User guide

The air chemistry model AirchemistryDNDC calculates deposition and throughfall of dry and wet N deposition

It considers:

washing off (throughfall) of nitrogen previously stored on foliage,
current dry and wet atmospheric N deposition,
allocation of washed‑off and newly deposited N to either foliage, soil, or surface water.

AirchemistryDNDC does not require further models but can run standalone.

Nitrogen throughfall of canopy N.

Existing NH4 and NO3 on foliage layers can be removed by throughfall water. We denote:

\( C_w \): amount of canopy water (snow + liquid)
\( T \): throughfall in the current time step
\( f_T \): fraction of canopy leaf nitrogen washed off by throughfall

If there is both canopy water and new throughfall, the fraction of leaf N that is washed off in the current step is given by

\[ f_T = \frac{T}{T + C_w} \]

This fraction \(f_T\) is then applied to the nitrogen content of each foliage layer to compute the amount of NH4 and NO3 that is transferred from the canopy to throughfall.

If either canopy water or throughfall is zero, all NH4 and NO3 currently stored on foliage layers are treated as throughfall and removed from the canopy in this step.

Nitrogen deposition over the canopy

Wet deposition is scaled by the current precipitation, while dry deposition is converted to the current time step using the model time resolution. For each nitrogen species X ∈ {NH4, NO3} we compute

\( D^{\text{wet}}_X \): wet deposition during the current time step
\( D^{\text{dry}}_X \): dry deposition during the current time step
\( D_X = D^{\text{wet}}_X + D^{\text{dry}}_X \): total deposition

If a canopy with non‑zero leaf area index is present, \(\mathrm{LAI} > 0\), the total deposition \(D_X\) is distributed over foliage layers in proportion to their layer‑wise LAI:

\(\mathrm{LAI}\): total stand leaf area index
\(\mathrm{LAI}_\ell\): leaf area index in canopy layer \(\ell\)
\(f_\ell = \mathrm{LAI}_\ell / \mathrm{LAI}\): fractional LAI of layer \(\ell\)

The amount of N deposited on layer \(\ell\) for species X is

\[ D_{X,\ell} = D_X \, f_\ell = D_X \, \frac{\mathrm{LAI}_\ell}{\mathrm{LAI}} \; . \]

If \(\mathrm{LAI} = 0\) (no foliage), the full deposition \(D_X\) bypasses the foliage and is added directly to the throughfall N budget, from where it is routed to the soil or surface water.

Allocation of throughfall N to soil or surface water

After deposition and canopy interactions, the nitrogen that reaches the ground as throughfall is allocated either to surface water/ice or to the soil:

\( T_{\mathrm{NH_4}} \): total NH4 throughfall flux reaching the surface
\( T_{\mathrm{NO_3}} \): total NO3 throughfall flux reaching the surface

If a surface water or snow layer is present, i.e. then \(T_{\mathrm{NH_4}}\) and \(T_{\mathrm{NO_3}}\) are added to the surface bulk layer (index 0):

\[ \text{NH4}^{\text{surf}}_0 \mathrel{+}= T_{\mathrm{NH_4}}, \qquad \text{NO3}^{\text{surf}}_0 \mathrel{+}= T_{\mathrm{NO_3}} \; . \]

Otherwise, if there is no surface water or ice, the throughfall nitrogen is added to the top soil layer (index 0):

\[ \text{NH4}^{\text{soil}}_0 \mathrel{+}= T_{\mathrm{NH_4}}, \qquad \text{NO3}^{\text{soil}}_0 \mathrel{+}= T_{\mathrm{NO_3}} \; . \]