Summary

This report presents measurements and calculations from the atmospheric part of NOVA 2003 and covers results for 2002. It summarises the main results concerning concentrations and depositions of nitrogen, phosphorus and sulphur compounds related to eutrofication and acidification and selected heavy metals Depositions of atmospheric compounds to Danish marine waters as well as land surface are presented.

Measurements
In 2002 the monitoring program consisted of nine stations. Wet deposition of ammonium, nitrate, phosphate (semi quantitatively) and sulphate were measured using bulk precipitation samplers at eight of the stations. At six stations additional measurements of atmospheric content of nitrogen, phosphorus, and sulphur compounds in gas and particulate phase were carried out using filter pack samplers. Filters were analysed at the National Environmental Research Institute. Finally, nitrogen dioxide was measured using nitrogen dioxide filter samplers and monitors at two monitoring stations and wet deposition and air concentrations of nine heavy metals were measured.

Model calculations
The measurements in the monitoring programme were supplemented with model calculations of concentrations and depositions of nitrogen and sulphur compounds to Danish land surfaces as well as marine waters, fjords and bays using the ACDEP model (Atmospheric Chemistry and Deposition). The model is a so-called trajectory model and simulates the physical and chemical processes in the atmosphere using meteorological and emission data as input. The advantage of combining measurements with model calculations is that the strengths of both methods can be utilised. Conclusions concerning:

- actual concentration levels at the monitoring stations,

- deposition at the monitoring stations,

- seasonal variations and

- long term trends in concentrations and depositions

are mainly based on the direct measurements. These are furthermore used to validate the results of the model calculations. Conclusions concerning

- depositions to land surfaces and to the individual marine waters,

- contributions from different emission sources and

- the contribution from Danish sources

are mainly based on the model calculations.

Acidifying and eutrofying nitrogen compounds

Nitrogen deposition to Danish waters
The model calculations for 2002 showed, that the total nitrogen deposition to the Danish waters, fjords and bays varied between 0.7 and 1.6 tonnes N/km2 with an average of about 1.0 tonnes N/km2 (Figure 2.11). This gives a total nitrogen deposition to the Danish waters of 107 ktonnes N.

2002 compared to 2001
In 2002 the nitrogen deposition was 9% lower compared to 2001. The reason for this decrease is mainly less precipitation to the Danish waters in 2002 compared to 2001. The calculated total depositions of nitrogen are given in Table 2.9 for the Danish main waters. Depositions to fjords and bays will later be available on the Internet. The depositions are highest to the fjords and coastal waters. 75% and 25% of the nitrogen deposition to the Danish waters are wet and dry deposited, respectively.

Nitrogen deposition to land surface
The annual nitrogen deposition to Danish land surfaces was between 1.5 and 2.0 tonnes/N km2 in 2002. The highest deposition is in Jutland and the lowest in the eastern part of Denmark. For the entire country the annual nitrogen deposition is calculated to 80 ktonnes, which is comparable to 2001. The geographical variation in the deposition is due to the geographical variation in the precipitation as well as differences in the distance to high emission areas in the northern part of the European continent and intensity of animal husbandry (and thereby ammonia emissions on a more local scale). The deposition per area is in general higher to land surfaces compared to the deposition to water surfaces, mainly due to higher depositions of ammonia emitted from local farms, higher deposition velocity of nitrogen dioxide to land surfaces compared to water surfaces and higher precipitation.

Land + water
The total deposition of nitrogen to the Danish land and water surfaces amounts in 2002 to 187 ktonnes N, which is some-what larger than the Danish emission of nitrogen. In 2001 the total emission was estimated to 146 ktonnes N (Illerup et al., 2003), and there is therefore a significant net import of nitrogen to Denmark.

Seasonal variation in 2002
In 2002 there are large monthly variations in the nitrogen deposition as well as large variations between the different monitoring stations. In general the deposition is highest during spring and lowest during winter. The general picture of the seasonal variations averaged over the monitoring period (1989-2002) has not changed since the previous annual report (Ellermann et al. 2002).

Sources of nitrogen deposition
Model calculations show that the nitrogen deposition to the Danish waters in 2002 originates from farming (about 40%) and combustion processes (about 60%). For areas located close to agricultural areas a somewhat higher contribution from farming is calculated. Some examples are Limfjorden where farming is responsible for about 57% of the deposition and Kattegat where farming is responsible for about 48% of the deposition. Moreover, the model calculations show that about 50% of the nitrogen deposition to land surfaces originate from farming. These results from the model calculations are in good agreement with the results from the monitoring stations, where the measurements show that farming accounts for 46-63% of the deposition to the coastal waters and 40-77% of the deposition to an average land surface (10 cm high vegetation).

Danish sources
The model calculations show that Danish sources contributes with 26% and 9% of the total nitrogen deposition to Kattegat and the Danish part of the North Sea, respectively. However, to some fjords and bays the contribution may be as high as 40%. The high Danish contribution to these areas is mainly due to deposition of locally emitted ammonia. On average the Danish contribution is calculated to 14% of the total nitrogen deposition to the Danish waters in 2002. On average the Danish sources contribute with 38% of the nitrogen depositions to land surfaces, which is comparable to the contributions to the waters. For the deposition to the land surfaces there are large geographical variations on a local scale due to emissions of ammonia from individual farms.

Long range transport
The majority of the nitrogen deposited in Denmark (62-86%) originates from foreign countries; this can be explained by the fact that nitrogen compounds can be transported over large distances in the atmosphere. Considering Denmark it is known from analyses of measurements and model calculations that the main part of the nitrogen compounds comes from the northern part of the European continent where there are high emissions of nitrogen compounds from farming as well as combustion processes.

Long term trends for the total nitrogen deposition
It is estimated that there has been a small decrease (about 17%) in the general total deposition of nitrogen to water surfaces for the period 1989-2002. This estimate is based on the fact that the total deposition of nitrogen mainly consists of wet deposition and that there has been a significant decrease in the average wet deposition to the Danish monitoring stations.

The total deposition to average land surfaces (corresponding to 10 cm high vegetation) consists of roughly equal parts of wet and dry deposition. Due to the large variability in ammonia concentrations and trends it is difficult to extract general trends for the total depositions of nitrogen. However, a conservative estimate is that no clear changes has occurred for the general deposition to Danish land surfaces during the period 1989-2002. The reason for this is most likely that an increase in precipitation and a decrease in the atmospheric sulphur content have counter balanced the decrease in the emissions. The years 1998-2000 have been very wet and on many of the monitoring stations a tendency for an increase in the amount of precipitation has been measured during the period 1989-2002. In general this will lead to an increase in the depositions of nitrogen. Besides this there has been a very large decrease in the sulphur emissions during the last decades. This may lead to a longer lifetime of ammonia in the atmosphere and thereby to an increase in the ambient ammonia concentration and subsequently an increase in the total nitrogen deposition. The changes in precipitation and sulphur emissions will therefore diminish the large effort, which has been carried out in order to reduce the nitrogen depositions by reducing the emissions.

Local changes
On a local scale the nitrogen deposition may of course be significantly modified due to local changes mainly in farming activities. As an example a statistical significant reduction of 35% in the ammonia concentration has been measured at the monitoring station at Tange during the period 1989-2002. This marked reduction in ammonia has lead to a significant reduction of the dry deposition of ammonia, which for Tange accounts for about one third of the total deposition.

The long-term trends in the concentrations of nitrogen compounds at the monitoring stations show the same general pattern as previously reported:

- For NHx (sum of gas phase ammonia and particulate ammonium) statistically significant reductions of 33-45% have been observed over the period 1989-2002. The reductions are mainly due to reduction in concentrations of particulate ammonium, which has been reduced with 41-48% during the period. For ammonia there are large variations between the monitoring stations. At Tange there has been a statistically significant reduction of 35%, while the trends at the other stations are insignificant.

- For sum-nitrate (sum of nitric acid and particulate nitrate) statistically significant reductions have been observed at all monitoring stations. The reductions were between 23 and 30% over the period 1989-2002.

- No long-term trends can be observed for nitrogen dioxide on the basis of the present time series. The large variations between the years conceal possible trends.

The long-term trends in particulate ammonia and sum-nitrate are at the same level for all monitoring stations. It is therefore estimated that these reductions are representative for Denmark as a whole. The reductions are mainly due to reductions in the emissions of ammonia and nitrogen dioxide in the northern part of the European continent. Also local Danish emission reductions contribute.

Ammonia
The ammonia concentration has not changed as much as the concentration of other compounds. This may be due to the pronounced decrease in atmospheric sulphur content or because local conditions at the sites of the six monitoring stations do not reflect the general picture for Denmark as a whole.

Uncertainties
The uncertainty in the model calculations of nitrogen deposition to Danish land and water surfaces is very difficult to estimate. On the basis of the comparison between model calculations and measurements it is estimated, that the total uncertainty for the calculations of nitrogen deposition to land surfaces may be as high as 50%. For the depositions to Kattegat it is estimated that the uncertainty is about 30%, which is assumed also to be true for the rest of the inner Danish waters. For the Danish part of the North Sea the uncertainty may be as high as 50%.

Deposition of phosphorous

It is estimated that the deposition of inorganic phosphorous to the inner Danish waters (area 31,500 km2) is about 130 tonnes P in 2002. This estimate is based on optimised methods for sampling and analysis. This is regarded as an upper limit It is estimated that the phosphorus deposition has not changed significantly during the period 1989-2002. This estimate is supported by measurements of the atmospheric content of particulate phosphorous and measurements carried out by Fyns Amt (2003).

Deposition of acidifying sulphur

Deposition of sulphur
In 2002 the sulphur deposition to Danish land surfaces is calculated to 0.3-0,9 tonnes S/km2. This leads to a total deposition of 27.000 tonnes S to Danish land surface. However, the model calculations give results, which are considerably higher than the measurements and it is therefore more realistic to estimate a total deposition of about 20,000 tonnes S. This is of the same magnitude as the annual Danish emissions of sulphur; 12.700 tonnes S (Illerup et al., 2003). The model calculations show a fairly even geographical distribution of the sulphur deposition, though higher depositions are calculated for the areas around the main cities and in areas with high precipitation. The lack of significant gradients in the deposition pattern is probably because sulphate originating from sea salt is not included in the ACDEP- model.

Sulphur from sea salt
The main part of the deposited sulphur arises from wet deposition (64 % in average) of which 10-25% originates from sea salt. The largest contribution from sea salt is measured in the western part of Jutland due to the short distance to the North Sea. In the eastern part of Denmark only a minor part of the wet deposition is due to sea salt

Sources
The main part of the deposited sulphur arises from wet deposition (64 % in average) of which 10-25% originates from sea salt. The largest contribution from sea salt is measured in the western part of Jutland due to the short distance to the North Sea. In the eastern part of Denmark only a minor part of the wet deposition is due to sea salt The main part of the sulphur deposition originates from long range transport of sulphur from the northern part of the European continent, where the emissions for combustion processes are high. The ACDEP-calculations show that only about 6 % of the sulphur deposition are due to emissions from Danish sources.

Long term trends
The long term trends for the sulphur depositions are more pronounced than the trends observed for the nitrogen deposition. For the period 1989-2002 significant and high decreases in the atmospheric content of sulphur dioxide and particulate sulphate as well as in the wet depositions of sulphur were measured at all the monitoring stations. The decrease in the total sulphur depositions are estimated to be about 50% during the period 1989-2002. This estimate is based on the decrease measured for the wet deposition, which account for the main part of the total deposition of sulphur. The back ground for the decrease is the large reductions in the emissions of sulphur dioxide in Denmark and the northern part of the European continent which has been carried out during the same period.

Uncertainty
As for the nitrogen deposition a considerable uncertainty is estimated for the model calculations. Based on the comparison of model calculations and measurements it is estimated that the ACDEP model calculates too high depositions. The error may be as high as 55% and it is therefore estimated that the uncertainty may be as high as 50%.

Deposition of heavy metals

In 2002 the annual average deposition of heavy metals to Danish background areas were Cr = 0.12, Ni = 0.22, Cu = 0.75, Zn = 7.4, As = 0.12, Cd = 0.04 and Pb = 1.0 (all in units of mg/m2).

During the period 1990 to 20002 there have been clear decreases in the concentrations and depositions of heavy metals. The levels of the heavy metals have decreased by a factor of two to three with the highest decreases for Pb and Cd. The decreases are due to the reductions in the emissions of heavy metals in Denmark and many of the European countries.

For year 2002 the deposition of heavy metals to the inner Danish waters with a total area of 31,500 km2 has been estimated to Cr = 4 tonnes, Ni = 7 tonnes, Cu = 24 tonnes, Zn = 236 tonnes, As = 4 tonnes, Cd = 1,1 tonnes and Pb = 32 tonnes. These estimates are based on measurements performed at all the monitoring stations. The uncertainty of the estimated depositions is about 30 %. When the atmospheric depositions are compared with land based runoff of heavy metals it is seen that the contributions from the atmosphere are of the same order of magnitude. For some of the compounds the atmospheric contributions are even larger than the runoff.

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