F.7 Dry deposition of inorganic nitrogen

Dry deposition of nitrogen includes the delivery of both ammonium and nitrate to the water column from atmospheric fallout. This fallout is typically thought of as being associated with dust, with the dry fallout dissolving on entry to the water column to become dissolved inorganic nitrogen. A single constant daily deposition rate of total nitrogen is specifiable, and the WQ Module splits this into separate ammonia and nitrate rates using the same fraction applied to the wet deposition total inorganic nitrogen concentration (Section F.6).

\[\begin{equation} R^{NH_4}_{atm-dry} = R^{TN}_{atm-dry} \times (1-f_{TN}^{NO_3}) \tag{F.17} \end{equation}\]

\[\begin{equation} R^{NO_3}_{atm-dry} = R^{TN}_{atm-dry} \times f_{TN}^{NO_3} \tag{F.18} \end{equation}\]

\(R^{TN}_{atm-dry}\) and \(f_{TN}^{NO_3}\) are the user specified rate of dry total inorganic nitrogen deposition, and the fraction that is nitrate, respectively. These individual rates \(R^{NH_4}_{atm-dry}\) and \(R^{NO_3}_{atm-dry}\) are multiplied by model timestep and the global model surface area to compute the dry mass flux of ammonium and nitrate (Equations (F.19) and (F.20), respectively). These masses are added to the uppermost model layer cells and corresponding changes to ambient concentrations computed.

\[\begin{equation} F_{atm-dry}^{NH_4} = R^{NH_4}_{atm-dry} \times \text{Timestep} \times \text{Global model area} \tag{F.19} \end{equation}\]

\[\begin{equation} F_{atm-dry}^{NO_3} = R^{NO_3}_{atm-dry} \times \text{Timestep} \times \text{Global model area} \tag{F.20} \end{equation}\]

This constant dry mass flux for each of ammonium and nitrate is applied globally.

The total atmospheric depositional flux of dissolved inorganic nitrogen (noting that dry atmospheric flux is assumed to dissolve on entry to the water column) is as per Equation (F.21).

\[\begin{equation} \href{AppDiags.html#WQDiagDINAtmFlx}{F_{atm-din\langle computed \rangle}^{N}} = F_{atm-wet}^{NH_4} + F_{atm-wet}^{NO_3} + F_{atm-dry}^{NH_4} + F_{atm-dry}^{NO_3} \tag{F.21} \end{equation}\]