The calculation of long term sulphur deposition in Europe

Abstract

A model based on statistical distributions of windspeed, wind direction and dispersion categories and of rainfall is used to calculate the annual wet and dry deposition of sulphur over Europe, using a detailed emission inventory. The model explicitly allows for vertical diffusion in the mixing layer and thereby the effect of source height. Wet and dry removal are taken into account as well as the conversion of sulphur dioxide to sulphate. In regions of Europe with heavy annual precipitation, such as southwest Norway, corrections to the deposition pattern are made. Other factors, such as the diurnal variation of mixing depth and large scale synoptic vertical motions, are investigated and are not thought to change the overall pattern of annual deposition. The calculated pattern compares favourably with measurements and calculations based on trajectory models. The results show that the highest total deposition rates occur over the major emission areas (10g m −2 y −1 SO 2)and decrease to low values (1 gm −2 y −1 SO 2) in outlying areas of the U.K., France, Italy and in Scandinavia. The wet deposition does not show such pronounced maxima close to emissions. The decrease in wet deposition is also modified in regions of heavy annual precipitation, such as southern Norway. A sulphur budget is constructed for a region of southern Norway. The calculated total deposition, which agrees fairly well with the measured total deposition, is analysed according to the sectors containing the main source areas. The contribution from each source sector is only estimated approximately (to within a factor of two) but shows the expected dependence on source strength and frequency of wind direction. The main uncertainty is a large contribution from the background concentration of sulphate in precipitation not directly attributable to man-made sources in Europe. The model calculation excludes the contribution from sources within about 100 km of a receptor site. The deposition at any sampling site is dependent on details of the source emissions within the neighbourhood of the site. However the average regional deposition from sources within the receptor region can be assessed. It is small for southern Norway because of the low emissions in the region. Source height strongly influences the local deposition, but on a larger scale is less important than other factors, such as the frequency of wind direction, rainfall and uncertainties in the emission inventory.