Global soil‐biogenic NOx emissions and the role of canopy processes

Soils are an important though uncertain source of oxidized nitrogen (NO x ) to the atmosphere. One of the main uncertainties in the source estimates is the role of the canopy interactions between NO x 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 NO x 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 NO x emissions to the atmosphere. Our new NO x 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 NO x emissions inventory of about 21 Tg N yr −1 . It appears that the enhancement of NO x 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 NO x 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 NO x is a limiting factor in ozone and hydroxyl chemistry, there are distinct differences between the multilayer and big leaf NO x surface fluxes. This justifies the use of more comprehensive atmosphere‐biosphere exchange descriptions in global models.