Total N and S deposition is calculated for a 5kmx5km grid square as the sum of wet, dry, cloud droplet and aerosol deposition with most of the deposition in the form of rain (wet deposition) or gases (dry deposition). Using the UK national measurement site concentration data a concentration map for the UK for each pollutant is derived - these are SO2, NO2, HNO3, NH3, SO4, NO3 and NH4.
Deposition is then the product of the concentration map and a process to deliver the pollutant from the atmosphere to the landscape. The wet deposition uses rainfall from the Met Office modified to account for the orographic enhancement of both rainfall volume and rain ion concentration. Dry deposition uses a modified big leaf model which basically estimates the transfer rates from the atmosphere to the canopy surface and then the uptake by various mechanisms within the plant canopy. Cloud droplet and aerosol deposition use a simpler version of the same mechanistic structure as dry deposition.
Where it is beneficial, eg for NO2 and NH3 concentrations, we use some extra model information from the emissions inventory to improve the spatial pattern of the concentration maps. The deposition is modelled to each land use separately so the differences between semi-natural (short vegetation) and woodland are related to the physics of the canopy structures and the biology of the plants in that typical land use.
There has been a an important methodological change in the modelling which has been applied to the data from 2004 onwards which means the new data are not comparable with previously published data for those years. Find out more.