In APIS background data are calculated on an annual basis but provided as rolling 3-year means. The 3-year mean data are ecosystem-specific providing two sets of values: (i) assuming moorland/short vegetation everywhere; (ii) assuming forest everywhere. Additionally grid average are provided as 3-year rolling means.
For Nitrogen deposition, Acid deposition and Ammonia concentration background values are calculated using the Concentration Based Estimated Deposition (CBED) methodology. CBED generates 5x5 km resolution gridded data of wet and dry deposition of sulphur, oxidised and reduced nitrogen, and base cations from measured concentrations of gases and particulate matter in air and measured concentrations of ions in precipitation.
These data are collected at sites in the UK Eutrophying and Acidifying Pollutants (UKEAP) network. The site-based measurements are first interpolated to generate maps of concentrations for the UK. The ion concentrations in precipitation are combined with an annual precipitation map from the UK Meteorological Office to generate values of wet deposition. Gas and particulate matter concentration maps are combined with spatially distributed estimates of habitat-specific deposition velocities to generate dry deposition for 5 land cover categories: forest, moorland, grassland, arable and urban. The deposition to the 5 land cover categories are combined, depending on the relative proportions of different land cover categories in the 5x5 km grid square, to generate values for grid square averaged deposition.
Ammonia concentrations are taken from a combination of the FRAME atmospheric chemical transport model and the annual measured concentrations from the UKEAP network, where the former generates the local scale variability that cannot be derived from the network measurement data on their own.
Significant inter-annual variations in deposition can occur due to the natural variability in annual weather patterns including precipitation which directly
influences wet deposition. Therefore, CBED deposition data used to calculate the exceedance of critical loads are therefore averaged over a three year period.
The Pollution Climate Mapping model (PCM) is used to produce background maps, 1x1 km grids of pollutant concentrations for SO2 and NOx. The annual mean background concentration maps for SO2 and NOx have been calculated by summing the contributions from:
- Distant sources (characterised by the rural background concentration)
- Large point sources
- Small point sources
- Local area sources
The area source model has been calibrated using data from the national automatic monitoring networks. See the report for full methodology.