Resistance model for dry deposition

It has proved useful to interpret the deposition process in terms 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 the quasi-laminar layer resistance rh, and the surface or canopy (the term canopy refers to the vegetation canopy) resistance r. The total resistance, r to deposition of a gaseous species is the sum of the three individual resistances and is, by definition, the inverse of the deposition velocity  

By solving the three independent equations for the three unknowns, Ci, Ct. C3, one obtains (19.2). 

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 upon contact so that the surface or canopy resistance r, = 0. In this case the vertical flux is 

Solving the two independent equations to eliminate C2 and C3 yields the resistance relation analogous to (19.2) for particle dry deposition. 

Therefore the deposition velocity of particles may be viewed in terms of electrical resistance as the reciprocal of three resistances in series (r , r/, and rc,ri,i ) and one in parallel (1 /i)J. The third resistance in series is a virtual resistance since it is a mathematical artifact of the equation manipulation and not an actual physical resistance. 

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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... 

Selecting accurate and representative estimates of 7d is a challenge because it bundles several competing processes. Figure 6.2 illustrates a conceptual resistance model for dry deposition of small and large particles. Small, low-density particles are mixed in the atmosphere like gases and do not fall under the influence of gravity. For these particles, C/d can be calculated as... 

FIGURE 19.7 Resistance schematic for dry deposition model of Wesely (1989). 

FIGURE 4-31 Dry deposition by absorption. Ca and Cs are chemical concentrations in bulk air and at the leaf surface, respectively. The chemical flux is determined in part by the thickness of the stagnant air boundary layer (shown for simplicity as having uniform thickness). This model is essentially the same as that applied to gas exchange between air and water for the case in which transport resistance is dominated by the air side (Section 2.3). 

A complicating factor in dry deposition measurements is the presence of sources of the depositing substance in the footprint of the measurement. Whereas the flux of S02 is nearly always unidirectional (downward) and the surface is a sink for S02, gases such as H2S, NH3, and NO may have surface sources. It may be possible in some cases to specify a surface emission rate. For N02, the situation appears to be even more complex than just adding a surface emission rate to the resistance model. As much as 50% of the N02 initially removed at the surface can reappear as NO as a result of surface emissions (Meyers and Baldocchi 1988). 

The limitation of the resistance model lies in its application only for sufficient homogeneous surfaces such as forests, lakes and grasslands. Therefore, in dispersion models dry deposition can be described by using partial areas or weighted partial areas within the grid. The aerodynamic resistance through the upper layer can be calculated using Eq. (4.312) and an approach for the turbulent diffusion coefficient (eddy diffusivity) = Ku,z (valid only for neutral conditions) ... 

Fig. 2 Process flow (a) Starting Material, (b)Deposit SisN (c)Deposit poly-silicon, (d) Deposit Al, (e) Resist coating, (f) Soft bake, (g) Exposure mask, (h) Develop resist (i) Poly-silicon RIE, (j) Alum etch and stripe resist, ion(k) Dry oxidation, (1) Poly-silicon nanogap pattern with pad Pt/Au fabrication( Electrical checking of the device can be performed on the fabricated pad)(Repeat step (a) to (j) for mask 2). Fig. 3 shows the circuit after serial impedance is measured, a simple resistor model is developed representing the substrate and polysilicon layer. The capacitor also found in series to describe the device with no liquid test...

The deposition velocity 1/ for vegetative surfaces is commonly estimated by an empirical modeling approach (often called inferential modeling), which uses meteorological data and information on the surface characteristics of the vegetation (Brook et al., 1997 Hicks et al., 1987). In the ADOM model, for example, the dry deposition velocity to vegetation is estimated as the inverse of the sum of three resistances aerodynamic, laminar-layer or diffusive, and canopy. The aerodynamic resistance is... 

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