Introduction
Nitrogen oxides or NOx (NO, NO2) play a key role in the photochemistry of the atmosphere and their removal by precipitation is a potential natural source of fixed nitrogen for the biosphere. Thus the rate of production of nitrogen oxides by natural and anthropogenic sources has been of great interest in recent years. It has been suggested that lightning is a major natural source of NOx, although there is considerable uncertainty over both the magnitude and mechanism of NOx production by the hot gases produced by a lightning discharge [Noxon, 1976; Tuck, 1976; Griffing, 1977; Chameides et al., 1977; Chameides, 1979; Hill, 1979; Hill et al., 1980; Dawson, 1980; Levine et al., 1981, 1984; Peyrous and Lapeyre, 1982; Drapcho et al. 1983; Borucki and Chameides, 1984; Franzblau and Popp, 1989; Liaw et al., 1990; Goldenbaum and Dickerson, 1993].
One approach to the calculation of the global production of NOx (NO + NO2) by lightning is to measure the global energy dissipation by this phenomenon and then multiply this by the amount of NOx generated per unit energy (P) by electrical discharges in controlled laboratory experiments [Chameides et al., 1977; Levine et al., 1981; Peyrous and Lapeyre, 1982]. Estimates of global NOx production of 3-30×1034 NOx molecules per year have been made using this technique [Borucki and Chameides, 1984]. The uncertainty is to an extent a result of the uncertainties in measuring P.
A potential problem with this method is that in terms of energy per unit length and total length of discharge, laboratory electrical discharges have tended to be smaller than atmospheric discharges. Atmospheric discharges can be over 104 m long, depositing 104-105 J m-1 [Dawson, 1980], while laboratory discharges range from 10-2 to 1 m with energy depositions of 101 to 105 J m-1 [Chameides et al., 1977; Levine et al., 1981; Peyrous and Lapeyre, 1982]. The theoretical analysis of Chameides et al. [1977, 1979] has attributed NOx formation by lightning to the heating of air in the vicinity of the discharge channel by the strong shock front generated by the discharge, with the NOx being formed via the Zel'dovich mechanism [Zel'dovich and Raizer, 1966]. A consequence of this analysis is that there should be a weak dependence of P on the energy per unit length of the discharge, justifying the use of laboratory determinations of P for atmospheric scale discharges. Chameides [1979] calculated that P would only increase by a factor of two, as the discharge energy per unit length increased from 1 to 104 J m-1.
However there has been some controversy recently as to whether NOx is formed in the shock front, with Hill et al. [1979, 1980] suggesting, on the basis of numerical simulations of lightning, that due to the long duration of the discharge, the resulting shock wave never travels sufficiently fast for the gas just behind the front to get hot enough to fix nitrogen. It was suggested that NOx formation was occurring as the gases in the hot channel slowly cool as they mix with surrounding air.
In the present work, the formation of nitrogen oxides by laboratory scale discharges has been examined, to try to help clarify the mechanism of formation by lightning, which is not as amenable to study. The velocity of the shock front generated by laboratory scale discharges has been examined experimentally. The close relationship that exists between shock front velocity and the temperature rise across the front allows the temperature generated by the shock waves to be determined.
The Zel'dovich mechanism assumes that as the hot gases cool, an equilibrium NO concentration for a high temperature is "frozen out", giving a significant NO yield. For this work, the NO freeze out concentration has been directly determined, by addition of NO to an N2:O2 mixtures in which a discharge is fired. It has been suggested recently [Goldenbaum and Dickerson, 1993] that NO is formed by the Zel'dovich reactions, but that the freeze out of NO is due to a rapid drop in density during the onset of the discharge, not a drop in temperature. This suggestion is discussed in the light of the results from the present study.
Lightning occurs in regions of the atmosphere that vary greatly in their water content and ambient pressure, so the effect of these variables on NO and NO2 formation by laboratory discharges was also examined, as was the effect of the trace gases CO2, N2O and CH4.