Investigation of the mechanism of NO x formation in atmospheric pressure pulsed microwave air plasma

Abstract Electrodeless microwave plasma is produced in nitrogen-oxygen mixtures at atmospheric pressure to investigate the formation of nitrogen oxides( NO x ) through systematic variations of N 2 / O 2 ratio, enabling us to disentangle reaction regimes and uncover how air plasma behaves intrinsically. Optical emission spectroscopy demonstrates that O 2 quenches N 2 ( C ) at low oxygen content while O atom recombination forms O 2 ( B 3 Σ u − ) that emits O 2 Schumann–Runge radiation at higher oxygen content, explaining the disappearance of N 2 ( C − B ) and emergence of strong O 2 * emission under air condition. NO and O atoms are extensively converted to excited NO 2 *, which relaxes to ground-state NO 2 , emitting spectra from 400 nm to 700 nm. Laser-induced fluorescence (LIF) reveals complete N atom depletion at the waveguide exit under air condition. O and NO signals increase sharply at low oxygen content and approach saturation at higher levels, with NO exhibiting a hollow radial distribution complementary to O. Fourier-transform infrared spectroscopy of steady-state products shows higher integrated NO content at elevated duty cycles despite lower LIF intensities, attributed to localized high temperature suppression of LIF signals with compensatory NO accumulation in lower temperature zones. Thermodynamic equilibrium simulations confirm temperature as the decisive factor governing NO, N, and O distributions, reveal substantial N atom presence near the discharge core (undetected at the exit), and demonstrate hollow O atom distributions in this region, which is different from that at the exit. Although NO density is lower near the core at higher duty cycles, broader temperature distributions and abundant particles ultimately yield greater NO production. These findings identify the physicochemical origins of observed air plasma characteristics, while simulations make a good agreement with experimental results and elucidate internal distribution of species that cannot be observed out of waveguide, providing fundamental insights for optimization of