Abstract — The gas phase reactions of NO-NO2and surface effects for oxygen atom recombination in a geometry typical of water-cooled stainless steel aerodynamic sampling probes are considered. The rate equations describing the system are summarized in a set of non-linear ordinary differential equations which are solved numerically. It is shown that the catalytic activity of the probe walls for atom recombination, the available surface to volume ratio, and the pressure are important parameters controlling the extent of NO2/NO reactions in the probe environment.