The impact of forest canopy structure on simulations of atmosphere-biosphere NO_X exchange

The concentrations and fluxes of reactive nitrogen species in the land-atmosphere system are controlled by complex interactions between emissions, turbulent transfer, dry deposition and chemical transformations. The forest canopy can significantly affect turbulent fluxes between the atmosphere, the canopy crown and the understory where most of the sinks or sources of mass and energy are located. Exchange processes depend on canopy homogeneity and isotropy, as well as morphological, aerodynamic and thermal characteristics. Therefore, it is anticipated that the forest canopy structure will play a significant role in the exchange of reactive nitrogen species. The goal of this study was to examine the impact of different forest structure types on the exchange of nitrogen oxides ([NOX] = nitric oxide [NO] + nitrogen dioxide [NO2]). Produced by soil bacteria in natural environments or transported from urban areas, NOX is major precursor for the production of tropospheric ozone (O3). However, in the presence of a forest canopy a fraction of the emitted NOX is removed by dry deposition process implying that the efficient atmosphere-biosphere exchange is substantially reduced compared to the soil NO emission flux for pristine sites. Polluted sites might even have an overall NOX forest deposition flux. Moreover because ozone is toxic to plants (and humans) it is important to understand the role of NOX sources and sinks inside the forest canopy in determining the actual O3 uptake by vegetation. In this study the Multi-Layer Canopy CHemistry Exchange Model (MLC-CHEM) is used. One of its applications is to study the exchange of reactive compounds and aerosols above and inside canopy air space for which the model is being constrained with observed micrometeorological and, preferentially observed SL concentrations. The model can also be applied to conduct more theoretical studies such as studies of the impact of the forest canopy structure on atmosphere-biosphere NOX exchange. Description of forest heterogeneity is introduced into the canopy parameterization using relationship between the leaf area index (LAI) and the leaf area density profile (LAD). As MLC-CHEM is a multi-layer model, the LAD values were calculated as an integral of LAD(z) over each layer. In this study four different three crown shapes and appropriate LAD profiles were distinguished as commonly used in environmental models. MLC-CHEM simulations for selected crown shapes were used to study the sensitivity of simulated turbulent fluxes of NO, NO2 and O3 to the LAD profile and forest efficiency in removing pollutants from the air.