Exchange of ozone and nitrogen oxides between the atmosphere and coniferous forest

The deposition flux of O₃ to a Douglas fir forest in the Netherlands was monitored by eddy correlation during nine months. At the same time the concentration gradients of NO, NO₂ and O₃ were determined over the forest. The canopy resistance to O₃ uptake was calculated from the measurements and it compared well with model estimates. The sensitivity of the stomatal resistance to humidity calculated in the model was adapted to improve the comparison. A multi-layered model of canopy exchange which included the influence of chemical reactions between NO and O₃ and soil emissions was used to interpret the results for NO₂. The observed fluxes of NO₂ away from the surface into the atmosphere were probably caused by soil emissions of NO. The soil-emitted NO was converted to NO₂ in the trunk space and vented into the atmosphere. The model showed that the NO₂ flux above the canopy was either away or towards the canopy depending on the strength of the soil emission and the amount of NO₂ taken up in the canopy. A canopy compensation point for NO₂ could be established above which deposition was the main process and below which emission was observed. The model calculations supported the observations which indicated a compensation point of approximately 10 ppb NO₂.