Content

 

Phenomena involved and geographical extent

Acid pollution is linked to human-induced emissions of SO2, NOx, NH3, which are deposited in part near the emission sources but also hundreds or even thousands of kilometers away from it (SO2 persists in the atmosphere for around 2 to 5 days). Eutrophication is mainly linked to the transformation of NOx and NH3 emissions into nitrogen.

The nitrogen is then deposited in aquatic environments in the air or in precipitation. SO2 and NOx are transformed respectively into sulphates and nitrates, and into sulphuric or nitric acid depending on conditions.

NH3, mainly released by farming activities, also contributes to acidification of the environment. The acidification potential of NH3 is equivalent to that of NOx.

This excess nitrogen due to NOx and NH3 emissions modifies the chemical balance of the ecosystems in which it is deposited.

Acid air pollution was identified many years ago in urban and/or industrial areas, where it is referred to as "London smog" or "acid smog". This type of smog has tended to disappear in recent years in Europe's large urban areas, but has become a characteristic of many urban areas in developing countries.

Large-scale acid pollution first became apparent in the 1970s with the acidification of Scandinavian and Canadian lake waters, where the increasingly low pH has caused sharp declines in the fish population. Whereas the natural pH of rainwater is 5.6, it has sometimes dropped to 3 or 4.

The illustration below shows the principles of pollutant transport.

Figure 1 : simplified diagram of pollutant transport and transformation [source CITEPA]

IV.1.3 Acidification Figure1

The chemical reactions involved can be summarised as follows:

Gaseous phase reactions:

2 SO2 + O2 → 2 SO3
SO3 + H2O  H2SO4
SO2 + OH + M  HOSO2 + M
HOSO2 + O2  SO3 + HO2

2 NO + O2  2 NO2
2 NO2 + H2O  HNO3 + HONO
NO2 + OH + M  HONO2 + M
NO2 + O3  O2 + NO3
NO2 + NO3  N2O5
N2O5 + H2O  2 HNO3

Liquid phase reactions, when pollutants have dissolved:

SO2 (g) + H2O → H2SO3 (liq)
H2SO3 (liq) + H2O2 (liq)  H2SO4 (liq) + H2O
H2SO3 (liq) + O3 (liq)  H2SO4 (liq) + O2
2 H2SO3 (liq) + O2 (liq)  2 H2SO4 (liq)

Liquid phase processes in the case of nitrogen oxides appear to be less marked.

These pollutants are transported over long distances, as shown in the diagrams below. France has an impact on more or less extensive areas in neighbouring countries, but the reverse is also true.

Figure 2 below shows France's impact on sulphur deposition in neighbouring countries (EMEP zone) [EMEP 2012a]. In 2010, 60% of French emissions were transported beyond its borders.

 

Figure 2 : France's impact on sulphur deposition in neighbouring countries (EMEP zone) and zones where sulphur released in France is deposited (MED, ATL, NOS, DE, IT and Others) [EMEP 2012a] 

 IV.1.3 Acidification Figure2

Figure 3 below shows the origins of sulphur deposition in France. Top left: sulphur deposition in France. Top right: origins of sulphur deposition in France. Bottom left: sulphur deposition of transboundary origin. Bottom right: fraction of transboundary deposition in all deposition. Emissions from outside its borders have a strong impact in France, with 70% of deposition originating elsewhere.

 

Figure 3 : origins of sulphur deposition in France in 2010 [EMEP 2012a]

IV.1.3 Acidification Figure3

Figure 4 below shows France's impact on nitrous oxide deposition in neighbouring countries (EMEP zone) [EMEP 2012a]. In 2010, 77% of nitrous oxide emissions in France were transported beyond its borders.

 

Figure 4 : France's impact on nitrous oxide deposition in neighbouring countries (EMEP zone) and zones where nitrous oxide released in France is deposited (MED, ATL, NOS, DE, IT and Others) [EMEP 2012a]

IV.1.3 Acidification Figure4

Figure 5 below shows the origins of nitrous oxide deposition in France. Top left: nitrogen deposition in France. Top right: origins of nitrogen deposition in France. Bottom left: nitrogen deposition of transboundary origin. Bottom right: fraction of transboundary deposition in all deposition. As with sulphur, a large proportion of deposition - 68% - is of external origin.

 

Figure 5 : origins of nitrous oxide deposition in France in 2010 [EMEP 2012a]

IV.1.3 Acidification Figure5

Figure 6 shows France's impact on reduced nitrogen deposition in neighbouring countries (EMEP zone) [EMEP 2012]. In 2010, 47% of reduced nitrogen emissions in France were deposited beyond its borders, a much lower proportion than for nitrous oxide (77%).

 

Figure 6 : France's impact on reduced nitrogen deposition in neighbouring countries (EMEP zone) and zones where reduced nitrogen released in France is deposited (MED, ATL, NOS, DE, IT and Others) [EMEP 2012a]

IV.1.3 Acidification Figure6

Figure 7 below shows the origins of reduced nitrogen deposition in France. Top left: reduced nitrogen deposition in France. Top right: origins of reduced nitrogen deposition in France. Bottom left: reduced nitrogen deposition of transboundary origin. Bottom right: fraction of transboundary deposition in all deposition. Deposition of external origin account for only 23% of the total as compared to 68% for nitrous oxide.

 

Figure 7 : Origins of reduced nitrogen deposition in France in 2010 [EMEP 2012a]

IV.1.3 Acidification Figure7

 

Critical loads

The critical load concept has been developed to measure environmental impacts and sensitivity.

Critical loads are determined on the basis of geological, pedological, hydrological and ecological criteria. The critical load for acid deposition is defined as follows: "the highest load of acid compounds that will not cause chemical dysfunction in soils leading to long-term harmful effects on the structure and functioning of ecological systems".

 

Acid deposition

Acid deposition has the following effects:

  • Acidification of lake waters: studies of certain fossils (certain aquatic animals are characteristic of a given pH range) have established that the pH of lake waters was constant until around 1950, after which it dropped abruptly. The acidification of lake waters is harmful to fish and a low pH can even destroy fish populations altogether.
  • Forest dieback, affecting both conifers and broadleaved species. The numerous studies carried out on forest dieback have shown that several phenomena are involved, acting in combination: effects of drought (which are reinforced by the presence of SO2 and ozone), dry or wet acid deposition that causes leaching of soil nutrients, direct action of SO2 or NOx on the physiology of plants.

The critical load for acid deposition is therefore the value that must not be exceeded in order to maintain the soil's capacity to neutralise the excess acid and thus maintain the vital parameters for flora and fauna. These parameters may be the pH of soils and of surface waters, dissolved calcium and aluminium concentrations and/or combinations of these parameters, and others.

Because the effects of acid deposition vary geographically depending on the sensitivity of ecosystems, the critical load is determined for each one. The mapping work required is therefore considerable.

The critical load concept forms the basis for the Protocol for reducing emissions of SO2, NOx, NH3 (as well as VOCs) adopted by the United Nations Economic Commission for Europe (UNECE) and for EU Directive 2001/81/EC on emission caps. Emission caps are determined for each country, with the aim of reducing the extent of the areas affected by acid deposition in excess of the critical load values.

The substantial reduction in European emissions of SO2, and also NOx have reduced acid deposition everywhere in Europe. Figure 8 shows progress achieved in Europe since 1990. The percentage of areas exposed to acidification risks dropped from 35% in 1990 to 9% in 2010 [EMEP2012b].

Figure 8 : Change in acid deposition from 1990 to 2010 (eq/hect.year) [EMEP2012b]

IV.1.3 Acidification Figure8

In 2005, for example, acid deposition was in excess of the critical load in forests covering 160 000 km2, or 12% of all forested areas in the EU-28. The figures are expected to improve within the next 10 years following the application of measures developed not only to reduce emissions of SO2 and NOx at source, but also to ensure compliance with undertakings to reduce GHG emissions [COMM2013]. By 2025, current regulations are expected to reduce exposed areas by 100 000 km2, leaving 50 000 km2 (or 4%) still in excess of critical loads.

The figure below shows the percentage of forested areas with acid deposition in excess of critical loads in 2005 and in 2025 according to the CLE (current legislation) [COMM2013] scenario.

Figure 9 : Percentage of forested areas with acid deposition in excess of critical loads in 2005 and in 2025 according to the CLE (current legislation) [COMM2013] scenario

IV.1.3 Acidification Figure9

 

Nitrogen deposition

Nitrogen deposition in an ecosystem gradually changes its plant composition: nitrophilous species, for example, will thrive at the expense of species that prefer poorer soils, thus reducing biodiversity.

The reductions in NOx and NH3 emissions achieved in Europe have in turn reduced nitrogen deposition in Europe. Figure 10 shows progress achieved in Europe since 1990. The percentage of areas exposed to nitrogen deposition risks dropped from 75% in 1990 to 62% in 2010 [EMEP2012b].

Figure 10 : Changes in the deposition of reduced nitrogen from 1990 to 2010 (mgN/m2) [EMEP2012b]

IV.1.3 Acidification Figure10

In 2005, for example, critical loads for eutrophication were exceeded across 1.1 million km2, or 66% of ecosystem areas in the 28 Member states. The emission reduction measures being applied by the different countries should reduce areas affected by excess nitrogen by 0.9 million km2 [COMM2013].

The figure below shows the percentage of ecosystem areas with eutrophication in excess of critical loads in 2005 and in 2025 according to the CLE (current legislation) scenario [COMM2013].

Figure 11 : percentage of ecosystem areas with eutrophication in excess of critical loads in 2005 and in 2025 according to the CLE (current legislation) [COMM2013] scenario.

IV.1.3 Acidification Figure11

Biodiversity is adversely affected by excess nitrogen. Biodiversity is under threat in 77% of the areas under Natura 2000 protection (423 000 km2). With planned reductions in NOx emissions, the figure should drop to 62%, but 343 000 km2 will still be under threat [COMM2013].

Figure 12 : percentage of Natura 2000 areas with eutrophication in excess of critical loads in 2005 and in 2025 according to the CLE (current legislation) [COMM2013] scenario.

IV.1.3 Acidification Figure12

 

References

[EMEP 2012a] M. Gauss, ´A. Ny´ıri, B. M. Steensen and H. Klein - EMEP/MSC-W Norwegian Meteorological Institute
"Transboundary air pollution by main pollutants (S, N, O3) and PM in 2010 - France" July 2012

[EMEP 2012b] EMEP Status Report 2012; July 18, 2012 Transboundary - Acidification, Eutrophication and Ground Level Ozone in Europe in 2010 - ISSN 1504-6109 (print) - EMEP/MSC-W Norwegian Meteorological Institute - ISSN 1504-6192 (on-line)

[COMM2013] M POSH – RIVM cite dans l'étude d'impact du paquet Air Commission européenne, SWD(2013)531