Numerical solutions of a typical boundary-layer seeding problem at low pressure are presented to demonstrate the important roles played by the interaction of heterogeneous fluid flow and chemical processes. A physicochemical model is developed for the investigation of the coupling of heterogeneous vaporization kinetics in a rarefied atmosphere, with the distribution of ionic and neutral species and homogeneous gas phase chemistry. It is found that the seeding effectiveness, as indicated by the evaporation efficiency and the degree of ionization for a given injection mass flow rate, is essentially controlled by the initial droplet size, the velocity and the direction of injection, kinetics of vaporization, and chemical processes.