Quasi-laminar layer resistance

It has proved useful to interpret the deposition process in terras of an electrical resistance analogy, in which the transport of material to the surface is assumed to be governed by three resistances in series the aerodynamic resistance ra, the quasi-laminar layer resistance rt, and the surface or canopy (the term canopy refers to the vegetation canopy) resistance rc. The total resistance, rt, to deposition of a gaseous species is the sum of the three individual resistances and is, by definition, the inverse of the deposition velocity ... 

The effects of dry deposition are included as a flux boundary condition in the vertical diffusion equation. Dry deposition velocities are calculated from a big leaf multiple resistance model (Wesely 1989 Zhang et al. 2002) with aerodynamic, quasi-laminar layer, and surface resistances acting in series. The process assumes 15 land-use types and takes snow cover into account. 

The resistance model for dry deposition postulates that adjacent to the surface exists a quasi-laminar layer, across which the resistance to transfer depends on molecular... 

A number of pathways are available from the quasi-laminar layer to the vegetation canopy or ground, including uptake by plant tissue inside leaf pores (stomata), the waxy skin of some leaves (cuticle), and mesophyll of leaves, deposition to the soil, and reactions with wetted surfaces. Each of these pathways can be represented by an appropriate resistance to transport the total canopy resistance rc is then determined by summing these resistances in parallel... 

Equation (19.2) is derived as follows. By reference to Figure 19.1, let C3 be the concentration at the top of the surface layer (the reference height referred to in (19.1)) C2 that at the top of the quasi-laminar sublayer C at the bottom of the quasi-laminar sublayer, and C() = 0 in the surface itself. The difference between C and Co accounts for the resistance offered by the surface itself. At steady state the overall flux is related to the concentration differences and resistances across the layers by... 

Each step contributes to Vd or the dry deposition resistance r = 1/Vd- According to the three layers, the total resistance r is given from the partial resistances (Fig. 4.22), the aerodynamic r, the quasi-laminar r and the surface resistance r ... 

The aerodynamic resistance grows proportionally with wind speed at the reference height and decreases with increasing roughness of the surface. The quasi-laminar resistance r is based on the idea that close to the surface exists a molecular diffusion sub-layer where the transport only depends on the diffusivity of the molecule and the surface characteristics of the surface but not on atmospheric parameters (such as wind speed). The flux under steady-state conditions is parameterized by B dimensionless transfer coefficient) ... 

The quasilaminar sublayer resistance / b describes the excess resistance for the transfer of matter from the atmosphere to the surfaces of the vegetation, that is, the difference between the resistance for matter and the resistance for momentum. It is primarily associated with molecular diffusion through quasi laminar boundary layers. Several parameterizations for Rb have been developed, but that employed by Brook et al. (1999), which like Equations 7.3 and 7.6 is valid for conditions of neutral atmospheric stability, is particularly easy to apply ... 

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