Nettles are favoured by nitrogen deposition. Meadow species such as orchids and birdsfoot trefoil are disadvantaged by nitrogen deposition. Photo: / BIOdiversity Heritage Library CC BY

Ecosystems hit by air pollutant fallout

Three-quarters of EU ecosystems are currently exposed to more nitrogen deposition than they can cope with and nearly one-tenth is still receiving too much acid fallout.

The concept of critical loads was introduced in the 1980s and has been used in Europe for developing cost-effective air pollution abatement strategies. Critical loads are scientific estimates of the amounts of pollutants that various ecosystems can tolerate without being harmed. They are sometimes referred to as the limits on what “nature can tolerate”. If pollutant depositions exceed the critical load limits, damage to sensitive ecosystems will by definition occur sooner or later.

The sensitivity of various ecosystems to exposure to acidifying and eutrophying air pollutants has been monitored and mapped for more than 25 years, and European countries have coordinated this work through the Coordination Centre for Effects (CCE) of the Convention on Long-range Transboundary Air Pollution (CLRTAP).

By comparing the critical load maps with data on air pollutant deposition, the CCE has also produced maps that show the extent to which European ecosystems are exposed to more air pollutant depositions than they can tolerate in the long term without damage, i.e. where the critical load limits for acidification and eutrophication are exceeded.

This information has, together with data on people’s exposure to air pollutants, been used as input for the development of fundamentally important air pollution policies, such as the CLRTAP’s Gothenburg Protocol and the EU’s National Emission Ceilings (NEC) directive.

Located at the Dutch National Institute for Public Health and the Environment (RIVM) in Bilthoven, the CCE has since 1990 been supported by the Netherlands. But due to lack of continued financial support this has now come to an end. Responsibility for the European ecosystem mapping activities will instead be taken over by Germany and its Environmental Protection Agency.

Late last year the CCE published its final critical loads mapping report, providing maps and country-by-country data on exceedance of critical loads for acidification and eutrophication as well as more preliminary information on biodiversity-critical loads.

Looking at the situation in 2005 – which is the base year applied in the 2012 Gothenburg Protocol and the 2016 NEC directive – the highest levels of exceedance of acidification critical loads were found in the Czech Republic, the Netherlands, Germany, Poland and Switzerland. For eutrophication, the countries exposed to the highest exceedance levels were the Netherlands, Luxembourg and Germany.

In that same year, acidity critical loads were being exceeded in 14 per cent of the ecosystems in the EU (11% in the whole of Europe). The area exposed to nitrogen overload, i.e. at risk of eutrophication, extended over 81 per cent of EU ecosystems (67% in Europe). For biodiversity, the figures were 29 and 27 per cent, respectively. See table and maps.

Following emission cuts over the last 40 years in the main acidifying air pollutants, especially sulphur dioxide (SO2), the area of sensitive ecosystems at risk of acidification in Europe has now shrunk significantly, to seven per cent, or 205,000 square kilometres (km2). For the EU, eight per cent of the ecosystem area (137,000 km2) received excess acid deposition in 2015.

Progress is however markedly slower for eutrophication, which is caused by excess nitrogen deposition resulting from emissions of nitrogen oxides (NOx) and ammonia (NH3). While the area at risk in Europe has shrunk somewhat since peaking around 1990, it still covers nearly 1.7 million km2 (63%). Specifically for the EU, 1.1 million km2 of the ecosystem area (77%) was exposed to excess nitrogen fallout in 2015.

It should be noted that these figures and maps give a snapshot of deposition versus ability to resist at a given point in time – they do not really reflect the true environmental situation right now. Environmental monitoring, experiments and calculations show that there may be considerable time lags in response (damage) to excess deposition exposure as well as in recovery once pollution levels come down. This means that the damage that has already been caused by excess air pollutant inputs may persist for decades, in some places even for centuries.

Clearly there is still a long way to go to actually achieve the long-term environmental objectives of the EU’s 7th Environmental Action Programme, one of which is that there should be no exceedance of the critical loads for acidification and eutrophication. The same objective is also enshrined in the CLRTAP Gothenburg Protocol and in the EU’s NEC directive.

Christer Ågren


European critical loads: database, biodiversity and ecosystems at risk. CCE Final Report 2017. By J-P Hettelingh, M. Posch, and J. Slootweg.

Modelling and mapping of the impacts of atmospheric deposition of nitrogen and sulphur: CCE Status Report 2015. By J. Slootweg, M. Posch, and J-P Hettelingh.


Personal communication with J-P Hettelingh and M. Posch (February 2018).

Figure: Areas where critical loads for acidification are exceeded by acid depositions (top) and areas where critical loads for eutrophication are exceeded (bottom) at 2005 (left) and 2015 (right) emission levels.


Table: Percentage area of ecosystems exposed to excess deposition of eutrophying and acidifying air pollutants in 2005 and 2015.

Note: There are some minor differences between the figures for 2005 in this table and those in CCE Final Report 2017, as the figures presented here are based on the most recently available updated emission data.


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