Estimating the exceedance of critical loads in Europe by considering localvariability in deposition

Estimates of the deposition of nitrogen and sulphate across Europe are avai lable from two sources: The mean deposition on each square of a 150 km squa re grid is estimated by an atmospheric dispersion model (ADM), and the actu al deposition is measured by a network of monitoring stations across the co ntinent. Critical loads are estimates of the deposition a natural or semi-n atural area can withstand without damage. These have been calculated for th e whole of Europe, resulting in a distribution of critical loads for each s quare on the grid. We estimate the area over which critical loads are excee ded in each square by three methods: (1) The true deposition is assumed to be equal to the ADM-predicted mean across each square. (2) The ADM-predicte d and monitored depositions are combined to obtain a kriged mean deposition , and this is assumed to be constant across each square. (3) The deposition field is modelled as a stochastic Gaussian field and its distribution calc ulated conditional on the measurements. The distribution (but not the depos ition) is assumed to be constant across each square. The area exceeded is t hen a stochastic variable, and we calculate its expectation and standard de viation. We find that the three methods give different estimates of the are a over which critical loads are exceeded. III some parts of Europe the diff erences are large. The third method should give the best estimate since it makes more reasonable assumptions about the variability of the deposition. We discuss potential improvements to our methodology and the need for bette r estimates of the local variability in depositions. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.