Modelling of the transport of nitrogen and sulphur contaminants to the Baltic Sea region

Abstract

In the work for this thesis, the 3D model Hilatar, which can be used to monitor and numerically forecast air pollution transport, chemical transformation and deposition in Europe and its sub-areas, has been developed. Model has been used to assess the sulphur and nitrogen concentrations and fluxes over the Baltic Sea and its surroundings in 1985, 1988 and 1993-2002 and to study their time variation caused by physical, meteorological and model structural factors, as well as other uncertainties connected to the 3D modelling. The model has been used e.g. in quantifying nitrogen flux to the Baltic Sea, and the factors that influence it, within the EU BASYS (Baltic Sea System Study) project and in studying of pollutants in episodic situations. The first model version has been used in calculating the Daiquiri-model unit matrixes for the Finnish Environmental Institute.

The temporal- and spatial variation of the concentration and deposition of air pollutants is high. Episodicity is to a large extent caused by meteorological factors, the time variation of emissions, high concentration gradients close to source areas and long-range transport from high-intensity emission areas. Considering the Nordic countries, the modelled deposition of oxidised nitrogen in Lapland is around 10 % of that in Denmark, the ammonium deposition being still smaller. Although sulphur deposition is high in the vicinity of the large industrial areas of the Kola Peninsula, is does not exceed the Central European level there. The wet flux of nitrogen to the Baltic Sea is strongest in winter and autumn. The dry nitrogen deposition share was highest in Southern Sweden and Denmark in summer due to high ammonium emissions. The effects of the decreasing emission trend could not be seen in Northern European N-deposition in 1993-98; S-deposition decreased slightly, however. The prevailing meteorological conditions were the primary cause of the inter-annual variation of regional deposition. Annual deposition to the open Baltic Sea area (391 000 km2) over the period 1993-1998 vary in the range 140-180 kt(N) for NOx and 100-120 kt(N) for NHx. The relative importance of the atmospheric load for algae growth in the Baltic Sea needs to be further studied since the sea is already rather badly eutrophicated and the primary production is largely driven by the nutrients that have already accumulated in the system.

Modelled NOx and SO2 concentrations are at their highest during winter, due to slower chemical and terrestrial deposition sinks, maximum emissions and more frequent inversions. Ammonia emissions are highest during the summer and the conversion of NH3 to ammonium-sulphate particles is stronger in winter when there is more sulphate in the air. HNO3 levels are low in winter due to low reactivity, short days and reactions with NH3. The critical deposition limits were exceeded, but the critical wintertime SO2 and annual NO2 concentrations were not exceeded in the Nordic background areas, although this occurred for SO2 over large areas in Europe especially in the cold winter of 1996.

Comparison with field campaign results and long-term measurements demonstrated a rather good description of both the spatial and temporal characteristics of the S and N pollution in the Baltic Sea region. Model results and meteorological input data are stored in two continuously updated databases. They can be used in studying simultaneous effect of meteorological and air pollution stresses on vegetation.