Aeronomie systems such as the negagive ion chemistry in the D region are characterized by numerous reactions involving positive, negative, and neutral species. Classical solutions of such systems do not necessarily point put the role played by each parameter. When the chemical system is considered as a graph, is is possible to apply the signal flow graph technique, which allows a quantitative evaluation of the various paths and loops. A negative ion model is obtained for daytime conditions by using the signal flow graph technique, which is briefly described. This reference model is analyzed in terms of transmittances path gains, and loop gains. An arbitrary variation of atomic oxygen and nitric oxide is introduced at a height of 65 km in order to show the induced effects on the negative ions and on the electron concentrations. Variations of the radio &lgr; between negative ions and electrons are also presented.