The physical mechanism of nitric oxide formation in simulated lightning

We report an experimental assessment of the contributions of the shockwave and the hot channel to the production of nitric oxide by simulated lightning. Lightning in the laboratory was simulated by a hot plasma generated with a pulsed Nd-YAG laser. The temporal evolution of electric breakdown in air at atmospheric pressure was studied from the nanosecond to the millisecond time scale by shadowgraphy and interferometry techniques. The shockwave front velocity was determined to be about 60 km s(-1) at 20 ns and the temperature behind the shock front was estimated to be about 10(5) K. The production yield of nitric oxide by shock heating is estimated to be: P(NO) (3 +/- 2) x 10(14) molecule J(-1). In contrast it was calculated that the production yield of NO by the hot channel is as much as P(NO) = (1.5 +/- 0.5) X 10(17) molecule J(-1). To the extent our simulation is an accurate representation of natural lightning, the hot channel is the dominant region for nitrogen fixation.