An analysis of measurements and modelling of air–surface exchange of NO–NO2–O3 over grass

This study lends support to the view that a dry deposition model for NO, NO2 and O3 should include the appropriate chemical reactions in the surface layer concentration equation. Here, we attempt to include this mechanism in a model that is based on a K-theory (a rough approximation that has been criticized in the literature), structured to be used within regional air quality models. The model accepts input of routine meteorological measurements and is evaluated using numerical sensitivity tests and flux measurements that were collected over grass in Northern Colorado and are available for about 20 summer days. A comparison of this model’s estimates of fluxes with observations shows that the impact on the NO and NO2 fluxes is strong when both the chemical reactions and the NO emissions are included at some low height in the surface layer and this impact increases with height. For O3 fluxes at the low height of 6 m this impact is not as large, primarily due to the existence of large O3 fluxes compared to NO+NO2. Numerical sensitivity tests show that it is necessary to include chemical reactions as well as NO emission for modelling the NO, NO2 and O3 dry deposition fluxes, particularly for NO and NO2. Under some circumstances when the O3 concentration is much larger than that of NO+NO2 and at a low reference height, the Big-Leaf model based on the constant flux assumption outperforms the K-theory model for O3 dry deposition but this is not the case for NO and NO2 where reasonably accurate NO emissions need to be made available. This conclusion is based on the limited observations of the present study and may not apply to chemically reaction models using higher-order closures.