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The Background Air Quality in Denmark 1978-1997

Niels Z. Heidam

NERI Technical Report No. 341


Monitoring in 20 years
In this report results from 20 years of air quality monitoring in rural areas in Denmark are presented.

Network history
The monitoring of background air quality in Denmark has been carried out since 1978. Initially the network comprised two stations with a quite simple measurement programme, but increasing demands and needs have over the years led to the development of a full national Background Air Quality Monitoring Programme (BOP). It is a framework programme, which covers both national needs and Denmark's obligations in international conventions and programmes.

Network stations
Since the mid-1980s BOP has comprised 6 main stations and a number of satellite precipitation stations. At the main stations both air and precipitation sampling takes place. Air samples are collected on filters over 24 hours by semi-automatic filterpack samplers and precipitation is collected as wet-only and in most cases also as bulk samples. Precipitation is usually collected on a ½-monthly basis, but at two of the stations wet-only measurements have been on a 24-hour basis. The precipitation satellite stations supplement the main stations with bulk samples on a ½-monthly basis.

Measurement programme
The components measured in BOP comprise sulphur and nitrogen compounds in air and precipitation, elements in airborne aerosols, and at some stations also nitrogen dioxide and ozone. For the ammonia/ammonium and nitric acid/nitrate systems the phase sums are determined. All these components may have both indigenous and foreign origins.

Network is both necessary and sufficient
It is the conclusion of this report that with respect to the measurement programme this network is both necessary and sufficient for a qualified descrip-tion of the air quality in the Danish rural areas. The stations are found to be representative both of the region where they are situated and of the type of their surroundings. The number of stations and the frequency of sampling are sufficient for describing the air and precipitation quality but also necessary for obtaining both the temporal variations over the years and the geographic variations across the country. The network density is however insufficient for a closer study of urban influences in the open countryside. The value of really long time series have been revealed from differences between conclusions based on series of 20 years compared to those possible from results that only span about 10 years.

Log-normal air concentrations
For the concentrations in air it has been found that most of the components are to a good approximation log-normally distributed; consequently the median rather than the mean is used extensively in this report as the most representative aggregate quantity. The main exception is ozone, which appears to be distributed normally.

Memory of past concentrations
Most components carry a memory of past concentrations, an effect that extends for about a week. The autocorrelation functions also show that periodicities of one or two weeks occur and, not surprisingly, that there are annual patterns in the concentrations.

Aerosol stoichiometry
The aerosol in Danish rural areas consists mainly of compounds containing ammonium, sulphate and nitrate. A stoichiometric analysis has shown that this aerosol is normally fully neutralised and consists of a stoichiometric mixture of ammonium sulphate and ammonium nitrate.

Geographic variations
The geographic variations of air concentrations are not large although there is some dependence on site surroundings, whether marine or agricultural. But a N-S gradient for SO2, S, TNHz, and TNO3 as well as the heavy metals Pb and Zn is clearly visible and is probably caused by LRTAP from the south. A weaker W-E gradient can also be discerned in many cases, but for TNHz the gradient is reversed.

The temporal variations occur on both long and short time scales. The long-term trends show that the air concentrations of most pollutants have fallen dramatically and continuously by factors of 3-10 over the last 20 years. These changes have been brought about by co-ordinated international efforts to reduce emissions in Europe and they have affected not only the average but also the maximally occurring concentrations. There is however no trend in the concentrations of the agriculturally related NH3, and TNO3 appears to have decreased only in eastern Denmark. For NO2 no trends can be seen although emissions on a European scale have dropped by almost 2% per year in the last 10 years. As expected no trends can be seen for ozone.

Reduced annual variations
The annual variations have for these reasons been reduced considerably so that the winter maxima of sulphur compounds and lead that occurred in the early 1980s have all but disappeared. In contrast the annual variations of nitrogen compounds have not changed much. Similarly ozone has consistently maintained a broad summer maximum.

Frequency and severity of episodes
On the short term scale episodes of the anthropogenic and/or long-range transported compounds still occur under special conditions, notably in situa-tions with cold stagnant air. But the severity of the episodes has decreased and the frequency of occurrence has also fallen from the original average of about 7 episodes per year to 3-4. For ozone, which is mostly caused by LRTAP from the southern part of Europe, the 24-h limit value of 65 µg·m-3 introduced by EU for protection of vegetation and adopted by Denmark have been and are still regularly exceeded 2-6 times a week in the growth season. The accumulated AOT40 limit for protection of vegetation is also regularly exceeded each year. These exceedances may cause substantial losses in crop yield.

Concentrations in precipitation
For concentrations in precipitation it is found that total nitrogen is quite evenly distributed among reduced and oxidised nitrogen, i.e. ammonium and nitrate. Also the predominantly anthropogenic non_sea_salt sulphur constitutes a major fraction of sulphur in precipitation. The concentrations are approximately log-normally distributed with the exception of Hydrogen H+, which seems to be neither log-normally nor normally distributed.

Concentrations and accumulated precipitation
Precipitation concentrations tend to decrease with increasing precipitation because already in the beginning of a precipitation event the atmosphere is cleaned quite efficiently by scavenging.

Geographic variations
The type of surroundings of the various stations influences the geographic variations of precipitation concentrations. At agricultural stations precipitation is less acid than at other sites, e.g. coastal sites, which may have higher concentrations of sulphate and nitrate, and/or sites exposed to long-range transport where the precipitation is more acid and has higher contents of ammonium and non-sea salt sulphate. But these variations are generally rather small.

Concentration trends
The concentrations of anthropogenic pollutants in precipitation have since the early 1980s fallen considerably, by 20-50 % and in some cases more. The main exceptions are the nitrate concentrations that cannot be seen to have changed.

Wet-only and bulk depositions
The wet depositions as monitored by wet-only and bulk collectors at the stations are approximately log-normally distributed with the exception of nitrate and H+. A comparison of deposition results from these two collector-types show that they agree quite well, both types collect precipitation equally well. But wet-only deposition is smaller than bulk deposition because of a contribution that is only present in the bulk samplers and which probably represents dry deposition.

Geographic variations
The geographic variations are rather similar to those found for the concentrations, which are under some influence from the station surroundings. The largest variations are found for sulphate, where bulk depositions vary considerably among stations as well as within stations; at the coastal stations marine sulphate constitutes up to 50% of the deposition. The depositions of non-sea_salt sulphate are more evenly distributed over the country but an LRTAP-induced N-S gradient, most pronounced over land, is evident. A similar variation is also found for ammonium, another LRTAP-component. The bulk depositions of nitrate are fairly uniform across the country although depositions at the coastal stations are somewhat smaller. Acidity depositions do not seem to have any systematic geographic variation.

Deposition trends
Both wet-only and bulk depositions of anthropogenic components have, with some reservation for nitrate, decreased markedly in the two decades, usually by factors of 2-5. This decrease is most pronounced for non-sea_salt sul-phate and acidity and is without doubt caused by the internationally agreed restrictions on emissions. For the long time series of 20 years for the wet-only depositions the negative trends are significant more often than for the shorter bulk series of 7-12 years. This agrees with the observation that the major decrease in the wet-only depositions occurred before the mid-1980s. A notable case is that of the base cation Calcium, which as the only dominantly natural component has been found to have a significant negative trend. This phenomenon, which may be caused by the increasing agricultural practice of green winter fields, may reduce the neutralisation of the precipitation and thereby offset the benefits from reduced sulphur emissions.

Total deposition by models
The total deposition of both wet and dry deposition to Danish land and water surfaces is calculated by the ACDEP model that simulates a number of physical and chemical atmospheric processes.

Sulphur deposition
The deposition of sulphur, which is environmentally important for land areas only, is dominated by dry deposition of SO2 and wet deposition of SO4=. The total deposition is composed of about equal amounts from dry and wet depo-sition and amounts on the average to an annual value of about 600 kg S·km-2. The largest depositions occur in the southern part of the country, indicating a large LRTAP-component.

Deposition trend
Because of the international emission reductions the deposition of sulphur has decreased markedly, by about 50%, in the period from the early 1980s to the middle of the 1990s.

Nitrogen deposition
The deposition of nitrogen compounds to water surfaces vary considerably from low values in the open (North) sea, about 500 kg N·km-2·yr-1, to depositions as high as 1300 kg N·km-2·yr-1 in the inner waters, notably the fjords and lakes. Depositions to land areas vary less and are largest in the south. About 66% of this deposition is caused by reduced nitrogen, in particular ammonia releases from nearby agricultural areas and ammonium of less local origin. The total depositions calculated by different models and in differ-ent years vary somewhat because of model improvements and improved emission estimates but unfortunately also because of varying area definitions.

Deposition trend
The total deposition of nitrogen to Danish waters amount to about (100 * 5) kt N annually and no trend can be discerned. This atmospheric contribution to eutrophying nitrogen inflow to Danish waters is very large, about 50% of the total supply.

Critical loads
The areas in Europe where critical loads for acidification and eutrophication are exceeded have been calculated within the EMEP programme.

For acidification the situation has improved considerably over the last 20 years because of the emission reductions. On the European level the areas of exceedance have decreased from about 25% of the total land area before 1990 to 8-13% in 1996-1997. In Denmark the corresponding figures reveal a decrease from 7-8% to about 2%.

The situation is considerably more serious for eutrophication where the areas of exceedance since the mid-1980s have remained virtually constant at about 20% in the EU countries, at 35% in Europe as a whole, and at 5-8% in Den-mark.

Atmospheric pollution transport from abroad
Much of the atmospheric pollution in Denmark has been transported here from abroad. An analysis of transport directions reveals that the atmospheric concentrations of sulphur, reduced nitrogen and heavy metals are enhanced in winds from east over south to south-west, i.e. from the European continent, compared to directions from the north-west or north. The maximum concentrations occur in the southeasterly winds, so the most polluted air arrives in Denmark from Eastern Europe. But when the frequencies of wind from the different directions are included it is found that the southwesterly wind sector contributes the most to the annual mean concentrations, the main dose of foreign air pollution arrives in Denmark from Western Europe. The variations over time of the air concentrations vary considerably with wind direction. In winds from Western Europe concentrations have been falling steadily through two decades, but in the south-easterly sector concentrations vary quite erratically around 1990, the period of political and economic up-heavals in Eastern Europe, and have only recently started to decrease more systematically.

Origin of deposited of pollutants
The depositions of pollutants in European countries, including Denmark can be traced back to the original place of emission. This is done regularly in the European co-operation under the EMEP programme by application of the large-scale EMEP models for transport and transformation. For Denmark in particular the Danish ACDEP model has also been used.

Depositions in Denmark dominated by foreign sources
In this way it has been shown that Danish sources contribute only about 15-20% to the depositions of sulphur and nitrogen in Denmark, the remaining 80-85% derive from foreign sources. The decrease of 50% in sulphur deposition over the last 15-20 years as well as the lack of such tendencies for nitrogen depositions are found in both domestic and foreign contributions. The major foreign contributions to nitrogen deposition consist of nitrates and ammonium salts that easily travel long distances in the atmosphere before being deposited. The Danish deposition contributions have a considerable geographic variation, for nitrogen from less than 5% in the North Sea to more than 60% in the inner Danish waters.

Pollutant "import" to Denmark
For the period 1985-1996 it has been estimated by EMEP that Denmark imported on the average 80% of the sulphur deposited in the country from a total of 13 foreign and neighbouring countries, mainly from Germany and the United Kingdom. Similarly 90% of the deposited oxidised nitrogen was imported from 15 foreign countries, and again with Germany and the United Kingdom as the main contributors. For reduced nitrogen that is prone to rapid deposition the import from neighbouring countries accounted for only about 22% of the deposition, the remainder was of domestic origin. Pollutant "export" from Denmark However, Denmark also exports pollution that is deposited outside the country. For the same 12-year period as above it has been estimated by EMEP that 91% of the Danish sulphur emissions are transported outside the country, notably to the east. The main part of this pollution (44%) is deposited in the Baltic where it has no effect and in 7 neighbouring countries, mainly in Norway and Sweden. The Danish contributions to the sulphur depositions in these countries are however of no great importance compared to the total deposition there. Similar figures and conclusions apply to the export of oxidised nitrogen whereas the export percentages for reduced ni-trogen are considerably lower, because a main part of the emissions is gaseous ammonia, that is efficiently deposited in the local area.

Net exports from Denmark
In conclusion it can be stated that along with most of the European countries Denmark is a net pollutant exporter of deposited sulphur and nitrogen. This is to a large degree caused by the small size of the country where the pollutants do not have far to go to cross a border. For sulphur the net result was in 1996 that 54 kt or 59% of the Danish emissions of 93 kt S left the country in 1996. For all nitrogen compounds, both oxidised and reduced, the net export in 1996 from Denmark was 104 kt corresponding to 61% of the total annual emissions of 169 kt N.

Bibliographic information: Heidam, N.Z. (2000): The Background Air Quality in Denmark 1978-1997. National Environmental Research Institute, Denmark. 192 pp. NERI Technical Report No. 341. Available electronic at

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