the gas cools by hydrodynamic expansion and turbulent mixing, the equilibrium composition of the gas changes from a plasma to a mixture of neutral atoms such as N and O and then to a mixture of molecular species and ultimately as the temperatures return to ambient to a mixture of N2, O2, H2O, and CO2 much like the background composition of the atmosphere. This variation in the equilibrium composition of air as a function of temperature is illustrated in Figure 6.1; note that as the temperatures fall below 5000 K the equilibrium shifts from N, O, H, and CO to NO, OH, and CO and then to N2, O2, H2O, and CO2. If the gas around the lightning discharge was always to remain in thermochemical equilibrium, the net effect of lightning on the atmospheric composition would be negligible; once the temperatire of the gas returned to its ambient level, its composition would be essentially the same as that of the background atmosphere's, and thus there would be no net production or destruction of atmospheric chemical species by lightning discharges. On the other hand, it is well known that laboratory sparks can have significant effects on the composition of air; most notable is the fixation of atmospheric nitrogen (N2) by sparks to produce nitric oxide (NO). Given the basic equivalence between laboratory sparks and lightning discharges it would seem reasonable to expect that lightning also affects the composition of air. In fact, the knowledge that NO is produced by laboratory sparks led von Liebig to propose in 1827 that the typically observed in rainwater arises from the fixation of atmospheric N2 by lightning discharges. This nineteenth century hypothesis of von Liebig's has only recently been qualitatively confirmed by direct observations of enhanced levels of NO and NO2 in and around active thunderclouds (Noxon, 1976, 1978; Davis and Chameides, 1984) and in the vicinity of a cloud-to-ground lightning flash (Drapcho et al., 1983). The identification of the mechanisms responsible for the net production of trace species such as NO by lightning and the quantification of their source rates on a global scale define the current frontier in the field of the chemistry of atmospheric lightning and will, therefore, be the major subject of this review. The discussion begins by focusing on the production by lightning of atmospheric NO, a species of special interest because of its central role in the photochemistry of the atmosphere (cf., Crutzen, 1983). Following the discussion of NO production by lightning, a more general presentation will be given of the production of other trace species in both the present and the prebiological, terrestrial atmosphere, as well as in other planetary atmospheres. A brief discussion is then presented on the possible effect of electrical discharges on the chemistry of cloudwater and the generation of acids in precipitation. Finally, a brief outline of the needs for future work in this area is presented. Figure 6.1 Equilibrium volume mixing ratios of selected atmospheric species as a function of temperatures in heated tropospheric air.