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

The deposition flux of O-3 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 O-3 were determined over the forest. The canopy resistance to O-3 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 O-3 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.