6.2. Nitrogene compounds

Table 6.8: Exceedances of EU and WHO Guide(line) values for NO2.

Trends

Figure 6.23-Figure 6.28 show, as examples, trends in NO2 data on an annual basis for some selected stations in different countries. The figures show the maximum, 98 percentile and median of 1 h data from Denmark, United Kingdom, the Netherlands, Belgium, Germany (only 98 percentile), Switzerland (mean instead of median), Spain and Greece, while Norwegian values are based on 24 h data.

Most stations show a relative stable NO2 level for the last 10 years, even though some stations might show slightly decreasing median and mean values over the last few years, for instance Aalborg, Rotterdam, Bruxelles and several Swiss towns. Rather high values, especially maximum values, are measured in Athens.

Figure 6.23: NO2 trend in Norway 1986-1994 (only winter data and 24 h values) (µg/m3). Data from national reports.

Figure 6.24: NO2 trend in Aalborg, Denmark 1982-1994 and Bridge Place (Greater London), UK 1990-93 (µg/m3). APIS data.

Figure 6.25: NO2 trend in Rotterdam, the Netherlands 1981-1994 and Bruxelles, Belgium 1980-1994 (µg/m3). APIS data.

Figure 6.26: NO2 trend in Bremen, Germany 1987-1993 (µg/m3). Data from Bremen State report.

Figure 6.27: NO2 trend in Switzerland 1984-1993 (µg/m3). The Swiss trend figures are based on œ h values, and the numbers given are the 95 percentile and mean values. Data from national report.

Figure 6.28: NO2 trend in Greece 1983-1993 (µg/m3). APIS data.

6.2.4. Regional nitrogen compound concentrations and deposition

The following presentation is based directly upon the 1993 EMEP data report (EMEP, 1995).

Concentrations

Nitrogen dioxide

The number of sites reporting NO2 data during 1993 were 30% less than the number of sites reporting SO2. For this reason and due to the fact that there are larger local NO2 concentrations, and larger inaccuracies in the measurements of NO2, the uncertainties in the NO2 concentrations in Figure 6.29 are generally much larger than for SO2. The reservations made for SO2 with respect to site location and presence of higher concentrations in industrialised, densely populated areas are also valid for NO2. In addition, even higher concentrations will be found in areas with heavy traffic.

In northern Scandinavia, the annual values are below 1 µg N/m3. For United Kingdom, no NO2 data are available for 1993. A decreasing gradient from England over the Netherlands to Germany is indicated by the Irish and the Continental results. The highest concentrations of NO2, >6 µg N/m3, are reported from four stations in Spain, one in Italy and one in the Netherlands. In central Europe the NO2 concentrations range from 2 to 6 µg N/m3.

Deposition

Nitrate in precipitation

Annual averages of nitrate in precipitation is presented in Figure 6.30. The pattern is somewhat similar to that of sulphate in precipitation. Most areas have annual concentrations lower than 0.6 mg N/l. In Portugal. western Ireland and northern Scandinavia, the annual averages are lower than 0.2 mg N/l. The areas with the highest levels are mainly located in eastern parts of Europe; Poland and towards south-east. High nitrate concentrations are also measured at stations in Germany, Italy and Spain. There is a sharp gradient over the Iberian peninsula, with low concentrations in the west and high concentrations in north-eastern Spain.

Ammonium in precipitation

The ammonium levels presented in Figure 6.31, are also relatively low. In contrast to the anthropogenic sources of NOx giving nitrate in precipitation after a series of photochemical reaction, the sources for ammonia, which give ammonium are mainly related to agricultural activities, including domestic animal keeping and the conversion from gaseous ammonia to ammonium is relatively rapid. The pattern for ammonium is usually quite different from that of nitrate and sulphate. Two maximum areas are indicated, one in south-western France and one in southern Poland, Ukraine and Romania. Concentrations of ammonium in precipitation higher than 1 mg N/l are measured at stations in Lithuania and Poland. Concentrations around 1 mg N/l are measured at stations in France and Croatia. In most parts of Europe the concentrations are lower than 0.7 mg N/l and in the outskirts of Europe, in northern Scandinavia, even below 0.2 mg N/l.

Figure 6.29: Nitrogen dioxide in rural areas 1993, annual average (µg N/m3).
Factor: 3.28.

Figure 6.30: Nitrate in precipitation 1993 (mg N/l).
Factor: 4.43.

Figure 6.31: Ammonium in precipitation 1993 (mg N/l).
Factor: 1.21.