Atmospheric deposition in European forests in 2019

Studying the effects of atmospheric pollution on forest ecosystems requires an evaluation of air quality and of the amount of pollutants carried to the forests by atmospheric deposition. Pollutant flux towards ecosystems through deposition mainly follows two pathways: wet deposition of compounds dissolved in rain, snow, sleet or similar, and dry deposition of particulate matter through gravity or adsorption on forest canopy for example. Pollutant deposition shows a relatively high local variability, related to the distribution of pollutant sources and the local topography, and in-situ measurement is needed to obtain accurate evaluations and to validate model estimates. In 2019, the chemical composition of atmospheric deposition under the tree canopy was measured in 290 ICP Forests Level II permanent plots throughout Europe. In this report, we focus on acidifying, buffering, and eutrophying compounds in canopy throughfall deposition. High values of nitrate deposition were mainly found in central Europe (Germany, Denmark, Belgium, Czechia, Switzerland, and eastern Austria), while for ammonium they were also found in northern Italy and Poland. While most of central Europe receives a moderate amount of sulphate deposition, high values are mainly found close to the largest point sources. In the southern part of Europe, sulphate deposition is also influenced by volcanic emission and by the episodic deposition of Saharan dust. The influence of marine aerosols was relevant at sites in coastal areas. Calcium and magnesium deposition can buffer the acidifying effect of atmospheric deposition. High values of calcium deposition are reported in southern Europe, mainly related to the deposition of Saharan dust, and in eastern Europe. The correction for the marine contribution of calcium matters mainly for sites in central Europe and in Spain. In the case of magnesium, however, the distribution of the highest values is markedly reduced by the sea salt correction.