Global soil-biogenic NOx emissions and the role of canopy processes -: art. no. 4298

[1] Soils are an important though uncertain source of oxidized nitrogen (NOx)to the atmosphere. One of the main uncertainties in the source estimates is the role of the canopy interactions between NOx emissions, dry deposition, turbulence, and chemistry. Previous studies, in which only dry deposition has been considered, indicate a reduction of about 50% of the globally emitted NOx by soils. We have implemented a multilayer trace gas exchange model in a chemistry general circulation model to explicitly calculate the role of canopy interactions in regulating the effective NOx emissions to the atmosphere. Our new NOx emission algorithm interactively calculates a global soil emission flux of about 12 Tg N yr(-1). For a sensitivity analysis we have also included a fixed global soil NOx emissions inventory of about 21 Tg N yr(-1). It appears that the enhancement of NOx and O-3 concentrations in response to the soil emission flux is suppressed by the compensating effect of dry deposition. For sites that are exposed to relatively large emission fluxes, our multilayer and the previously used big leaf approach, which does not consider canopy interactions, calculate similar surface NOx fluxes. This confirms the validity of the big leaf approach for most polluted regions at midlatitudes. However, for relatively pristine sites in the subtropics and tropics, where NOx is a limiting factor in ozone and hydroxyl chemistry, there are distinct differences between the multilayer and big leaf NOx surface fluxes. This justifies the use of more comprehensive atmosphere-biosphere exchange descriptions in global models.