3.4. transboundary air pollution

133Transboundary air pollution

3.4. Transboundary air pollution

Transboundary air pollution (generated in one country and impacting in others) makes amajor contribution to acidification and summer smog (caused by tropospheric ozone), andalso to eutrophication of soil and water and dispersion of hazardous substances. The mainsources of this pollution are energy use and transport in which international shipping is ofgrowing importance. The cost-effectiveness of measures to reduce emissions from shipshas been demonstrated by the European Commission in its strategy to combatacidification. However, sufficient measures are yet to be implemented.

Major emission reductions for sulphur dioxide and nitrogen dioxide adopted under theConvention on Long-Range Transboundary Air Pollution (CLRTAP) and EU legislation havereduced the harmful effects of transboundary air pollution. Projected further reductionsfall short of EU targets for 2000 and 2010 and further intiatives are needed in theframework of integrated abatement strategies.

Projections for 2010 suggest that, despite the projected emission reductions, areas of theEU and (especially) the Accession Countries will continue to be affected by acid andnitrogen deposition above the level defined as the ‘critical load’. Ecosystems in the EUstill receive 7% acid deposition and 39% nitrogen deposition above their critical loads.European countries where over 70% of ecosystems will still be affected by excessnitrogen deposition include the Czech Republic, Lithuania, Poland, the Slovak Republicand Switzerland.

With respect to ozone, despite the considerable efforts in precursor emission reductions,the long-term objective for protection of crops is expected to be reached only in thenorth-west parts of Europe (Ireland, Scandinavia). The health-related ozone thresholdconcentration will continue to be exceeded 50 days per year. The highest number ofexceedances will be found in the more densely-populated north-western part of Europe(Netherlands, Belgium and northern France).

Moreover, despite reductions in precursor emissions, smog will remain a health threat dueto increases in ozone worldwide: this calls for action on a global scale to reduce emissionsof carbon dioxide, nitrogen oxides and methane.

Main findings

1. Transboundary air pollution: a complex problem

1.1. It is all intertwinedTransboundary air pollution is a pan-Euro-pean problem, requiring pan-Europeansolutions. The main causes are emissionsfrom transport and energy usage of sulphurdioxide (SO2), nitrogen oxides (NOx),volatile organic compounds (VOCs) andvarious toxic materials such as heavy metalsand persistent organic pollutants (POPs).These pollutants (particularly carbon mon-oxide (CO) and methane (CH4)) canremain in the atmosphere sufficiently longto be transported thousands of kilometresand thus to spread over the whole of Europe,across national borders, far from the originalsources of polluting emissions.

The main effects are manifested in acidifica-tion of water and soil, summer smog causedby tropospheric ozone, eutrophication ofsoils and waters, and dispersion of hazardoussubstances (see also Chapter 3.3). Theenvironmental impacts caused by the mainpollutants SO2, NOx, VOCs, ammonia (NH3)and toxic substances are summarised inTable 3.4.1.

Environmental Impact Caused by

Acidification SO2, NOx, NH3

Eutrophication NOx, NH3

Ozone VOCs, NOx

Bioaccumulation of toxic substances Heavy metals, POPs

Table 3.4.1.Main air pollutants and their environmental impacts

Environmental Issues134

Table 3.4.1 is of course a simplified summary,which abstracts from a reality of complexinteractions. For instance, all the acidifyingcompounds are precursors of particulateswhich are harmful to human health, andVOC species such as benzene also have toxiceffects and contribute to the formation ofharmful particulates. VOC and NOx emissioncontrols vary in their effectiveness in reducingozone concentrations. As a rule of thumb,ozone concentrations in highly polluted areasare most efficiently reduced by a combinationof NOx and VOC controls. In regions with lowNOx concentrations, both NOx and VOCcontrol reduces ozone levels but NOx controlis more effective. In parts of north-westEurope, reductions in NOx emissions alonewill actually increase ozone levels, although areduction in NOx emissions is still desirable tomitigate effects of acidification andeutrophication and to reduce ozone forma-tion on the hemispheric and global scale.

Further complicating factors are the multi-ple environmental impacts of human activi-ties. Thus, for example, energy-savingmeasures to reduce CO2 emissions and theensuing pollutants released mitigate climate

change (see Chapter 3.1) and also reduceSO2, NOx and CO emissions, with beneficialimpacts on acidification, tropospheric ozoneand urban air quality (Figure 3.4.1).

1.2. Targets and policies at European levelThe UNECE Convention on Long-RangeTransboundary Air Pollution (CLTRAP)signed in Geneva in 1979 was the first multi-lateral treaty concerning air pollution. It hasprovided an important demonstration thatinternational co-operation can achieveresults. During its first 10-15 years, theCLRTAP resulted in protocols aiming at areduction in acidifying emissions and abate-ment of ozone concentrations. Protocols onheavy metals and persistent organic pollutants(1998) aim at a reduction of emissions ofthese toxic materials. A second protocol onNOx and NH3 emissions is under negotiation.

Parties to the CLRTAP were committed to astabilisation of NOx emissions at the 1987level by December 1994. The VOC protocolsigned in 1991 requires emissions to beeither stabilised or reduced by at least 30%from the base year (usually 1988) in 1999.The second Sulphur Protocol signed in 1994

CO

CH4

NOX

VOCs

NH3 NOX

CO

NOX SO2 PM

VOCs

Urban airquality

Climate change

Fewer droughts, floods, storms,and agricultural changes etc.

Reduced ill healthReduced losses of fishbiodiversity and amenity

Eutrophication

CO2CH4 N2O

Reduced damage to forests,soils, fish and buildings

Reduced ill health,agricultural losses

TrophosphericOzoneAcidification

NH3 NOX SO2

to multiple benefits

Lower emissions lead

Energy Transport

Agriculture

Household Industry

Figure 3.4.1 Multi-pollutants, multi-effects

Source: EEA


has the objective to reduce the extent bywhich critical loads are exceeded in Europeby 60% in the year 2000; the various coun-tries are required to reduce their emissionsby different amounts. The Protocol also hasobligatory clauses regarding emissionsstandards, including application of ‘BestAvailable Technology’ emission standards fornew plants. A further requirement is areduction in sulphur content to 0.2% and to0.05% respectively in gas oil for stationarysources and in diesel fuel for road traffic.

EU limit, guide or target values for airquality concentration levels of pollutantsincluding SO2, NO2 and ozone are nowbeing revised through so-called ‘daughterdirectives’ to the Air Quality FrameworkDirective (96/62/EC).

The EU Fifth Environmental Action Pro-gramme (5EAP) set emission targets up tothe end of the century for reduction ofacidifying compounds and ozone precursors.Further targets, up to 2010, were proposedin the 1997 Community Strategy to combatacidification (European Commission, 1997;Table 3.4.2). This strategy emphasised themulti-pollutant multi-effect framework andshowed that, in terms of cost-effectiveness,there is more potential for reducing sulphurrather than nitrogen emissions; this isbecause SO2 is mainly emitted from a smallnumber of relatively controllable largesources (power plants) whereas NOx is alsoemitted from a wide range of smallersources, including vehicles.

New strategies for emission abatement facethe challenge of satisfying protection re-quirements from all inter-related effectscaused by tropospheric ozone, acidificationand eutrophication. The Commission iscurrently developing an ozone-abatementstrategy. In line with the 5EAP, the aim is toaddress the ‘symptoms’ by setting interimand long-term air quality objectives and the‘causes’ by developing strategies and meas-ures to reduce polluting emissions and alsoto stimulate changes in society’s patterns ofbehaviour. The new strategy will be linkedwith provisional national emission ceilingsfor SO2, NOx, NH3 and VOC.

More specific measures for abating precur-sor emissions are defined in a number of EUDirectives:

• Large Combustion Plants Directive.• Directives on Sulphur Content of Cer-

tain Liquid Fuels.

• Integrated Pollution Prevention andControl Directive.

• Implementation of measures developedthrough the Auto-Oil Programme:• A two-step tightening of vehicle

emission limit values for passengercars and light commercial vehicleswith the first step in the year 2000 andthe second step in 2005;

• new environmental specifications forpetrol and diesel fuels to take effectfrom the year 2000; very low-sulphurfuels to be mandatory from 2005;

• provision made for earlier phase-in ofvery low-sulphur fuels;

• phase-out of leaded fuels by 2000.• The Solvent Directive and the Directive

to reduce emissions from storage anddistribution of petrol aim at reduction ofVOC emissions from stationary sourcesand at all stages of the chain of petrolstorage, distribution and usage.

Guided by the work done by the WHO onhuman-health effects and by UNECE oneffects on vegetation, the Commission hasproposed new ozone reference levels (Table3.4.3). In addition to the long-term objectivefor the protection of human health, thresh-old values to inform both the general andinformed population have been defined. Forthe protection of human health, short-termconcentrations (peak values over one toeight hours) are of concern. For protectionof agricultural crops and forests, ozoneconcentrations integrated over the growingseason are of importance.

2. Trends in emissions

2.1. Emissions of sulphur dioxide (SO2)Since 1980, SO2 emissions in Europe havebeen reduced considerably: by approximately50% over the period 1980-1996 (Figure 3.4.2)although in comparisons between countries,allowance should be made for differences ininitial emission rates and economic circum-stances. In the EU, the 5EAP target of 35%

EU air emissions reduction targets Table 3.4.2.

SO2 NOx VOCs

5EAP 35% (1985-2000) 30% (1990-2000) 30% (1990-99)

CLRTAP 62% (1980-2000) stabilise 1987 emission 30% (1987-99)levels by 1994

COM(97)88 84% (1990-2010) 55% (1990-2010)(proposed targets)

Source: European Commssion, 1997; UN-ECE CLRTAP, 1998


Table 3.4.3.

Ozone concentrations integrated over a certain period of time are indicated as AOTxx-values where xx is a threshold valueexpressed in ppb (1 ppb O3 = 2 µg/m3 O3). The AOT40, for example, is defined as the sum of all excess concentrations abovethe threshold of 40 ppb (80 µg/m3). It is calculated by subtracting 80 µg/m3 from all hourly concentrations and subsequentlysumming all positive values.

* In addition to the Long-term Objective, a Target value has been proposed: by the year 2011 a daily eight-hour maximumof 120 mg/m3 may not be exceeded more than 20-25 times per calendar year averaged over 3 years. Current legislation (92/72/EC) has the limit value 110 mg/m3 average over a fixed eight -hour period.

** In addition to the Long-term Objective, a Target value has been proposed: by the year 2011 the AOT40 value may not exceed18 000 mg/m3.h averaged over 5 years. Current legislation (92/72/EC) has the limit value 65 mg/m3 average over 24 hours.

Ozone air quality reference levels proposed in the working EU draft Ozone Directive

Concern

Health

Damage to crops andvegetation

Description

Long Term Objectivevalue for the protectionof human health

Alert threshold sensitivepopulation

Alert threshold generalpopulation

Long Term Objectivevalue for the protectionof vegetation

Reference levelconcerning visibledamage on forests

Metric

Moving 8 hours averageconcentration

1 hour averageconcentration

1 hour averageconcentration

AOT40, May-July, 08.00-20.00 CET

AOT40, April-September,08.00-20.00 CET

Value

120 mg/m3 *

180 mg/m3

240 mg/m3

6 000 mg/m3.h**

20 000 mg/m3.h

Source: EuropaenCommission, 1999b

reduction from 1985 levels by the year 2000was met in 1994. EU emissions were reducedfrom 19 600 ktonnes in 1985 to 10 700ktonnes in 1995. The reduction, which willcontinue, is expected to meet the commit-ments of the Second Sulphur Protocol.

The decrease in emissions from the Acces-sion Countries is most notable since 1990,largely caused by the economic reconstruc-

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CRLTAPTarget

tion in these countries. Since 1994, the 5EAPtarget of 35% reduction has also been metby Accession Countries, as result of thegeneral European overfulfilment of the 30%emission reduction objective set by the FirstSulphur Protocol. At present, the relativereduction in emissions from the AccessionCountries is similar to that in the EU, whiletheir total emissions amounts to approxi-mately one-quarter of EU emissions.

Figure 3.4.2 Sulphur dioxide emissions were strongly reduced between 1980 and 1996 (1980 = 100%)

In general, Parties to the1985 LRTAP-Sulphur

Protocol (on the left) haveachieved a larger reduction

than other countries (on theright). Data officially

submitted is in dark colour,estimates are in light colour.

Source: EMEP/MSC-W,Report 1/98


2.2. Emissions of nitrogen oxides (NOx) and ammonia (NH3)Emission targets for nitrogen oxides set inthe 5EAP are far more restrictive than thestabilisation at 1987 levels required by theFirst NOx Protocol of the LRTAP Conven-tion. By 1994, the average NOx emissionreduction for the EEA and the AccessionCountries was 13%, thus complying with thecommitments of the First NOx Protocol(Figure 3.4.3). However, the EU (which hada reduction of 11%) is unlikely to achievethe 5th EAP emission target for 2000 of a30% reduction; in contrast, the AccessionCountries have already met this targetaccording to the most recent official emis-sion estimate.

Reported ammonia emissions decreased by14% in the EEA and the Accession Coun-tries, from 5 000 ktonnes in 1990 to 4 300ktonnes in 1996. Again, emission reductionsin Accession Countries are considerablyhigher (28%) than those reported from theEU (8%). At present, there are no emissiontargets for ammonia, but these are expectedto be specified in the new multi-effect multi-pollutant directives and protocols.

2.3. Emissions of volatile organic compounds (VOCs)The requirements for reduction of emissionsof non-methane volatile organic compounds(VOCs) by 30% in 1999 are different on thechoice of the base year: 1990 for the 5EAP

and 1987 for the VOC Protocol. For the year1995, Accession Countries reported emis-sions well on the way to complying with theVOC Protocol, while VOC emission reduc-tion in the EU amounted to 11% withrespect to 1990 levels and 15% with respectto 1987 levels. EU emissions were reportedto be 11 500 ktonnes in 1995, that is, sixtimes higher than the reported emissionsfrom the Accession Countries (Figure 3.4.4).Therefore, as in the case of NOx emissions, itis unlikely that the 5EAP VOC target emis-sion reduction can be reached by 1999.

In contrast to other pollutants, a large pro-portion of VOCs are emitted by naturalprocesses. Biogenic VOC emission stronglydepends on temperature. During ozone smogepisodes, the natural contribution might beas high as 60% (Slanina et al., 1998) while onan annual basis, biogenic emission contrib-utes 40% of total VOC emissions over Europe(Simpson et al., 1999).

3. Photochemical smog

Photochemical smog, or simply summer smog,is formed in the lowest few kilometres of theatmosphere. It has adverse effects on humanhealth, agricultural crops, natural vegeta-tion, and materials. The main component ofsmog is ozone, although levels of otheraggressive oxidants are elevated as well.There is, however, only limited information

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Party to 1988 LRTAP NOx protocol (estimated)

Other countries


CRLTAPTarget

Parties to the 1988 NOx

Protocol of the Conventionon LRTAP are on the left inthe diagram. Othercountries are on the right.Data officially submitted is ina dark colour, estimates arein a light colour.

Source: EMEP/MSC-WReport 1/98

Figure 3.4.3Compared with 1987 (=100%) emissions of nitrogen oxides in 1996 show a reduction in most countries.


Parties to the CLRTAP VOCProtocol are on the left

(officially submitted data indark blue, estimates in light

blue). Non-parties to theProtocol are on the right

(officially submitted data inyellow, estimates in brown).

Source: EMEP MSC-WReport 1/98

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Figure 3.4.4 Compared with 1988 (=100%) most countries have achieved the reduction in VOC emissions

available on concentrations and effects ofthese other oxidants. As no significant effectshave been observed at current ambientlevels, no international guidelines have beenset for the other photochemical oxidantsfound in summer smog.

Ozone and its precursors have an atmos-pheric residence time of several days ormore. Under average summer conditions, airpollutants are transported over distances of400-500 km per day. Besides thetransboundary character, summer smog mayshow a more local character. For example, inthe Mediterranean Basin, air masses re-entering the source regions have beenshown to raise pollution levels (Millan et al.,1997).

High levels of ozone occur during shortperiods of two to four days. Threshold valuesset for the protection of human health areexceeded regularly over large parts ofEurope. Smog episodes are often foundduring summer over most parts of thecontinent, during periods with clear skies,increased solar radiation and elevatedtemperatures (Map 3.4.1). The occurrenceand duration of these episodes variesstrongly from one year to the next.

3.1. Accumulated exposureBased on a series of controlled exposureexperiments, the concept of AOT40 (accu-mulated ozone hourly concentrations above80 µg/m3) is chosen as an indicator forexposure of ecosystems. The great advantageof the AOT concept is that it is indicative of

the ozone situation during the relevantgrowing season. The AOT is a practicalindicator for ozone effects on vegetation. Itmust be realised, however, that it is only afirst, crude estimate of the real ozone expo-sure of vegetation. Due to its simplifications,the link between AOT and actual vegetationdamage might be weak.

First of all, the AOT gives a potential but notan actual exposure of vegetation. Under dryand hot conditions, the plant protects itselfby closing its stomata. Although ozoneconcentrations are likely to be high underthese conditions, the closed stomata preventuptake of ozone and the damaging effects ofozone will be small. In the AOT concept thiscan be included by adding a constraint onrelative humidity. This makes it, however,more complex and less transparent.

In the newly proposed daughter directive onozone, the integration time of AOT is clearlydefined: May to July using all one-hourlyvalues measured between 8 AM and 8 PMCET. This period may differ from the activeperiod of the plants. Daylight hours, definedas solar radiation above a certain minimummight be more appropriate. Also the grow-ing season varies all over Europe and inplaces extends beyond the three monthsspecified in the directive.

The simple and clear definition of AOTsuggests that monitoring will be easy. Unfor-tunately, there are a number of complicatingfactors which make the observed AOT valuesensitive to monitoring conditions:


more than 104

92 – 10480 – 92

68 – 80

less than 68

0 1000 km

Exceedance in µg/m³of the threshold 120 µg/m³,accumulated hourly values

in the EMEP 150 – grid

Mean of dailysummer maximum

concentration of ozone

Modelled from 1990 level emissions.Meteorology from selected years.

• the measuring height: monitoring ozoneat a height of 1 m (crops) or at 10 m (atcrown level of trees) might result in afactor of 2-3 difference in AOT;

• missing data will lead to systematicunderestimation of the AOT; an agreedcorrection method will be required.

For practical reasons, the time resolutionapplied in the current atmospheric models issix hours, which is too coarse to calculate anAOT without further assumption on thediurnal variation of ozone. This problem willbe solved in the next generation of models.Notwithstanding these drawbacks, theAOT40 is, according to current knowledge,the best indicator for vegetation exposure.

3.2. Threshold values for ozone are exceededAll EU threshold values set for ozone underthe current Ozone Directive (92/72/EC)have been exceeded since 1994, the year theDirective came into force. Thresholds set forthe protection of human health and vegeta-tion are exceeded in all Member States everyyear. Short-term thresholds for protection ofvegetation and the threshold values forpublic information are exceeded in mostMember States. The threshold concentrationfor warning the population is exceeded onlyoccasionally (Beck et al., 1998).

One of the 5EAP targets is that air qualityguidelines recommended by the WHOshould be mandatory by 1998. Routinemonitoring (Hjellbrekke, 1997; de Leeuwand de Paus, 1998) indicates that the WHOvalue of 120 µg/m3 eight-hour average isexceeded in all EU Member States and in allother European states which report data.Model calculations, allowing for estimationof exceedance in the whole of the Europeanterritory (Map 3.4.2) indicate that 99% ofEU inhabitants are exposed to at least oneexceedance per year. For the development ofabatement strategies, the AOT60 (accumu-lated excess above 120 µg/m3) has beentaken as indicative of ozone health impacts.

According to WHO recommendations, theAOT40 for daylight hours in May-July shouldnot exceed 6 000 µg/m3.h for the protectionof natural vegetation and the prevention ofmore than 5% crop yield loss (this is also thecritical level for ozone protection of cropsunder CLRTAP). According to observations,this AOT40 parameter exceeds 6 000 µg/m3.h in most of the EU, with the exceptionof northern parts of both Scandinavia andthe UK (Hjellbrekke, 1997). Modelled resultsindicate that, averaged over five years, 94%

less than 200200 – 2 000

2 000 – 6 000

6 000 – 12 000

more than 12 000

0 1000 km

Concentration in µg/m³/hexceeding the threshold

of 120 µg/m³/hin the EMEP 150 – grid

Accumulated excessof ozone concentration

areas withno exceedance

Modelled from 1990 level emissions.Meteorology from selected years.

Map 3.4.2Almost the entire population of Europe is exposed to ozone levels exceeding the WHOguideline for protection of human health. This map shows the average accumulated excess ofozone concentration above a level of 60 ppb (=120 mg/m3) modelled over five years.

Source: Simpson et al., 1997

of arable land is exposed to exceedance ofthis WHO guideline.

For protection of forests, the WHO andCLRTAP have set a guideline/critical level ofAOT40, 24 hours, April-September of 20.000µg/m3.h. Both monitoring and modelleddata indicate that Scandinavia, Ireland andthe UK are below the critical level mostyears. In the rest of Europe, the level isexceeded by up to a factor of three. Calcula-tions indicate that about 35% of Europe’s

Map 3.4.1The model indicates a cleargradient over Europe:summertime concentrationsranging from 60 mg/m3 inthe northern part to morethan 100 mg/m3 in thecentral part and in theMediterranean region (1 ppbO3 » 2 mg/m3).

Source: Simpson et al., 1997

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coniferous forests and 70% of broadleafforests experience exceedances.

3.3. Ozone levels vary from year to year and over EuropeOzone concentrations, particularly peakvalues, vary considerably from year to yeardue to specific meteorological conditions,whose occurrence is very variable in timeand space (Figure 3.4.5). Typical variationsfrom year to year for peak concentrations(98 percentile) are 10-15%, while AOT40may show variations of about a factor ofthree.

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Num

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Exceedance of threshold 65 ug/m3 (24h)

Threshold values defined in the EU Ozone Directive(92/72/EC) are exceeded frequently and widely

Figure 3.4.5

This figure shows thenumber of days as a function

of the minimum number ofMember States where at

least one exceedance of athreshold value was

observed in 1997 (e.g. on174 days the threshold valueof 110 mg/m3 was exceeded

in at least four MemberStates). Note: information

on exceedances of 65 mg/m3 and 110 mg/m3 were

submitted by 14 MemberStates only.

Source: de Leeuw and dePaus, 1998

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Distribution of ozone concentrations overEurope also differs considerably from year toyear. While the mean daily maximum value ishighest in Italy and France, exceeding 104µg/m3 (see Map 3.4.1), the AOT60 is highestin northern France, Belgium and southwestern Germany (see Map 3.4.2).

Average ozone concentration in summershows an increase from north-west to centralEurope. In mountainous regions, averageozone concentrations are higher than inlowland plains. There are indications, still tobe confirmed over the long-term, that peakozone levels are higher in southern Europethan in the north. Characteristics of ozoneformation and transport in major Mediterra-nean cities and coastal areas differ fromthose normally occurring in northerncontinental areas (Millan et al., 1997).

3.4. Present trends in ozone are small, and hard to detectIn the late 1990s, rural ozone levels are threetimes higher than in the pre-industrial era.Around the year 1880, average ozone levelsat a rural site near Paris were about 20 µg/m3, but by 1950, rural ozone levels hadincreased to 30-40 µg/m3, and to 60 µg/m3

around 1980. However, trends in rural ozonein the last decade are statistically insignifi-cant (Beck et al., 1998). Studies areunderway to try to explain these trends fromthe analysis of the trends in the emissions ofozone precursors (Lindskog et al., 1998).

Trends in ozone episodes are even moredifficult to assess because of annual variabil-ity of peak ozone concentrations (Figure3.4.6). A recent analysis of peak ozone datasubmitted under the Ozone Directive (deLeeuw et al., 1997) showed for 68 stationsover the period 1989-1997 a significantupward trend at only two stations and adownward trend at 22 stations. For themajority of the 68 stations analysed, nosignificant trend was detected.

A detailed analysis of ozone data in the UK(PORG, 1997) showed a decrease in peakozone levels. The hourly maximum ozoneconcentrations, averaged per month overthe period 1986-1995, are 40-60 µg/m3 belowthat during the period 1972-1985. At nineout of 16 rural stations, there is however asignificant positive trend in annual meanconcentrations.

Urban ozone showed downward trends inCentral London between 1973 and 1992,while monthly means increased by 15% per


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Ozone concentrations differ widely betweenmonitoring stations Figure 3.4.6

The reason for thisdifference is the local impactof NOx sources: in urbanareas and especially instreets the locally emittedtraffic emissions result in atemporary reduction ofozone levels. Furtherdownwind, ozone levels willresume previous high levels.

Source: de Leeuw et al.,1997

year at a suburban station in Athens in theperiod 1984-1989.

Ozone concentrations in ‘unpolluted’ airmasses arriving over the North Atlanticregion at the Irish station Mace Head ap-peared to increase over the 1990-1996period with 0.18 µg/m3 per year equivalentto 0.2% per year (PORG, 1997). Modelcalculations on a global scale support thefindings of an ongoing change in chemicalcomposition of tropospheric background air(Parrish et al., 1993; Collins et al., 1998;Berntsen et al., 1997; Hov and Flatøy, 1997).Current daily mean concentrations of ozoneare three to four times higher than in thepre-industrial era mainly as a result of theconsiderable growth in NOx emissions fromindustry and the transport sector.

4. Acidification

4.1. Critical loadsAcid deposition originates largely from man-made emissions of sulphur dioxide (SO2),nitrogen oxides (NOx) and ammonia (NH3).Major sources of these pollutants are theusage of fossil fuels for power generation,transport and agricultural practice. Deposi-tion of these three primary components andtheir secondary-reaction products leads toacidification (sulphur and nitrogen deposi-tion) and euthrophication (nitrogen deposi-tion). Acidification is causing damage tofreshwater systems, forest soils and naturalecosystems in large areas of Europe. Theeffects of acidification have been evident invarious different ways, including defoliationand reduced vitality of trees, declining fishstocks and decreased diversity in acid-sensitivelakes, rivers and streams and changes in soilchemistry. Nitrogen contributes also to theeutrophication of terrestrial and marineecosystems and it is an important precursor ofground-level ozone. Along with these effectson ecosystems, there are also corrosion effectson materials, cultural monuments andbuildings. In addition, there is growingconcern about the adverse health effectsrelated to secondary-reaction products ofacidifying and eutrophying pollutants.

Since the signature of the 1979 LRTAPConvention, significant reductions in harmfulemissions have been achieved. Increasingattention is now being given to the analysis ofeffects of emissions, in order to differentiatereduction commitments according to thevarying sensitivity of the natural environment.There is now an increasing focus on the

concept of ‘critical load’, defined as thehighest deposition of chemical compoundsthat will not cause long-term harmful effectson the ecosystem structure and function, inorder to target emission reductions towardsthe actual damage. The critical load dependson characteristics of the soil and ecosystemand varies strongly over Europe. Lake systemsin Scandinavia are very sensitive (low criticalload) whereas the Iberian Peninsula is lesssensitive – partly because the deposition ofbase cations (e.g. Sahara dust) neutralises theacid deposition to some extent.

The effects of acidification on forests andfreshwater ecosystems and the effects ofeutrophication on terrestrial ecosystems areincluded in the present definition of criticalloads. Other effects, such as corrosiondamage to monuments, eutrophication ofmarine ecosystems, or health effects, are notincluded within the critical load approach.The new strategies for emission reductionface the challenge of satisfying protectionrequirements from all these inter-relatedeffects.


4.2. Exceedances of critical loadsSulphur and nitrogen deposition combinedcontributes to exceedance of critical loads(Maps 3.4.3, 3.4.4, and 3.4.5; excess deposi-tion was calculated as the percentage ofecosystems in each grid with acid depositionabove the critical loads, for scenario calcula-tions in preparation for the Second CLRTAPSulphur Protocol. The excess depositionover the fifth percentile was used as the maincriterion driving emission reductions.)

Pollution ‘hot spots’ can be identified, suchas the so-called Black Triangle (Box 3.4.1).Exceedances of critical loads of acidificationand eutrophication over Europe are atpresent mostly dominated by nitrogendeposition, the relative importance of whichis increasing, showing the need for a newnitrogen protocol. There are different waysto determine critical loads and theirexceedances, because the amount of nitro-gen deposition that an ecosystem maytolerate without harmful effects also dependson the level of the sulphur deposition andvice versa. The critical load function for eachseparate ecosystem can be approximatedtaking account of sulphur deposition,acidifying nitrogen and eutrofying nitrogen.Exceedances are recorded for each particu-lar ecosystem whenever the actual deposi-tions fall outside the function.

In preparation for the EU acidificationstrategy, an ‘area of exceedance’ (AE)concept was developed for scenario analysis.The alternative way to calculate exceedanceswas to consider the area occupied by theecosystems, rather than the number ofecosystems with depositions exceeding theircritical loads.

Recently, a further refined measure forevaluating ecosystem protection has beendeveloped. The so-called ‘average accumu-lated exceedance’ (AAE) actually calculatesthe exceedance of critical loads for allecosystems in a given grid cell (Posch et al.,1997). Exceedances of individual criticalloads are added for all ecosystems andaveraged in terms of the area covered byeach individual ecosystem in the grid cell.

30˚ 20˚ 10˚ 0˚ 10˚ 20˚ 30˚ 40˚ 50˚ 60˚

60˚

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N o r t hS e a


At

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ic

Oc

ea

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C h a n n e l

more than 2 000

0 1000 km

Loads in eq/hain the EMEP 150 – grid

Exceedanceof critical loads

for sulphur in 1996

1 000 – 2 000

400 – 1 000

200 – 400

40 – 200


less than 40

30˚ 20˚ 10˚ 0˚ 10˚ 20˚ 30˚ 40˚ 50˚ 60˚

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Exceedanceof critical loadsfor eutrophyingnitrogen in 1996

more than 1 000

400 – 1 000

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less than 40

30˚ 20˚ 10˚ 0˚ 10˚ 20˚ 30˚ 40˚ 50˚ 60˚

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Exceedanceof critical loads

for acidifying nitrogenin 1996

more than 1 000

400 – 1 000

200 – 400

40 – 200


less than 40

Map 3.4.3

Map 3.4.4

Map 3.4.5

Source: EMEP


Box 3.4.1. Air pollution in the Black Triangle

On a Europe-wide scale, problems with acidifica-tion are most serious within the Black Triangle.Exceedances of critical loads show their Europeanmaximum over this area and they are primarilyrelated to sulphur deposition, despite effectivereductions in past years. The brown coal belt, whichforms the heart of the Black Triangle, stretches fromLower Silesia (Poland) to Southern Saxony(Germany) and Northern Bohemia (the CzechRepublic). It covers a mountainous area with 6.4million inhabitants. Climate, soils, geomorphology,and geology vary substantially. Poor and acid soilsare common and low levels of critical loads ofsulphur characterise these areas.

The long-term annual precipitation totals are in arange of 500 mm in the basin to 1 350 mm athigher altitudes. Annually, there are more than 50days of continuous fog and 70% of the days arecloudy. Situations of stagnant air often occur duringwinter, causing accumulations and the highestconcentrations of air pollutants.

The intensive exploitation of lignite began in the19th century and the development of mining, heavyand chemical industry intensified after World War II.The most dense concentration of large emissionsources in the Black Triangle region is in industrialareas of Most, Chomutov and Ústí nad Labem inthe Czech Republic and Zwickau, Chemnitz,Dresden, Plauen and Elsterber in Germany.

Between 1989 and 1996, the air emissions from thelargest Black Triangle sources were reducedsignificantly (Figure 3.4.7). Sulphur dioxideemissions were reduced by 50% in Poland and theCzech Republic and to 30% in Germany. Thenitrogen oxides emissions also decreased by 50% inthe Czech Republic and Germany. The improve-ment of air quality in the Black Triangle region isthe result of emission abatement of German andCzech sources, as well as the reconstruction ofTurów Power Plant in Poland. There is also aremarkable decrease of coal consumption by smallsources and households, due to substitution withgas and other types of fuel.

Concentrations of sulphate and nitrates inprecipitation show decreasing trends similar to those

Source: PHARE Topic link -ETC/AQ

0 50 km

Deposition of sulphurin the Black Triangle,

1997

2 000 – 3 000

1 000 – 1 500

500 – 750less than 500

750 – 1 000

1 500 – 2 000

more than 3 000

border of BlackTriangle

Loads in eqH/ha/yr

Map 3.4.6The demarcation is that ofthe EU PHARE-funded BlackTriangle ProjectSource: Phare Topic Link/AQ

for emissions and air pollution. However, thedeposition of sulphur and other acidifying substancesin the Black Triangle area is still fairly high (Map 3.4.6).Sulphur deposition is expected to further decline fromabout 9 g/m2/year in 1990 to 1 g/m2/year in 2010.

The long-term exceedances of critical loads byatmospheric deposition of sulphur are highest on topof the mountains. These are the areas with the worstforest damage. During the period 1972-1989, about50% of the coniferous forests in the Ore Mountainsdisappeared. The most affected areas were on slopesfacing south and south-east. The absolutedeforestation rate decreased from 26.7 km2/year inthe beginning of the period to 7.8 km2/year in 1989.Only 26% of the Bohemian forest, 45% of theSaxonian forest, and 22% of the Silesian forestremains undamaged (Hlobil and Holub, 1997). Airpollution has changed the ecosystem in NorthBohemia, which resulted in soil acidification, loss ofbase cations, changes in plant communitycomposition and forest damage. Deforestationnegatively influences availability and quality of waterand soil erosion.

0

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1989 1990 1991 1992 1993 1994 1995 1996

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0A

nnua

l em

issi

ons

/ k

tonn

es

Czech RepublicGermanyPoland

Czech part of Black TriangleGerman part of Black TrianglePolish part of Black Triangle

Annual emissions Air pollution concentration

Air

po

llutio

n co

ncen

trat

ion

/ ug

/m3

Figure 3.4.7Time courses of total annual emissions and airpollution by sulphur dioxide in the Black Triangle

��

��

��

15˚ 16˚ 17˚14˚13˚12˚

15˚ 16˚ 17˚14˚13˚12˚

50˚

51˚

50˚

51˚

Prague

Plzen

Zwickau

Chemnitz

Dresden

Cottbus

Leipzig

Halle

Gera

Wroc aw

Liberec

Most

Ústí nadLabem

Dêcín

Pardubice

HradecKrálové

Trutnov

Wa brzych

KarlovyVary

S

ax

on

yS

il

es

ia

Bo

h e m i a

G E R M A N YP O L A N D

C Z E C H R E P U B L I C


The various approaches to calculatingexceedances of critical loads have implica-tions for reaching the environmental goalsset by present policies. The improved accu-racy of the AAE approach allows quantifica-tion of environmental improvements thatwere largely underestimated by the calcula-tions based on the AE of critical loads. Forexample, recent calculations (Amann et al.,1998) indicate that scenarios aiming at a90% reduction of the AAE exceedance ofcritical loads (90% AAE gap closure) did notreach a 50% reduction of the areaexceedance in all places(50% AE gap clo-sure), because the AE calculations hadoverestimated the actual exceedances.

The present critical loads evaluate damageto soils, vegetation and freshwater. Theactual types of ecosystems considered in thecalculations vary considerably from countryto country. In all countries, the incorpora-tion of other effects, such as adverse healtheffects or corrosion damage to materials inthe critical load analysis is an importantissue for integrated assessment, as newpolicies and responses will necessarily focuson the diverse effects of closely linkedatmospheric pollutants. In addition, theconsequences for the critical loads resultingfrom the interaction of different pollutantsare not yet well understood. The study ofdamaging effects should combine theanalysis of both tropospheric ozone andacidifying pollutants.

5. Effects on flora and fauna

The assessment of adverse effects on vegeta-tion caused by ozone exposure is based onthe critical level concept. In a series ofinternational workshops organised underthe auspices of UNECE CLRTAP, criticallevels of ozone were agreed upon to preventdamage to the most sensitive crops, forestsand natural vegetation.

The UNECE International Co-operativeprogramme on assessment and monitoringof air pollution effects on Forests (ICPForest) has observed the forests of Europewhere tree crown condition has been dete-riorating on a large scale over several years.Distinct areas with heavily damaged treesexist in various parts of Europe (Box 3.4.2).The deterioration is most severe in theregions of central Europe where sulphur andnitrogen depositions are highest. In some ofthose regions, Scots pine recovered after adecrease in air pollution and improved

weather conditions. Crown conditions areaffected by many stress factors and thetrends of the most common species arerelated to soil and humus types. However,large-scale deterioration over more than adecade is not readily explicable by naturalstresses alone (UNECE, 1998). On a largescale, air pollution is considered as a predis-posing or triggering factor (see also Chapter3.11).

Nitrogen deposition affects ecosystems inparticular in nutrient-poor areas. Theincreasing domination of grass species onmany lowland dry-heathlands in westernEurope can largely be attributed to the effectof excess nutrient deposition. There is in factgrowing evidence that excess nitrogen inputincreases frost and drought sensitivity ofheather.

Transboundary transport and deposition ofheavy metals lead to accumulation of thesemetals in the ecosystem. Acidification alsoincreases the bio-availability of metals. Thelong-term accumulation of metals in thetopsoil and vegetation also leads to increasedconcentrations in animals. It may be con-cluded that high concentrations of cadmiumand lead found in birds and mammals(reindeer, moose) in remote areas areattributable to long-range transport. Thereported concentrations are below the lowestacute effect-levels but sub-lethal effects(impaired vision, co-ordination and bodymovement) might well occur (AMAP,1997).

The tendency of persistent organic pollut-ants to bio-accumulate and bio-magnifyresults in significant exposure levels fororganisms at highest trophic levels, such ashumans and marine mammals. It is knownthat POP exposure affects reproductioneither by diminishing survival of offspring orby disrupting reproductive function andreproductive cycles of adult animals (see alsoChapters 3.3 and 3.11).

Short-term exposure of crops during smogepisodes may lead to visible injuries. How-ever, the largest effect on agricultural cropsis caused by chronic exposure during thegrowing season which might lead to reduc-tion in crop yield. For the Netherlandsalone, it is estimated that a 30% reduction inaverage ozone concentrations during thegrowing season will yield an annual benefitof 200 million euros (Tonneijck et al., 1998).This compares with an estimated cost of halfa million euros per year.


Box 3.4.2 Defoliation

There is a distinct cluster of deterioration in centraland eastern Europe and in Spain. It is well knownthat the air pollution differs strongly between thetwo regions, indicating that other factors than airpollution alone must be responsible for thedecrease. There is however, a clear coincidencebetween the high air pollution in central Europeand the high deposition of acidity. In this areacritical loads are strongly exceeded (Klap et al.,1997). The temporal changes in defoliation for theNorway Spruce, one of the most commonconiferous trees over Europe (Figure 3.4.8) shows a

significant improvement

Trend in defoliationfor all tree species,

1989–1995

0 500 km

improvementno clear trenddeteriorationsignificant deterioration

Map 3.4.7

Source: SC-DLO

slow deterioration in crown conditions between1989 and 1995 with a peak in 1992. The fraction oftrees with a defoliation of more than 50% increasedsteadily over the period. The fraction of healthytrees (defoliation of 10% or less) decreased overthe period from 47% in 1989 to 39% in 1995.However, the deterioration slowed down after 1992with little change in crown condition occurring inthe period 1993-1995 (see also Chapter 3.11).

As an example of a typical Mediterranean tree, theresults for Holm oak (Quercus ilex) are shown in

... /...

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C h a n n e l

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a

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S e a

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B l a c k

S e a

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6. Future trends

6.1. Total emissionsImplementation of policies in the pipeline islikely to substantially reduce emissions inEurope by 2010 (European Commission,1999). For the EU and AC10 combined theSO2 emissions are expected to decrease by65% in 2010 from 1990 levels. NOx emissionsare expected to decline by 40% and VOCemissions by 43%. It is anticipated thatreductions in SO2, NOx and VOC emissionswill be larger in the EU than in the Acces-sion Countries (Table 3.4.4). The projectedreduction in ammonia emissions is lowerthan for the other pollutants (14%). Ger-many and the Netherlands are projected tomake significant contributions to theseoverall reductions. A small 1% reduction ofNH3 emissions is expected in the AC10 forthe period 1990 to 2010, while substantialincreases in NH3 emissions are projected insome AC10 countries.

The implications of EU emission reductionsagainst its targets are summarised in Table3.4.5.

SO2 emissions are mainly originating fromthe energy sector (power generation) andindustry, while the most important sources ofNOx emissions are the transport sectorfollowed by the energy and industry sector.The main source of NH3 emissions is agricul-ture.

Under the baseline scenario, the share of SO2

emissions from energy is projected to de-crease, due to measures such as fuel shiftfrom coal to natural gas, while the share fromindustry is projected to increase (Table 3.4.6).However, all sectors are expected to substan-tially decrease SO2 emissions in absoluteterms. Also, NOx emissions from transport,energy and industry are projected to decreasesubstantially, resulting in a decrease in theshare from transport. Only small reductions

Figure 3.4.9. This species presents the most seriousdeterioration of all species considered by theUNECE International Co-operative programme onForest. While crown condition improved between1989 and 1990, it subsequently declined sharply. In1989 more than 70% of Holm oaks were considerednot defoliated whereas in 1995, nearly 80% of thetrees showed some sign of defoliation (Müller-Edzards et al., 1997).

0

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40

50

1989 1990 1991 1992 1993 1994 1995

% o

f tre

es

Norway Spruce

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70

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90

100

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21-30

11-20

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% of defoliation

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% o

f tre

es

Holm Oak

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70

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90

100

51-100

41-50

31-40

21-30

11-20

0-10

% of defoliation

Figure 3.4.8Temporal changes in defoliation for the NorwaySpruce

Source: SC-DLO

Figure 3.4.9Temporal changes in defoliation for the HolmOak

Source: SC-DLO


Table 3.4.4.

EU15 Accession Countries

Pollutant 1990 2010 reduction 1990 2010 reduction

SO2 16 300 4 800 71% 10 300 4 300 59%

NOx 13 200 7 300 45% 3 500 2 600 27%

NH3 3 600 2 900 14% 1 400 1 390 1%

VOC 14 000 7 200 49% 2 600 2 300 10%

Source: European Commission, 1999

Projected emission reductions in Europe (in ktonnes)

target year expected level targetAcidification in target year

SO2 emissions 2000 53* 60

2010 29 16**

NOx emissions 2000 81* 70

2010 55 45

NMVOC emissions 1999 81* 70

Table 3.4.5.EU progress in achieving key EU environmentaltargets (1990 = 100)

*) based on current reduction plans of Member States

**) proposed target within the Acidification Strategy (European Commission, 1977) will berevised in the forthcoming National Emission Ceiling Direktive, linked to the forthcomingcombined EU ozone/acidification strategy.

Source: EEA

of NH3 emissions from agriculture are pro-jected under the baseline scenario.

Transport and other sectors, mainly solventuse in industry and households, are the mainsources of NMVOC emissions. Under thebaseline scenario, emission reductions aremainly expected in the transport sector,resulting in a lower share in 2010.

6.2. Emissions from the transport sectorThe transport sector accounts for a substan-tial proportion of VOC and NOx emissions,and is therefore a main contributor to theproblem of tropospheric ozone and to alesser extend acidification (EEA, 1998).Total passenger-km travelled is projected toincrease by 40% between 1990 and 2010(Figure 3.4.10) (the figures for the EFTAand AC10 are 230% and 115% respectively).A 50% increase is projected in freighttransport tonne/km between 1990 and 2010;projections also show 85% growth in roadfreight with an increase in fuel consumptionof 70% – indicating some gains in fuelefficiency (for further details of these projec-tions, see Chapter 2.2).

6.3. Emissions from ship trafficThe European Commission’s acidificationstrategy (European Commission, 1997)recognises the cost-effectiveness of reducingemissions from ship traffic in the North Seaand the north-eastern Atlantic Ocean rela-tive to reduction of land-base emissions.

In absolute values, the emissions of SO2 andNOx from international ship traffic in 1995were similar in magnitude to the contribu-tion of individual large countries. Thecontribution of emissions from internationalship traffic sources to the total depositionover western Europe is about 10-15% (Tsyroand Berge, 1997).

Sector SO2 NOx NH3 NMVOC

1995 2010 1995 2010 1995 2010 1995 2010

Energy 58 49 19 19 0 0 0 0

Industry 25 32 12 17 2 2 8 13

Agriculture 0 0 0 0 94 95 *) *)

Transport 7 7 63 55 4 3 40 29

Other 10 12 6 10 0 0 52*) 58*)

Table 3.4.6.Contribution to EU emissions of SO2, NOx, NH3 and NMVOC, per sector, in 1995 and in 2010

*) VOCs from agriculture is included in ‘other’. The dominating contribution in the sector ‘other’ is from solventuse within industry and households.

Source: EuropeanCommission, 1999

The relative importance of internationalship traffic emissions will increase if nomeasures are taken to control this type ofemission. Since the total reported emissionsof SO2 and NOx in European countriesdecreased markedly from 1990 to 1995, therelative contribution of international shiptraffic increased in the period to 3.5% forSO2 and 7.5% for NOx. If no further reduc-tions are accomplished, it is expected thatthe relative contribution of emissions from


international ship traffic will have doubledby the year 2010 (Figure 3.4.11).

As a consequence, reductions of emissionsfrom international ship traffic would trans-late into considerable reductions of thepressures/depositions in western Europeancountries, with the additional advantage ofreducing the costs for reaching the estab-lished environmental targets.

In 1997, the International Convention forthe Prevention of Pollution from Ships(MARPOL) proposed a new protocol toreduce pollution from ships’ exhaust emis-sions. Reducing emission from internationalship traffic could be more cost-effective thanreducing land-based sources (Amann et al.,1997). The estimated savings are about 1 000million euros/year. While the costs forlimiting the sulphur content of marinebunkers in the North Sea and the Baltic to1.5% (the maximum value accepted byMARPOL) were estimated at about 87million euros/year, land-based sourceswould experience a decline in their costs ofabout 1 150 million euros/year.

Between 1990 and 2010,passenger and freight

transport will grow by about40% and 80% respectively.

Consumption of fuel willgrow less, with the

introduction of moreefficient engines. With the

introduction of technical‘end-of-pipe’ measures, a

further reduction inemissions will be achieved.

For NOx and VOC, areduction substantially

below the 1990 levels isprojected.

Source: EuropeanCommission, 1999

200

150

100

50

0Passengertransport

Freighttransport

Ind

ex re

lativ

e to

199

0(1

990

=10

0)

Volume of passenger/freight transportFuel consumptionVOC emissionsNOx emissions

Indexed EU development of various road transportparameters in 2010, relative to 1990

Figure 3.4.10

10

5

0

15

% o

f to

tal

Eur

op

ean

emis

sio

ns

1990 1995 2010

NOx SO2

Contribution from international shipping in theNorth Sea and north-east Atlantic Ocean to totalEuropean acidifying emissions

Figure 3.4.11

Nitrogen compounds andsulphur compounds eachcontribute about 50% to

ongoing acidification.

Source: EMEP

These values are presently under re-evalua-tion, taking into account revised emissionestimates calculated by Lloyd’s Register ofShipping. Since the latest emission estimatesare more than a factor of two larger thanbefore, the estimated cost-benefit could beeven higher than 1 000 million euros/year.

6.4. Emissions from aircraftAlthough aircraft NOx emissions are inabsolute terms small when compared with theNOx emissions from road transport, theirimpact on ozone formation is relatively large.In the main flight corridors about 1-4% of theozone in the upper troposphere and lowerstratosphere is formed from aircraft emissions(Brasseur et al., 1998). According to aircraftscenarios the enhanced ozone formation willincrease between 1990 and 2015 by 50-70%(Valks and Velders, 1999).

6.5. Damage to ecosystems and healthUnder the baseline scenario, significantimprovements in the area of ecosystemswhich are protected against acidification andeutrophication will be achieved (Figure3.14.12). The acidification impact reductionsare concentrated in northern Europe: theUK, northern France, Belgium, the Nether-lands, Germany, Poland, Czech Republic,Slovak Republic and Austria (see section 4.3in Chapter 3.11). For eutrophication, theimprovements are more dispersed acrossEurope.

In the EU, ecosystems that receive aciddeposition above their critical loads coulddecrease from 25% of the total (38.6 millionha) in 1990 to 7% in 2010 (10.6 million ha).Ecosystems in several countries, such asFrance, Ireland, Italy, Spain are virtually nolonger exposed to exceedances of criticalload. Other countries, where the proportionof ecosystems affected by acidification washigh in 1990 show considerable improve-ment. In 2010, the two countries with thehighest proportion of ecosystems affected byacidification remain Germany and theNetherlands, although considerable im-provement will be experienced; there will bereductions from 84% above critical loads to33% in Germany and 89% to 45% in theNetherlands.

In the AC10, the area of ecosystems with adeposition above the critical load for acidifi-cation could decrease from 44% of allnatural and semi-natural ecosystems (18.1million ha) to 6%. In Norway and Switzer-land, these improvements are from 25% to13% and from 38% to 4% respectively. Very


large improvements in the impact of acidifi-cation are projected for some AccessionCountries. For example, the proportion ofecosystems adversely affected by acidificationdrops from 91% to 19% in the Czech Repub-lic, from 44% to 4% in Lithuania, and from73% to 8% in Poland.

In the EU, the area of ecosystems thatreceives nitrogen deposition above criticalloads for eutrophication could decreasefrom 55% (68.0 million ha) in 1990 to 39%in 2010 (48.8 million ha). Countries where ahigh proportion of ecosystems remainsadversely affected by eutrophication in 2010include Belgium, France, Germany, Luxem-bourg and the Netherlands.

Eutrophication impact levels, however,remain high in most of the EFTA countriesand in the AC10. In the AC10, theeutrophication indicator improves to a lesserdegree from 84% of the area (33.4 millionha) having a nitrogen deposition exceedingthe critical load to 72%. Countries whereover 70% of ecosystems will still be affectedby excess nitrogen deposition include theCzech Republic, Lithuania, Poland, theSlovak Republic and Switzerland.

With respect to ozone, the reductions inprecursor emissions will lead to a reductionof about 25% in AOT40 levels for crops(Figure 3.4.13). Despite the considerableefforts in emission reductions, the long-termobjective of 6 000 µg/m3.h will only be

60

40

30

20

10

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50

Acidificationin 1990

Acidificationin 2010

Eutrophicationin 1990

Eutrophicationin 2010

EUNon EUEurope

% o

f eco

syst

ems

with

dep

osi

tions

abo

ve c

ritic

al lo

adSource: EuropeanCommission, 1999

Ecosystem damage in Europe: fraction of eco-systems with depositions above their critical loads Figure 3.4.12

Aus

tria

0

Bel

giu

m

Den

mar

k

Finl

and

Fran

ce

Ger

man

y

Gre

ece

Irela

nd

Ital

y

Luxe

mb

our

g

The

Net

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nds

Port

ugal

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Kin

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15

2

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18

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AO

T40

(pp

m.h

)

19902010

Cro

p a

vera

ge

ozo

ne e

xpo

sure

Source: EMEP

Figure 3.4.13Reduction in exposure of crops in 2010 in EU compared with the situation in 1990

reached in the north-west outskirts of Eu-rope (Ireland, Scandinavia). The targetvalue of 16 000 µg/m3.h will still be ex-ceeded in seven EU Member States.

The reduction in the number of days withozone above the health-related threshold of120 mg/m3 varies over Europe (Map 3.4.8).Under the assumption of the most unfavour-able meteorological conditions, the numberof days above the health-related thresholdfalls from 67 in 1990 to 49 in 2010. There ishowever, a shift in location: in 1990 thehighest numbers of exceedances were foundin southern Italy; in 2010 the highest numberare found in the more densely populatednorth-western part of EU (the Netherlands,Belgium, and northern France).


7. Future acidification and summer smog policies

7.1. Emission-oriented policiesAn optimal emission-reduction strategyrequires coordination of SO2, NOx, NH3 andVOC emission controls. At present, environ-mental problems are treated separately, sothat acidification-related strategies empha-sise reduction in SO2 and nitrogen emis-sions, ozone-related strategies prioritise NOx

and VOC measures, while eutrophicationstrategies suggest the importance of NH3

emission reductions. The focus on acidifica-tion abatement is presently on nitrogencompounds. NOx emissions are majorcontributors to the acidification andeutrophication of ecosystems and are at thesame time a precursor of ground-levelozone. This implies that any emission reduc-tion scenario should combine the evaluationof the consequences on health and vegeta-tion due to ozone exposure together withtheir impacts in acidification.

Both the EU and UNECE have been workingsince 1997 on a combined acidification/eutrophication/ozone abatement strategy,seeking cost-effective solutions to minimisethe gap between critical load/level andambient levels. With the use of the new AAEconcept to calculate average accumulatedexceedance to critical loads, significant andcost-effective reductions appear possible.

The analysis of all these effects combinedincreases the demand for further NOx

reductions, while the requirements on SO2

and VOC controls are somewhat relaxed and

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Map 3.4.8Number of days the WHO

criterion is exceeded foremissions in 1990 and for

projected emissions in 2010.Calculations represent the

maximum of the three-yearmoving averages for the

five-year period considered

Souce: EuropeanCommission, 1999

the requirements for ammonia emissionreductions remain almost unchanged(Amann et al., 1998).

There is a – sometimes complex – relationbetween precursor emissions and environ-mental quality. If a target on environmentalquality is set, such as full protection of allecosystems from acidification, emissiontargets are implicitly set. So far, this interrela-tion has been included in the developmentof EU policies using expert judgements.Only recently have both the EU and UNECEadopted a broad approach which shouldhelp to abate the problems of acidification,eutrophication, as well as those of photo-chemical oxidants. This approach will seekto establish national emission ceilings for therelevant components that take into accountthe effect of the emission reduction in thevarious problem areas and the costs involvedin emission abatement. For the first time,agreements on abating NH3 emissions will bemade at an international level.

7.2. Air quality, deposition standards, and target levelsFor the air pollutants causing acidificationand summer smog, the WHO has recom-mended air quality guidelines for protectionof human health and ecosystems (WHO,1995 and 1996 a, b). These recommenda-tions are applied to policy use in the Euro-pean Union as well as the UN-ECE. Atpresent the EU Council of Ministers hasagreed on a common position on the pro-posed daughter directives on ozone, PM10,SO2, NO2 and other pollutants in the frame-work of the Air Quality Framework Directive


(European Community, 1996). Onceadopted, this will lead to more stringent airquality targets.

For ambient SO2 and NO2 concentration,targets have been set both for protection ofhuman health and to avoid damage toecosystems. Generally, the requirements onemissions reduction to meet the depositiontargets are more stringent than reductionsneeded to meet the concentration targets.However, close to sources, high concentra-tions may occur, such as in an industrial areaor in a very busy street (see Chapter 3.12).Current air quality thresholds for the EU aredefined in the Ozone Directive (92/72/EC).In 1999, a new ozone daughter directive wasproposed by the Commission. A similarsituation to the case with acidificationabatement (section 5 of this chapter)evolved in defining an ozone-abatementstrategy in the EU. To attain ambient ozoneconcentrations at a level below which ad-verse effects on human health and ecosys-tems are unlikely: European-wide emissionreduction of 80% or more will be needed.Even with major improvements in currenttechnologies, this will hardly be achievableand large changes in societal behaviourwould be needed. Therefore, interim targetvalues have been defined, taking into accountrealistic options for emission abatement.

A monitoring strategy is defined in the AirQuality Framework Directive (EuropeanCommunity, 1996) and the proposed daugh-ter directive on ozone. This strategy isdesigned to assess the current ozone levelsrelevant for human health and ecosystemsbut it can only partially answer other impor-tant questions:

• Is the ozone situation improving? Trendsin ozone concentrations are obscured bythe large year-to-year variations causedby different weather conditions. Todetect a possible trend, long-termmeasurements of high quality areneeded.

• Is there compliance with agreed emis-sion reductions? Ozone is a secondarypollutant, in the sense that it is formedby chemical processes from other sub-stances emitted to the environment.Information on its precursor levelscannot be detected by monitoringozone. Moreover, there is a strong non-linearity between ozone levels andprecursor emissions: for a certain reduc-tion in ozone levels, at least a threefoldreduction in precursor emissions is

needed. Monitoring of NOx and VOC incombination with ozone monitoring isneeded to follow the implementation ofabatement measures.

8. Emerging Issues

8.1. Global tropospheric ozoneDuring the summer, there is a generalblanket of medium to high ozone levels overEurope at least twice as high as during the1850s (EEA, 1998). These background levelsfrequently exceed the concentrations atwhich damage to vegetation is expected.Model calculations indicate that the increasein these tropospheric background concen-trations will continue. The steady growth inozone is caused by increases in backgroundlevels of nitrogen oxides, carbon monoxide,as well as methane (see for exampleBerntsen et al., 1997).

Ozone precursors may be transported overdistances of hundreds to thousands ofkilometres, resulting in ozone formation farfrom its source. An abatement policy atEuropean level is therefore a key issue.Long-range intercontinental scale ozoneformation and transport in the lower tropo-sphere has also been postulated (Parrish etal., 1993; Collins et al., 1998): fringes ofozone leaving the east coast of NorthAmerica spread out over the North Atlantictowards Europe; the European ozoneplumes stretch out eastwards over thecontinent and join with the ozone peaks overthe densely populated areas in Asia. It mightbe that the global background increasepartly reverses the advantages of a Europeanreduction program. The overall success ofpan-European abatement policies willdepend on actions on a hemispheric scale, aEuropean scale and an urban scale.

On a global scale, production and use offossil fuels are important sources of methaneand CO emissions. Biomass burning, wastetreatment and intensive cattle breeding areadditional sources. An overview of globalemissions is presented in Table 3.4.7.

Calculations using global-scale models(Collins et al., 1998) indicate that ozoneconcentrations within Europe are more orless stable. The reduced ozone formationcaused by reduction in NOx emissions withinEurope appears to be just enough to coun-terbalance the increased ozone input fromthe global background. Actions on a globalscale, in particular aiming at reductions of


Table 3.4.7.

Methane CO NOx (1) VOC (2)

Fossil fuel production and 94 263 72 69combustion

Biofuel 14 181 5 31

Industrial processes 0.8 35 5 34

Land use/waste treatment 211 496 20 44

Total anthropogenic 320 974 102 177.5

European contribution (%)2 25 15 26 21

Natural emissions 160 70-280 23-131 1100

Global emissions in 1990 (Tg)

(1) in Tg NO2(2) incl. Newly Independant States (of former USSR)

Source: Olivier et al., 1996; IPCC, 1996; EEA, 1995

emissions of CO, methane and NOx will benecessary to control ozone concentrations inEurope.

Ozone is also a greenhouse gas. The increasein tropospheric ozone has potentially impor-tant consequences for radiative forcing (SeeChapter 3.1).

8.2. Is our cultural heritage more in danger than our ecosystems?Exposure to acidifying and photo-oxidant airpollution increases the corrosion rate ofmaterials. Gaseous SO2 has been consideredto be the main explanatory factor for corro-sion damage to buildings and buildingmaterials. Ozone also affects materials suchas natural and synthetic rubbers, coatings

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and textiles. Recently, synergistic effects ofozone in combination with the acidifyingcomponents SO2 and NO2 have been re-ported to lead to increased corrosion. Thisaffects in particular the materials most widelyused in our cultural heritage, such as mar-ble, calcareous stone and rendering, medi-eval glass, copper, bronze and most construc-tion metals.

Recent results (ETSU, 1996) indicate thatthe average cost of material damage isapproximately 1 130 euros per tonne SO2

emitted, which implies a total damage cost of13.5 billion euros per year for the EU in1995. This probably underestimates the costof the damages if the enhanced corrosionlevels resulting from the combined exposureto O3 and acidifying compounds were to beconsidered. There are two main reasons forthis expected underestimation. First, thecombination of SO2 and O3 increases theactual corrosive attack on materials. Second,the geographical distribution of potentialcorrosion is expected to change whensynergetic effects are considered. Introduc-ing the effect of ozone will probably enhancethe corrosion levels in southern Europe, thatis, in areas with an extensive cultural herit-age where the impact of corrosion will causemore damage.

The distribution of copper corrosion ratesillustrates well the above-mentioned geo-graphical effect of the combined exposure toSO2 and O3 air concentrations. Coppercorrosion has been calculated from 1996European levels of SO2, NO2 and O3. Theactual corrosion levels have been comparedto the corrosion caused by exposure tobackground air concentration. Corrosionrates approximately 25% higher than back-ground corrosion are considered acceptable,above this corrosion levels are considereddamaging. Acceptable corrosion rates areexceeded all over Europe (Map 3.4.9), anddecay rates are doubled in most centralEuropean regions. The substantial emissionreductions during recent years have reducedcorrosion damage in Scandinavia. In centralEurope we still see the effects of high levelsof SO2 pollution in areas of Germany, Polandand the Czech Republic where the corrosionrate still is more than three times the generalbackground for Europe. In southern Europethe increased corrosion rate is mostly gener-ated by the high ozone level, although SO2

may still contribute in some areas.

Comparing the geographical distribution ofexceedances of acceptable corrosion rates

Map 3.4.9Note that the calculation will

underestimate the totalcorrosion in the larger cities.

Source: NILU


and the distribution of exceedances ofcritical loads for acidification andeutrophication brings the focus towardssouthern and eastern Europe. This intro-duces new elements in the discussion of theharmful effects of transboundary air pollu-tion that will be worth analysing in thefuture.

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Amann, M. et al., (1998) Cost-effective Control ofAcidification and Ground-Level Ozone. Fifth InterimReport, IIASA, A-2361 Laxenburg Austria

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Berntsen, T.K. et al. (1997) Effects of anthropogenicemissions on tropospheric ozone and its radiativeforcing. J. Geophys. Res. 102, 28101-28126

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Collins W.J., Stevenson D.S., Johnson C.E., andDerwent R.G. (1998) The European regional ozonedistribution and its links with the global scale for theyears 1992 and 2015, to be published.

De Leeuw F., Sluyter R. and van Zantvoort E. (1997)Air pollution by ozone in the European Union;exceedances of ozone threshold values in 1996 andsummer 1997. EEA Topic Report 7-1997. EEA,Copenhagen.

de Leeuw and de Paus, 1998. Air Pollution by ozonein Europe; 1997 - summer 1998. EEA Topic Report inpress, EEA, Copenhagen.

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Millan, M.M., Salvador, R., Mantilla, E., Kallos, G.(1997) Photooxidant dynamics in the Mediterraneanbasin in summer: results from European researchprojects. J. Geophys. Res., 102, 8811-8823.

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Olivier J.G.J., Bouwman A.F., van der Maas C.W.M.,Berdowski J.J.M., Veldt C., Bloos J.P.J., VisschedijkA.J.H., Zandveld P.Y.J. and Haverslag J.L. (1996)Description of EDGAR, version 2.0. Report 771060002, RIVM, Bilthoven.

Parrish D.D. et al., (1993) Export of North Americanozone pollution to the North Atlantic Ocean. Science,259, 1436-1439.

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3.4. transboundary air pollution

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