Temporal and radial evolution of nitrogen and oxygen atom number densities and formation of nitric oxide in lightning return stroke channel

Lightning is a major natural source of NOx in nature. It plays an important role in the global nitrogen cycle, atmospheric chemistry, and climate change. Based on time-resolved lightning spectra, by using plasma diagnostic techniques and the law of heat conduction, the temporal and radial evolution of lightning channel temperature is obtained. Combining the dissociation equilibrium of molecules and the ionization equilibrium of atoms and ions with mass conservation and charge conservation equations, the temporal and radial evolution of the number densities for particles are investigated. On this basis, the production and distribution of NO during a single return stroke process of lightning are estimated. The results show that NO is mainly produced during the decay stage after the return stroke current. The NO number density varies nonlinearly along the radial direction, increases to a peak, and then decreases sharply. The peak NO number density occurs in the outer corona channel. For the return strokes studied, it is in the radial range of about 3–8 cm. During the cooling process of the channel, the radius corresponding to the peak NO number density gradually shrinks. The total NO production in the 0–500 μs of a single return stroke ranges from 150 to 286 mol. The NO production per meter of the return stroke channel ranges from 0.034 to 0.085 mol. The amount of NO produced is positively correlated with discharge intensity, number of return strokes, and discharge duration. This work provides reference data and a feasible approach for studying NO production and its microscopic mechanisms during lightning processes.