Abstract. A simple model (2layer) was constructed that describes the exchange of the reactive gases NO, NO₂ and O₃ between forest and the atmosphere. The model uses standard equations to describe exchange processes and uptake of gases. It also takes into account reactions taking place in the trunk space between NO and O₃ and photolysis of NO₂. All equations are solved analytically leading to a scheme efficient enough to allow implementation in a large scale dispersion model such as the EMEP model.

The model is tested on two comprehensive datasets obtained in a coniferous forest and a deciduous forest. The model calculations of NO₂ and O₃ fluxes to the forest were compared with observations of these fluxes. Although the comparison is often not perfect some of the striking features of the observed fluxes i.e. upward fluxes of NO₂ were simulated quite well. The impact of chemical reactions between O₃, NO and NO₂ in the trunk space appear to have a significant effect on the deposition rate of O₂. This is especially true during the night and more so for forests emitting large amounts of NO.