The similarity theory proposed by Monin-Obukhov for the atmospheric surface layer is extended so that it can be used to study the flux-gradient relationships of chemically reactive species. A second-order model is developed in order to calculate the flux-gradient relationships and the (co-)variances of momentum, temperature, and concentration. The equations of the statistical quantities of the chemical species take chemical reactions into account. The closure constants are adjusted for a near-neutral surface layer using micrometeorological measurements. All model equations are made nondimensional and without chemistry can be solved as a function of a nondimensional parameter which accounts for the atmospheric stability z/L only. In the case of an irreversible second-order chemical reaction, three new dimensionless scaling parameters are introduced by the chemical processes: two Damk¿hler numbers (the ratio of the turbulent timescale to the chemical reaction timescale) and the ratio of the fluxes of the chemical species. Model calculations of the momentum and temperature (co-)variances are in good agreement with the observations over the whole range of the atmospheric stability. If the concentration of one chemical species (abundant) is much higher than the concentration of the other species (scarce), then one finds the largest deviations from the flux-gradent relationships and the other statistical quantities for the scarce chemical species with respect to the flux-gradient relationships and the statistical quantities of temperature.

The magnitude of these deviations depends on the Damk¿hler numbers and on the direction of the transport of the two chemical species, i.e., both deposited/emitted or one deposited and the other emitted. The abundant chemical species behaves as a passive scalar, i.e, temperature or moisture. Therefore in the case of scarce chemical species and Damk¿hler numbers close to unity (moderate chemistry), one cannot use the same flux-gradient relationship for temperature and for chemically reactive species. A similar behavior is found if one takes into account a backward chemical reaction (photodissociation) to the second-order chemical reaction. However, the deviations of the flux-gradient relationships of the scarce chemical species from the flux-gradient relationship of temperature are less severe if the photodissociation rate is increased. In summary, the model contains all the most important equations for solving the behavior of chemical species over the entire stability range of the atmospheric surface layer. Therefore it can be used to calculate the fluxes and (co-)variances equations of chemical species in atmospheric chemistry models. ¿ American Geophysical Union 1995