where C is an empirical instrument parameter, which depends on the state of the instrument and the temperature and pressure of the experiment, the parameter can be considered to be constant in the same experiment. Xi and Di are the mole fraction and the mass discrimination factor for species i, respectively, 𝜎 𝑖 is the photoionization cross-sections of target species at photon energy E, and the 𝜑(𝐸) is the photon flux. By taking nitric oxide as the reference species, the expression for converting the signal into the photoionization cross-sections is as: