Conclusions

The results reported here support the conclusion of Hill et al. [1979, 1980] that NO_x formation by electrical discharges occurs via the Zel'dovich mechanism in the hot channel region as it cools by mixing with surrounding air. Therefore we would tend to support the calculation of Borucki and Chameides [1984] for the yield of NO_x molecules per unit energy for lightning discharges of (9±2)×10¹⁶ NO molecules per joule, which was based on a cooling rate for the gases in the hot channel of ca. 400 s^-1 measured by Picone et al. [1981] and their estimate that producing 1 cm³ of air at 3000 K required 1 joule of energy (equivalent to assuming that approximately 40% of the discharge energy was used to heat the gases in the hot channel to 3000 K).

The variation with spark gap, pressure and electrode size of the yield of NO_x per unit energy stored on the capacitor highlights the problems of loss of energy from the hot channel gases to the electrodes. This is a potential problem for all experimental determinations of P, but is exacerbated by using smaller spark gaps, therefore we would place more reliance on the measurements of P for the larger discharges of Chameides et al. [1977] (1 m gap, P = (8±4)×10¹⁶ molecules/J) and Levine et al. [1981] (ca. 12 cm gap, P = (5±2)×10¹⁶ molecules/J). Any future experimental work on the determination of P should directly address the problem of the fraction of electrical energy that actually heats the discharge gases to ca. 3000 K.

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