A study is made of the various reactions in which nitrogen oxides are involved in the chemosphere. The hydrogen compounds do not play an important role, and it is found that the essential aeronomic reactions depend on ozone and atomic oxygen. Thus, the ratio of nitrogen dioxide to nitric oxide can be determined. The absolute values of the NO2 and NO concentrations depend on the dissociation of molecular nitrogen in the chemosphere. The chemical conditions cannot be applied in the mesosphere, since the lifetime of NO is relatively long, and a downward transport is involved. Very special assumptions about chemical reactions would be necessary to reconcile the photochemical picture and the observational results. The introduction of ionic reactions, considered in an accompanying paper, will lead to a correct interpretation.