Regional estimation of pollutant gas dry deposition in the UK: model description, sensitivity analyses and outputs

A “big-leaf” resistance analogy model for dry deposition of sulphur dioxide, nitrogen dioxide, ammonia and nitric acid is described with a stomatal compensation point included to allow bi-directional exchange of ammonia. The model derivation is constrained by measurement data and it is parameterized for UK conditions. Monthly average dry deposition estimates are provided at the 5 km×5 km spatial scale. The model uses data available nationally at the appropriate spatial and temporal scales, such as gas concentration, land use, wind speed, temperature, rainfall and vapour pressure. A method is presented to overcome the lack of suitable solar radiation data. The effect of uncertainty in model inputs and in model parameterization is explored using sensitivity analyses. SO2 deposition is sensitive to variation in gas concentration, wind speed and wet surface uptake parameters. NO2 deposition is sensitive to parameters and inputs regulating stomatal behaviour, including solar radiation and temperature, as well as to gas concentration. The use of monthly or annual average NO2 concentrations may underestimate deposition substantially in some areas. HNO3 dry deposition is sensitive to wind speed and concentration. NH3 dry deposition to moorland and forest land uses, where the majority of deposition occurs, is sensitive to concentration, wind speed and choice of canopy resistance parameters. For arable and grassland areas, with both deposition and emission of NH3, the model is sensitive to all the model inputs and parameter choices. A full uncertainty analysis requires further work on the reliability of input variables and model parameter choices but these results quantitatively focus on the important areas of the model for each gas. Estimated dry deposition to the UK (excluding Northern Ireland) of SO2 is 135 Gg S yr−1 for 1996, for NH3 is 97 Gg N yr−1, for NO2 is 26 Gg N yr−1 and the preliminary estimate for HNO3 is 42 Gg N yr−1. For sulphur and reduced nitrogen, estimated dry deposition accounts for 40% of total deposition, including wet and cloud droplet deposition. NO2 dry deposition only accounts for 15% of total oxidised nitrogen deposition, but another 25% may come from the dry deposition of HNO3, giving a similar 40% overall by dry deposition. The sensitivity of the model to parameter values and the comparisons of modelled output with measurements show that parameter choices may be valid only at the scale of European countries rather than the whole continent.