Oxidized nitrogen and ozone interaction with forests. II: Multi‐layer process‐oriented modelling results and a sensitivity study for Douglas fir

An existing process‐oriented multi‐layer canopy model is applied to data from an intensive NOx and O3 surface exchange experiment, and a sensitivity study is conducted. The canopy was a mature 22.5 m Douglas fir stand. Comparison of measured data and model results shows that the model represents the concentration and fluxes of ozone well above the canopy, with adequate accuracy for concentration and fluxes below the canopy. A similar pattern is demonstrated for NO2 concentration above and below a forest canopy, with the fluxes being calculated correctly to within at least an order of magnitude below the canopy, and more accurately above. The model can reproduce the processes leading to the observed NO2 emission from forest stands. The sensitivity study demonstrates the complex interdependence of oxidized nitrogen flux controlling variables within the canopy, with NO2 emission favoured by high NO soil emission and canopy resistance and by low global radiation, leaf area index and ambient NO2 concentrations. Realistic alterations of these variables can cause reversal of the NO2 flux, leading to an ‘ecosystem compensation point’ between 17 and 35 ppbv at night, and 5 to 10 ppbv during the day for the forest canopy investigated. This highlights our improved understanding of the controls on NO2 flux, explaining and quantifying some previously reported reversals in NO2 flux above forest canopies. The effect of soil NO emission on ozone flux is investigated. During the day, reaction with NO may account for only 10% of observed ozone deposition, however, at night, this figure can rise to around 50%. The effect of volatile organic compounds on forest ozone deposition was found not to be large. Copyright © 2004 Royal Meteorological Society.