Abstract Title of Dissertation: EXPERIMENTAL STUDIES OF THE PRODUCTION OF NITROGEN OXIDES BY SIMULATED LIGHTNING SPARKS

Title of Dissertation: EXPERIMENTAL STUDIES OF THE PRODUCTION OF NITROGEN OXIDES BY SIMULATED LIGHTNING SPARKS Yujin Wang Doctor of Philosophy, 1998 Dissertation directed by: Alan DeSilva Professor Institute for Physical Science and Technology Department of Physics Lightning flashes have been simulated with arcs at ambient conditions. Each simulated flash consisted of a weakly ionized leader channel initialized by a 250 kV pulse about one half microsecond long from a Marx generator followed by a high current stroke from the discharge of a 1035 μF main capacitor bank through the preionized leader channel. The leader channel was initialized up to 20 cm under atmospheric pressure. The arcs were successfully created up to 4 cm under atmospheric pressure and longer sparks were available under lower pressure. The experimental arcs matched the cloud-ground (CG) strokes in current waveform, current amplitude and energy dissipation. The current waveform of arcs, primarily determined by an external resistor, agreed well with the overall of CG strokes on the global average. Arc current arrived at its peak from zero in about 30 μs and decayed thereafter with a RC decay time constant of about 350 μs. The current amplitude of the simulated lightning depended on the charge voltage on the main capacitor bank. The peak current increased from 5.0 kA to 30 kA as the main capacitor was charged from 3.0 kV to 10.0 kV, which covers about 50% of the global CG stroke in peak current. Experimental measurements have shown that the energy dissipation of the simulated lightning fell in the range of a typical CG lightning stroke. The energy deposited per unit length of spark increased from 3 to 8×10 J/m as the spark peak current changed from 3.0 kA to 10.0 kA discharging at atmospheric pressure. NOx production by the simulated lightning has been measured with chemiluminescent techniques. The results showed that the NOx yield of a lightning flash not only depends on its energy deposition but also strongly depends on the dynamic discharge process. The NO production rate per unit of energy increased with the energy dissipated, rather than remaining constant as previously assumed in most estimates of the global NOx production by lightning. The NO production per unit length of the simulated lightning strongly depended on the spark peak current, and it was proportional to the density of the initial air in the range that corresponds to the atmospheric density from the ground to a cloud. About 11% of the NO produced by a spark was oxidized into NO2 after a discharge by the atmospheric O3 and the O3 produced during the discharge. The global NOx production by lightning was estimated with the FlashExtrapolation-Approach (FEA) from our observation of the NO production by the simulated lightning in combination with the lighting frequency data observed in the United States. The NOx yield by a global "average" CG flash was estimated as 1.51× 10 NOx molecules, in which the multiplicity of strokes in a flash and the tortuosity of a flash channel were included. We calculated that the global NOx production by lightning as 2.84 to 11.4 TgN/year by taking the global lightning frequency as 30 to 100 flashes/second respectively. This result shows that the lightning may have been overestimated as a source of the global NOx emission into the atmosphere. EXPERIMENTAL STUDIES OF THE PRODUCTION OF NITROGEN OXIDES BY SIMULATED LIGHTNING SPARKS