Sulfur deposition onto European forests: throughfall data and model estimates

The assessment of atmospheric sulfur deposition to forest is difficult because of its complex aerodynamic structure. Therefore, atmospheric deposition of sulfur to forest is often estimated by means of measuring throughfall fluxes onto the forest floor. In this paper, reported measurements of throughfall fluxes in European forests are analyzed. These fluxes are compared to deposition to bulk collectors located in nearby open land, to get an idea of the filtering efficiency of forests. In addition, fluxes are compared with deposition estimates from a long-range transport model of air pollutants, linked to an emission generation model. According to reported measurements from 52 European conifer stands, we found that the sulfur flux was 3.8 ± 2.3 times greater onto the forest floor than onto precipitation collectors. In a similar data set of 13 deciduous stands this ratio was 2.3 ± 0.9. The ratio of throughfall flux to model estimate was 1.8 ± 0.9 in coniferous stands and 0.9 ± 0.3 in deciduous stands. For sites that are located in moderately to highly sulfur polluted areas, it is assumed that throughfall fluxes give a good estimation of the atmospheric sulfur deposition. We conclude that (1) sulfur deposition to forests is 1.5 to 6 times higher than deposition to smooth receptor surfaces due to an efficient filtering by the forest canopy, (2) average annual sulfur deposition at a given location is 50—100% greater on conifers than on deciduous trees, (3) the existing European scale model that links sulfur deposition to the pollution generation processes is quite accurate as für as deciduous forests are concerned, and (4) the model underestimates deposition to coniferous forests.