Dry Deposition Fluxes and Timescales of Turbulent Deposition over Tall Vegetative Canopies

Dry deposition of atmospheric aerosols is strongly influenced by turbulence particularly close to the ground where the roughness elements present on the surface considerably amplify the atmosphere surface exchange fluxes. Vegetation elements induce turbulence in the canopy flow which extends vertically above the canopy creating a roughness layer in which turbulent eddies are present. Aerosol particles entering this layer are brought rapidly in contact with vegetation elements increasing deposition. We present a new approach to modelling dry deposition to high-roughness vegetated surfaces. As an initial step, the domain of analysis is one-dimensional (1D), extending in the vertical direction from the ground up to five times the height of the canopy top. In this domain, a time dependent conservation law is solved by constructing several flux components. The turbulent flux generated by vegetation elements is constructed using multiple scales in accordance with the size of vegetation elements. The removal flux is computed as a fraction of the turbulent flux using perfect collection efficiency. The gravitational flux is then calculated based on particle size and turbulence intensity. Once all the fluxes are known the conservation law is advanced in time. Multiple runs are necessary for completing the concentration profile for a given forest canopy structure and wind intensity. Once the concentration profile is determined, deposition velocities are obtained. Extending this method to two space dimensions represents the next step which requires a more complete characterization of the canopy, to include the horizontal vegetation. Several other improvements are under consideration for instance; less than perfect capturing efficiency and particle rebound, inclusion of other fluxes resulting from particle dynamic or chemical processes.

Keywords Turbulent flux, roughness sub-layer, aerosol dry deposition, tall vegetation