The Concentration Based Estimated Deposition (CBED) methodology 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. Dry deposition includes deposition of gases (Sulphur Dioxide (SO2), Nitric Acid (HNO3), Nitrogen Dioxide (NO2) and Ammonia (NH3) and particulate matter (sulphate, nitrate, ammonium, calcium and magnesium) to vegetation. For critical load exceedance calculations, deposition values for moorland are applied to all non-woodland habitats, and deposition values for forest are applied to all woodland habitats.
The values for SO2 concentration are calculated from rural measurements of SO2 and uses an urban enhancement factor. For oxidised nitrogen dry deposition, nitric acid concentrations are calculated by interpolation of measurements from about 30 sites and NO2 concentrations are taken for the Pollution Concentration Mapping (PCM) model (Stedman et al., 2007). This latter data set includes a combination of interpolation of measurements from rural sites combined with modelling concentrations from point sources and line sources. Ammonia concentrations are taken from a combination of the FRAME atmospheric chemical transport model (Singles et al., 1998; Fournier et al., 2003; Dore et al., 2007) 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.
Wet deposition includes deposition from precipitation as well as direct deposition of cloud droplets to vegetation (known as ‘occult’ deposition) and is mapped for sulphate, ammonium, nitrate, calcium, magnesium, and acidity (hydrogen ion). The separation of anthropogenic (non-seasalt) and total components of sulphur and calcium is calculated using ion ratios relative to sodium in sea water. Mapping wet deposition includes an orographic enhancement factor for the concentration of precipitation in upland regions due to the seeder-feeder effect. The enhancement factor is taken from observations of the increase in ion concentrations with altitude observed at Great Dun Fell in the Northern Pennines (Fowler et al., 1988) and subsequently confirmed by measurements at Holme Moss in the southern Pennines (Dore et al., 2001 ; Beswick et al., 2003).
Significant inter-annual variations in deposition can occur due to the natural variability in annual weather patterns including precipitation which directly influences wet deposition. The general circulation of the air, temperature and precipitation also affect emissions, atmospheric chemistry and transport, so adjusting the influence of different pollution sources on the measured concentrations at the network sites. The CBED deposition data used to calculate the exceedance of critical loads are therefore averaged over a three year period. This has been demonstrated to be a suitable time period to smooth out some of the inter-annual variations in deposition.
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.
Beswick, K.M., Choularton, T.W., Inglis, D.W.F., Dore, A.J. & Fowler, D. 2003. Influences on long-term trends in ion concentration and deposition at Holme Moss. Atmospheric Environment, 37 (14), 1927-1940.
Dore, A.J., Choularton, T.W. & Inglis, D.W.F. 2001. Monitoring studies of precipitation and cap cloud chemistry at Holme Moss in the Southern Pennines. Water, Air and Soil Pollution: Focus 1; 381-390.
Dore, A. J.; Vieno, M.; Tang, Y. S.; Dragosits, U.; Dosio, A.; Weston, K. J.; Sutton, M. A.. (2007) Modelling the atmospheric transport and deposition of sulphur and nitrogen over the United Kingdom and assessment of the influence of SO2 emissions from international shipping. Atmospheric Environment, 41 (11). 2355-2367. doi:10.1016/j.atmosenv.2006.11.013
Fournier, N.; Pais V.A.; Sutton M.A.; Weston K.J.; Dragosits U.; Tang S.Y. and Aherne J. (2003) Parallelisation and application of a multi-layer atmospheric transport model to quantify dispersion and deposition of ammonia over the British Isles. Environmental Pollution, 116(1), 95-107.
Fowler, D., Cape, J.N., Leith, I.D., Choularton, T.W., Gay, M.J. & Jones, A. 1988. The influence of altitude on rainfall composition at Great Dun Fell. Atmospheric Environment, 22, 1355-1362.
RoTAP. 2012. Review of Transboundary Air Pollution: Acidifcation, Eutrophication, Ground Level Ozone and Heavy Metals in the UK. Contract Report to the Department for Environment, Food and Rural Affairs. Centre for Ecology and Hydrology. www.rotap.ceh.ac.uk
Singles, R., M.A. Sutton & K.J. Weston (1998) A multi-layer model to describe the atmospheric transport and deposition of ammonia in Great Britain. Atmos. Environ., 32, 393-399.
Smith, R.I., Fowler, D., Sutton, M.A., Flechard, C. & Coyle, M. 2000. Regional estimation of pollutant gas deposition in the UK: model description, sensitivity analyses and outputs. Atmospheric Environment, 34, 3757-3777.
Stedman, J.R., Kent, A.J., Grice, S., Bush, T.J. & Derwent, R.G. 2007. A consistent method for modelling PM10 and PM2.5 concentrations across the UK.