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Despite significant reductions in nitrogen emissions achieved in Europe during the last three decades, eutrophication remains an environmental concern in the Baltic Sea basin. Recently, a number of comprehensive modelling studies have been conducted for the HELCOM Commission to inform the 2021 update of the Baltic Sea Action Plan. The calculations have focused on trends in airborne nitrogen deposition to the Baltic Sea and its nine sub-basins during the 2000-2017 period, the identification and ranking of the main contributors to deposition, as well as future projections for 2030, assuming compliance with the Gothenburg Protocol and the EU NEC Directive. This paper synthesizes the main results from these studies and puts them into the context of maximum allowable nutrient inputs to the Baltic Sea. According to our results, the airborne annual deposition to the Baltic Sea in 2017 amounted to 122.6 Gg(N) of oxidized nitrogen and 105.3 Gg(N) of reduced nitrogen, corresponding to a decrease since 2000 by, respectively, 39% and 11%. In order to filter out the large inter-annual variability due to meteorology and to better reflect trends in emissions, weather-normalized depositions of nitrogen have been calculated as well, according to which the decreases since 2000 amount to 35%, 7% and 25% for oxidized, reduced and total nitrogen, respectively. In 2017, Germany, Poland and Denmark were the most important contributors to airborne deposition of total (oxidized + reduced) nitrogen to the Baltic Sea. Agriculture contributed most to reduced nitrogen deposition, while the transport sector contributed most to oxidized nitrogen deposition. Agriculture in Germany was the single-most important contributor to nitrogen deposition to the Baltic Sea basin in 2017 (accounting for about 15% of the total), but there are numerous other important sectoral contributions. Emissions of nitrogen from the nine HELCOM Contracting Parties contributed 49%, 76% and 61% to oxidized, reduced and total nitrogen deposition, respectively. Assuming full compliance with the EU NEC Directive and the Gothenburg Protocol, significant further reductions in nitrogen deposition can be achieved by 2030, down to an annual deposition of 72.7 Gg(N) and 84.7 Gg(N) of oxidized and reduced nitrogen, respectively.

Within the framework of the collaborative project between IIASA and the Norwegian Meteorological Institute in Oslo, a new version of the Heavy Metals Eulerian Transport (HMET) model has been developed. This new version is presented in this paper, as well as the results of the model simulation for one year period of transport (1992). The main difference between the old version (here termed as HNIET-150), and a new version called HNIET-50, is the model grid size reduced from 150km in the old approach, to 50km in the latest model version. Higher spatial resolution is important for estimating atmospheric load of heavy metals to European soils. The HMET-50 model was used to simulate atmospheric transport and deposition in Europe of arsenic (As), cadmium (Cd), lead (Pb) and zinc (Zn). Necessary emission data for this simulation were compiled at IIASA. Meteorological input for the HMET-50 model was prepared at the Norwegian Meteorological Institute in Oslo. Maps of computed total depositions in Europe for all metals are presented. In addition, concentration maps, as well as dry and wet deposition maps, for all metals are also shown in the paper. The change of the model grid size influenced the locations of the deposition maxima. Because of the better resolution of emissions, computed concentrations are slightly closer to measured values. However, this fact has to be confirmed when more measurements for 1992 become available.

This paper presents a preliminary estimate of atmospheric emissions of cadmium, lead and zinc in Europe during the period 1955-1987. The emission data are used as input to the IIASA's atmospheric transport model, TRACE m a c e toxic Air concentrations in Europe), to compute cumulative deposition loads of heavy metals onto European soils during the investigated time period. To the authors' knowledge, this is the first attempt of this kind in the open literature. The computed with the TRACE model maxima of cumulative depositions for the three metals are approximately: 60 mg/m2 for Cd, 1450 mg/m2 for Pb and 2600 mg/m2 in the case of zinc. The results presented should be considered first-order approximations. Major uncertainties embedded in such calculations are discussed. Heavy metals enter the soils from atmospheric load and with the application of fertilizers and sewage sludge. Once in the soil the metals can be mobilized, leading to plant and ground water contamination. This threat is particularly valid for heavily polluted regions in Central Europe. The results of this study can be applied in assessing environmental and health effects of heavy metals, and therefore are important for scientists as well as policy makers.

In recent years there has been an increased interest in trace metals in the atmosphere and the environmental effects of their deposition. This is to large extent because heavy metals can accumulate in the biosphere and may be toxic to living systems. On the basis of IIASA's TRACE model, the total (wet plus dry) deposition of As, Cd, Pb and Zn has been estimated for Eastern Europe. These are annual averages for rural areas, and relate to the situation in Europe in the mid-1980s. The maximum deposition value is 3.5 mg m^{-2} yr^{-1} for As, 1.5 mg m^{-2} yr^{-1} for Cd and 50 mg m^{-2} yr^{-1} for Zn. All these maxima occur in Southern Poland. The highest total deposition of Pb (15.0-20.0 mg m^{-2} yr^{-1}) has been computed for western Czechoslovakia and also for southern Poland. Deposition levels throughout most of Eastern Europe are at least one or two orders of magnitude greater than observed in remote parts of the world. The annual average concentration of metals in some rural areas are lower, but within a factor of two of drinking water guidelines. This is cause for concern because some short-term concentrations are almost assuredly much higher than the annual average. Because of long-range transport, there is a very significant transboundary exchange of heavy metals within Eastern Europe. As with acid-causing pollutants, the problem of heavy metals contamination in the region depends on the reduction of this transboundary pollution.

This report investigates the composite uncertainty of a long range transport model of sulfur in Europe. This composite uncertainty includes the effect on model output of uncertain transport wind, meteorological forcing functions, parameters, and spatial distribution of emissions. Other sources of uncertainty are omitted in the analysis. Stochastic simulation is used for computations. The combined effect of these uncertainties on total annual sulfur deposition at three receptors ranged from about 10 to 20% (coefficient of variation). In comparing the effects of different uncertainties on annual model output, meteorological forcing functions were found to be the least important because of their frequent temporal variation in the model. The stochastic procedure was also used to compute the uncertainty of transfer coefficients for 30 source-receptor combinations; their relative uncertainty ranged from about 10 to 30% and was not correlated with distance. However, their absolute uncertainty (standard deviation) was strongly correlated with distance and was found to be proportional to the values of transfer coefficients themselves. This insight was used to develop a simple method for estimating the uncertainty of sulfur deposition calculated with transfer matrix. This method was then used to evaluate the `reliability' of emission reduction scenarios in reducing deposition.