2.4 Emissions and deposition

  Metphomod does not include any code to simulate emissions in the current version. Emission values therefore have to be taken from an external emissions inventory and given as input data to the model. Dry deposition of trace gases at the surface were modeled using a simple resistor model with three resistors R_a , R_b , and R_c (Erisman et al.; 1994; Eugster and Hesterberg; 1996; Pahl; 1990; Wesely; 1989) where R_a is the aerodynamic resistance of the turbulent atmosphere between the center of the lowest grid cell and the local topographic surface, R_b is the laminar boundary-layer resistance of the surface roughness elements, and R_c is the canopy layer resistance. R_a was computed assuming a diabatic wind profile,

$${\frac{ln(z/z_0)-\Psi_H(z/L)}{k u_*^2}}$$ (32)

where u_* is the friction velocity derived from momentum flux ( u_* = $\sqrt{-\overline{u'w'}}$ ), z is the height of the surface layer grid point over local topographic surface, z₀ is the roughness length (input to Metphomod ), z/L is the Monin-Obukhov stability parameter (Monin and Obukhov; 1954), k is the von-Kármán constant (0.40 was used), and $\Psi_{H}^{}$(z/L) is the stability correction function. We used the parameterization by Businger et al. (1971) for $\Psi_{H}^{}$ ,

$$\Psi_{H}^{} \left\{ \begin{array} {lr} 2 \ln \frac{1+0.74\Phi_H^{-1}}{2}, & z/L \leq 0 \\ -6.35\frac{z}{L}, & z/L \gt 0 \end{array} \right.\;$$ (33)

To estimate the laminar boundary layer resistance R_b the model by Wesely and Hicks (1977) was used,

$${\textstyle\frac{1}{k u_*}} \cdot \left( \frac{\kappa}{D_C} \right)^ {\textstyle\frac{2}{3}}$$ (34)

where $\kappa$ is the thermal diffusivity of air, and D_C diffusivity of chemical species C .

For canopy layer resistance R_c the model by Arritt et al. (1988) that is based on Walcek et al. (1986) and Chang et al. (1987) was used. This is a parametric model which uses tabulated values for R_c,Max and R_c,Min , the maximum and minimum canopy resistances expected for each land-use or surface type,

 

$$\left\{ \begin{array} {lc} R_{c,Max} & Q_s = 0 \\ R_{c,Min} + (... ...}{3}}\right] & 0 < Q_s < 400 \\ R_{c,Min} & Q_s \geq 400 \end{array}\right.$$ (35)

Where short-wave incoming radiation Q_s (global radiation) is used as the only driving force to model stomatal openings. R_c is replaced with a constant tabulated value at night where Q_s = 0 W m ^{- 2} and where the stomata of the plants are expected to be completely closed and in wet conditions (e.g. lake surfaces). The tabulated resistances are valid for SO ₂ and converted for other chemical species by a constant factor. The tables can be found in Perego (1996) and Pahl (1990).