6.2. Nitrogen compounds

6.2.1. Behaviour, effects, emissions

Nitrogen oxides (NOx) has natural and anthropogenic sources. The natural sources (bacterial and volcanic action, and lightening) cause a low-level background NO2 concentration. The main anthropogenic source is combustion of fossil fuel, which causes elevated concentrations in near-source areas. NOx is usually considered the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2). In the anthropogenic emissions, NO dominates (typically 80-90%). In the atmosphere, NO reacts quickly with O3 to form NO2. Further reactions depend mainly upon the concentrations of VOC in the air, and the intensity of sunlight. There is a photochemical equilibrium condition between NO, NO2 and O3 with the equilibrium concentrations dependent upon the concentration of OH and other radicals resulting from photochemical reactions. NO2 is eventually converted to HNO3 by reaction with OH radicals. HNO3 is then removed from the atmosphere by wet deposition, and to a lesser extent by dry depositions, adding to the acid deposition problem.

Major sources in most urban areas today are motor vehicle exhaust, which dominates, and combustion of fossil fuels in stationary installations. Large power plants with tall stacks are significant sources of NOx which, however, seldom causes high exposures in nearby areas.

Urban annual average NO2 concentrations are generally in the range 20-90 µg/m3, and peak daily and hourly concentrations may reach several hundred and more than 500 µg/m3 respectively in periods of poor dispersion.

NO2 affects the respiratory system, and a range of effects have been reported from short-term or long-term exposures from controlled human exposure experiments and epidemiological studies: altered lung functions and symptomatic effects, increased prevalence of acute respiratory illness and symptoms, lung tissue damage (at high exposures), increased susceptibility to infections. Young children and individuals with respiratory system decease, such as asthmatics and other chronic respiratory illnesses are those most sensitive to NO2 exposures.

NOx and NO2 contribute to the acid deposition problem, and cause visible injury to vegetation at concentration higher than those occurring in ambient air in Europe, but may occur near a few point sources.

The trend in NOx emissions in Europe (see Figure 6.19) has been calculated and reported by EMEP (Berge et al., 1995). NOx emissions in the EMEP area changed little prior to 1985. Later, emission data show a very rapid increase, culminating in 1989, and after that it has declined down to the pre-1985 emission level. It should be noted that the accuracy of emission numbers before 1985, or even before 1990, may be limited, especially for Eastern Europe. After 1990, the decline is mostly due to reported reduced emissions in East European countries, but also decreasing emissions in West European countries, partly due to the introduction of catalyst cars.

Figure 6.19: Trend in annual NOx emissions in the EMEP area.
(Berge et al., 1995).

6.2.2. Air Quality Limit and Guide Values

EU Limit and Guide values and WHO Guideline values for NO2 are given in Table 6.5. The WHO Guideline values are maximum values and are not to be exceeded.

Table 6.5: EU Limit and Guide Values for NO2 (µg/m3) EU Council Directive 85/203/EEC WHO Guideline values for NO2 (µg/m3).

Limit values Median of
1 h values
98 percentile of
<1 h values
Year   200
Guide values    
Year 50 135
WHO Guideline values Maximum
1 h average
24 h average
  400 150

6.2.3. Urban and local NO2 concentrations

In Appendix B NO2 data from 17 countries are given. The stations are ranged according to the 98 percentile of hourly values. The stations with the highest values are given in Table 6.6. Also the maximum

and median hourly values are given.

Figure 6.20 show maximum, 98 percentile and median hourly values for selected stations and cities. The figure does not show much lower concentrations in the Nordic countries as the SO2 data do. 98 percentile and median values do not vary much over Europe, except for higher values in Spain and Greece. As shown in Table 6.6 and Appendix B, stations in Italy and Portugal also have high 98 percentile values.

NO2 data from the selected cities in Figure 5.1/Table 5.2 are shown in Figure 6.21 and Figure 6.22. These figures give the median and 98 percentile of hourly values, respectively. These figures, together with Table 6.6 and Appendix B, show the highest NO2 values in southern Europe, and some large cities like London and Paris.

Table 6.6: 1 h maximum, 98 percentile and median NO2 values for 1993 for European cities ranged according to the 98 percentile hourly values (µg/m3).








Name Class Station
Italy Bologna   Via Marconi   348  
Portugal Lisboa   Entrecampos   311 69
Italy Milano   Statuto   289  
Italy Piacenza   Via Giordani   264  
Italy Milano prov.   Monza   256  
Italy Milano prov.   Sesto s. g. comune   252  
Greece Athens 1 Patission 147 554 247 98
Italy Milano   Senato   247  
Italy Milano   Juvara   246  
Italy Milano   Zavattari   233  
Italy Milano prov.   Lainate   226  
Italy Milano   Marche   222  
Italy Milano   Liguria   218  
Italy Bologna   Via Matteotti   214  
Italy Bologna   Via Irnerio I   211  
Italy Milano prov.   Legnano Tosi   202  
Italy Roma   Fermi   198  
Italy Torino   Via madama Cristina   198  
Italy Torino   Via Consolata   197  
Italy Genova   C. SO Europa   195  
Italy Roma   Gondar   195  
Italy Genova   Acquasola   193  
Italy Milano prov.   Villasanta   192  
Italy Milano prov.   Agrate   192  
Italy Milano prov.   Limito   192  
Italy Bologna   Via Emilia Ponente   190  
Italy Roma   M. Grecia   190  
Italy Venezia (ind. zone)   Stab. Montedipe 1   190  
Italy Milano prov.   Settimo   188  
Italy Torino   Corso Vercelli   188  
Italy Torino   Via Guido Reni   188  
Spain Madrid 1 Plaza España 470 187 57
Italy Venezia (ind. zone)   Piazza s. Antonio (Marghera)   186  
Italy Milano prov.   Inzago   186  
Italy Milano prov.   Corsico   184  
Italy Milano prov.   Melegnano   183  
Portugal Carregado   Cast. Ribatejo   181  
Italy Milano prov.   Cuggiono   181  
Italy Roma   Preneste   179  
Italy Genova   Villa Serra   178  
Italy Piacenza   Via Taverna   178  
Spain Madrid 1 Cuatro Caminos 448 177 72
Italy Milano prov.   Rivolta   176  
Italy Milano prov.   Robecchetto   176  
Italy Firenze   Mosse   174  
Italy Piacenza   Via Alberoni   174  
Italy Milano prov.   Legnano com.   173  
France Paris 1 R.N 13 Rue Malmaison 424 172 69
Italy Bologna   Fiera   171  
Italy Bologna   P. Za dei Martiri   170  

Figure 6.20: 1 h maximum, 98 percentile and median NO2 values for 1993 for selected stations and cities (µg/m3).

Figure 6.21: NO2 median 1h values in selected cities (µg/m3).

Figure 6.22: NO2 98 percentile 1h values in selected cities (µg/m3).


Table 6.7 shows stations with exceedances of the EU Limit value for NO2
(98 percentile of hourly values, 200 µg/m3). This limit value is rather high. It is exceeded at one station in Greece and Portugal and at a number of stations in Italy. Some stations in Austria have rather high maximum 1/2h NO2 values, but 98 percentile values are not given. Probably the EU Limit value is not exceeded in Austria. From Eastern Europe there are no available NO2 data to be compared with the EU Limit value.

Table 6.8 shows exceedances of EU Guide values and WHO Guideline values for NO2. WHO Guideline values are much higher than EU Guide values and are exceeded only in a few countries. EU Guide values seems to be exceeded commonly in Western Europe. Data from Eastern Europe are not available.

Table 6.7: Exceedances of EU Limit Values for NO2.






Station name

1h values
1 year
Greece Athens Patission 147


Italy Genova Via Cantore


    Piazza Masnata


  Bologna Via Marconi


    Via Matteotti


    Via Irnerio I


  Firenze Roselli


  Milano Marche


















  Milano (province) Sesto s. g. Comune












    Legnano Tosi


  Piacenza Via Giordani


Portugal Lisboa Entrecampos


** Data availability <75%.

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







EU Guide Values WHO Guideline Values
Median 1h > 50 P98 1h >135 Max. 1h >400 Max. 24 h >150
Austria Graz Süd       151
(mean values) Linz 24-er-Turm       172
    BH-Urfahr       155
    Kleinmünchen       162
    ORF-Zentrum       167
  Salzburg Rudolfplatz 71      
  Wien Hietzinger Kai 74      
France Lyon Lyon Berthelot 80 160 452 166
    Lyon Garibaldi 65 162 481  
    Lyon Puits Gaillot 67 164 403  
  Paris Neuilly/Seine (92) 58   418 177
    Paris Ch. de Mars 52   435 166
    Paris Tour St-Jacques 63 150   167
    R.N 13 Rue Malmaison 69 172 424 197
    Rue Danzig (15ème) 61 145 437 159
    Tour Eiffel (1er et.) 50 154   172
  Strasbourg rue du 22 novembre 50      
Germany Berlin Charlottenburg 56 155    
  Düsseldorf Düsseldorf-Mörsenbroich 61      
  Erfurt Erfurt, Krämpf.   137    
  Essen Essen-Ost 53      
  Gera Gera, Frieder.   141    
  Hamburg Stresemannstrasse 58 142    
  Hannover Göttinger strasse   136    
  Karlsruhe Karlsruhe-Mitte 51      
  Koblenz Friedrich-Ebert-Ring 57      
  Ludwigshafen-Frankental Frankental 57      
    Mundenheim 51      
  Mainz-Mombach Parcusstrasse 67 163    
  Wiesbaden W-Ringkirche   136    
Greece Athens Patission 147 98 247   253
    Pireas Platia Dimotikou 67 136    
Italy Bologna Fiera   171    
    P. Za dei Martiri   170    
    Via Emilia Ponente   190    
    Via Irnerio I   211    
    Via Marconi   348    
    Via Matteotti   214    
  Firenze Mosse   174    
    Via Bassi   145    
  Genova Acquasola   193    
    C. SO Europa   195    
    Villa Serra   178    






EU Guide Values WHO Guideline Values
Median 1h > 50 P98 1h >135 Max. 1h >400 Max. 24 h >150
Italy (contd.) Milano Juvara   246    
    Liguria   218    
    Marche   222    
    Senato   247    
    Statuto   289    
    Zavattari   233    
  Milano prov. Agrate   192    
    Cassano d’Adda   169    
    Corsico   184    
    Cuggiono   181    
    Galliate   166    
    Inzago   186    
    Lainate   226    
    Legnano com.   173    
    Legnano Tosi   202    
    Limito   192    
    Lodi   159    
    Melegnano   183    
    Monza   256    
    Rivolta   176    
    Robecchetto   176    
    Sesto s. g. comune   252    
    Settimo   188    
    Turbigo   143    
    Villasanta   192    
  Modena Cavour   141    
    Via Giardini   167    
  Piacenza Via Alberoni   174    
    Via Giordani   264    
    Via Taverna   178    
  Ravenna S.A.P.I.R   136    
  Roma Fermi   198    
    Francia   159    
    Gondar   195    
    M. Grecia   190    
    Preneste   179    
  Torino Corso Vercelli   188    
    Parco Lingotto   160    
    Via Consolata   197    
    Via Guido Reni   188    
    Via madama Cristina   198    
  Trento Trento   163    
    Trento Nord   144    
  Venezia Piazza s. Antonio (Marghera)   186    
  (ind. zone) Stab. Montedipe 1   190    
The Netherlands Utrecht Witte Vrouwenstraat 58      






EU Guide Values WHO Guideline Values
Median 1h > 50 P98 1h >135 Max. 1h >400 Max. 24 h >150
Norway Bergen CMI 59 135   204
(based upon 24h Drammen Engene 74      
values) Fredrikstad Broch street 50      
  Oslo St. Olavs square 50      
  Skien Kings street 57      
  Stavanger Handelens hus 63      
  Trondheim Torget 57      
Portugal Carregado Cast. Ribatejo   181    
  Faro Faro 80 141    
  Lisboa Benéfica 69 151    
    Entrecampos 69 311    
    Rua da Prata 52 137    
  Setúbal Setubal-Cicade   167    
Spain Barcelona Molina Pl. 55      
  Madrid Arturo Soria 53 159    
    Carlos V 57 151   154
    Cuatro Caminos 72 177 448 184
    Plaza Castilla 58 141    
    Plaza España 57 187 470 189
    Villaverde 72 151    
Switzerland Basel Feldbergstrasse 57      
(mean values) Bern Bern 57      
  Genève Ile 60      
  Lausanne Lausanne 50      
    Rue César Roux 2 56      
  Zürich Schimmelstrasse 57      
United Kingdom Edinburgh Edinburgh centre 50      
  Leeds Leeds centre 50      
  Liverpool Liverpool centre 50      
  London Bridge Place 63 136   192
    London Bloomsbury 65      
  Newcastle Newcastle centre 54      


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).


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.


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.

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