Canopy 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 ... 

For particles, the model is identical to that for gases except that particle settling operates in parallel with the three resistances in series. The sedimentation flux is equal to the particle settling velocity, vs, multiplied by the particle concentration. It is usually assumed that particles adhere to the surface on contact so that the surface or canopy resistance rc = 0. In this case the vertical flux is... 

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... 

Wesely (1989) recommends an alternate surface resistance equation when the ground surface is wet from rain or dew. For surfaces covered with dew, the upper-canopy resistances for S02 and 03 are calculated from... 

When it is raining, the S02 and ozone upper-canopy resistance is calculated from... 

The canopy resistance is usually the most difficult of the three flux resistances to evaluate theoretically. Studies of these pathways constitute a major area of dry deposition... 

A three resistance model has been developed to model the deposition of gases to vegetation canopies. These three resistances, which are aligned in series, describe the turbulent transfer from the atmospheric boundary layer into the canopy (the bulk aerodynamic resistance Ra), the transfer from the air in the canopy to the surface of the vegetation (the quasilaminar sublayer resistance Rb), and the transfer from the surface of the vegetation to reservoir or sink for the chemical in the vegetation (the canopy resistance Ra). These resistances are related to the mass transfer coefficient kg according to... 

The canopy resistance Rc depends on the location of the reservoirs/sinks for the chemical in the vegetation, as well as on the properties of the chemical. Chemicals... 

Particle impaction/interception may also be described with the three resistance model. The aerodynamic resistance (i a) is the same as for gases or Brownian diffusion. The quasilaminar sublayer resistance (i b) and canopy resistance (Ro) are, however, complex functions of both aerosol size and the properties of the canopy surfaces. Since particle size-specific information is typically not available for organic contaminants, it is expedient to combine Rb and Ro for both Brownian diffusion and impaction/interception into a single resistance Rd%. 

Comparing the results. Method B yields a value for kg that is 3.5 times higher than the value from Method A. Possible explanations include an underestimation by Method A, for instance because the measured data were based on net, not gross, accumulation in the grass, or an overestimation by Method B, for instance due to the assumption of neutral atmospheric stability, or because the canopy resistance Ro was not negligible in this case. 

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