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THE ROLE OF LIGHTNING IN THE CHEMISTRY OF THE ATMOSPHERE 76 organic precursor that ultimately led to the chemical evolution of life on Earth (Miller and Urey, 1959). Both laboratory and theoretical calculations indicate that in a highly reducing atmosphere, rich in hydrocarbons, lightning could have produced HCN in copious quantities, possibly large enough to allow HCN levels in ponds and ocean water to build to levels large enough to trigger the formation of peptide chains and similar precursors to amino acids. On the other hand, the calculations of Chameides and Walker (1981) indicate that the HCN yield rapidly decreases as the atmosphere becomes less reducing. For an atmosphere where C is primarily in the form of CO, the HCN yield decreases by about 3 orders of magnitude from that of a hydrocarbon atmosphere, and it decreases by an additional 3 orders of magnitude for a CO2 atmosphere. Thus in order to better understand the role of lightning in the evolution of life, studies are needed to better determine the relative amounts of CH4, CO, and CO2 in the primitive atmosphere. THE POSSIBLE ROLE OF ATMOSPHERIC DISCHARGES IN CLOUD CHEMISTRY In recent years the growing concern over the possible deleterious effects of acidic precipitation on lakes, forest ecosystems, and crops has led to an increased interest in the chemistry of clouds, a region where acids can be efficiently generated and incorporated into rainwater. One aspect of cloud chemistry that has yet to be adequately studied is the role of atmospheric electrical phenomena in acid generation in electrified clouds. One possible effect of electrical discharges on cloud chemistry is briefly described below. Suppose, under the appropriate conditions, continuous, low-level positive point coronal discharges from droplets occurred in a cloud. These discharges would cause 1 electron to be deposited on the droplet and 1 positive ion (most often ) to be produced in the gas phase for each ion pair produced (Loeb, 1965). The electrons deposited on the droplet would be incorporated into the droplet and become hydrated electrons [i.e., (eÂ¯)aq]. In the presence of dissolved O2, these hydrated electrons would rapidly form via The species is related to the aqueous-phase HO2 radical by the acid-base equilibrium reaction The ions produced in the gas phase would lead to the eventual formation of an OH radical and a hydrated oxonium ion, H3O+ (Good et al., 1970). Heterogeneous scavenging of the H3O+ Â· nH2O ion and its incorporation into the droplets to form H+ would maintain the nominal charge neutrality of the droplets. A sizable fraction of the gas- phase OH radicals produced by the discharge would also be scavenged, either as OH or HO2 in the gas phase, and incorporated into the droplet representing an additional radical source to the aqueous phase. Calculations similar to those of Chameides (1984) indicate that about 1.5 aqueous-phase HO2 free radicals would be produced for each ion pair generated. The HO2 radicals thus produced in the aqueous phase would rapidly react to form dissolved H2O2 via reactions such as Since aqueous-phase H2O2 is believed to be, in many cases, the most important oxidant of dissolved SO2 in cloud droplets leading to the production of sulfuric acid (Martin, 1983), it is conceivable that this electrical process could play a significant role in the generation of acids in clouds. CONCLUSION The agreement between theoretical calculations and experimental determinations of chemical yields from discharges for a wide range of gaseous species and a wide range of atmospheres suggests that the basic chemical mechanism by which trace species are produced by lightning is fairly well understood. However, in order to infer global production rates from these chemical yields, accurate values for the rate at which energy is dissipated by lightning is needed. Because these dissipation rates are not well known (uncertainty factors of 10 for the Earth and much larger for other planets are estimated), the role of lightning in the global budgets of species such as NO remains uncertain. To reduce this uncertainty, studies are needed to characterize more accurately the energy and frequency of lightning strokes on a global scale. Another area where research is needed concerns coronal discharges and their role as local sources of trace species. Mechanisms exist, for instance, by which positive-point corona from cloud droplets cloud lead to enhanced generation of sulfuric acid in cloudwater. To determine if this and similar processes occur at a significant rate, the magnitude of low-level coronal currents in clouds needs to be more accurately established. References Bar Nun, A. (1975). Thunderstorms on Jupiter, Icarus 24 , 86-94 . Bar Nun, A. (1980). Production of nitrogen and carbon species by thunderstorms on Venus, Icarus 42 , 338-342 . Bar Nun, A., and A. Shaviv (1975). Dynamics of the chemical evolution of Earth's primitive atmosphere, Icarus 24 , 197-211 .
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