Analysis of ion mobility and diffusion in atmospheric gaseous mixtures from Monte Carlo simulation and macroscopic laws

Abstract

The reduced mobility and diffusion coefficients of and O− are calculated with a Monte Carlo simulation for the gas mixtures N2–H2O (50%, 50%) and O2–N2 (80%N2, 20%O2), respectively, from measured and calculated elastic and inelastic cross sections. These mobility and longitudinal diffusion coefficients have been compared with the standard Blanc's law and with the common mean energy (CME) procedure. Good agreement between these three calculation methods was found for the mobility and diffusion of in the N2–H2O mixture at high reduced fields where inelastic processes are relatively uninfluential. However, a strong deviation between Blanc's law and both CME procedure and our Monte Carlo calculations for the reduced mobility and the diffusion coefficient of in this gas mixture N2–H2O was observed at low reduced fields, because inelastic processes are significant. On the contrary, for the case of the N2–O2 mixture, where inelastic processes are small over the reduced electric field range 1–8000 Td, the three calculation methods led to similar results. The elastic collision cross sections used were determined from a semi-classical JWKB approximation by using a rigid core potential model for both symmetric and asymmetric , O−/O2 and O−/N2 ion–neutral systems. Moreover, the inelastic cross sections were extended to low energies from appropriate approximations. These cross section sets were validated from the good agreement between our Monte Carlo calculated reduced mobilities in N2 and H2O, O− in O2 and N2 and either measured values for the systems and O−/O2 or physical properties of the systems and O−/N2.