Using the outcomes of nonlinear chemical reactions, describing the formation of photooxidants and of nitric acid/nitrate from hydrocarbons, oxides of nitrogen and carbon monoxide in a Lagrangian trajectory model, ‘linear’ reaction rates are derived for the reaction NO x( = NO + NO 2) → N(5) (= HNO 3 + nitrates), which are valid for different conditions. The reaction rate depends on the O 3 concentration and the photon flux during the day essentially. During the night the main factors are the O 3 concentration, the reaction rate between the NO 3- radical as well as the N 2O 5 molecule, and wet aerosols, and the local source strength of NO. The ‘linear’ rate constants during the day and during the night are given by regression equations between the constants and the specified variables. Some examples, describing real situations, are given.