REVIEW OF
TRANSBOUNDARY AIR
POLLUTION (RoTAP)
Acidification, Eutrophication, Ground
Level Ozone and Heavy Metals in the UK
SUMMARY FOR
POLICY MAKERS
DRAFT FOR REVIEW
http://www.rotap.ceh.ac.uk
© Crown copyright 2009
1
About this Review Exercise
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Summary
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This report is an extract from a draft version of a report which is due to be
published by Defra in Spring 2010. A draft version of the full report is
available at http://www.rotap.ceh.ac.uk
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Defra commissioned the Centre for Ecology and Hydrology (CEH) to produce a
report entitled the Review of Transboundary Air Pollution (RoTAP). This report
has been prepared using input and expertise from various members of the
relevant scientific community. The names of contributors are available on the
RoTAP website http://www.rotap.ceh.ac.uk.
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A Core RoTAP group was formed comprising the lead authors for each chapter
in the report, with additional chapter-specific input received from members of
the Associate RoTAP group. Additional input was received from a group of
RoTAP
Advisors,
comprising
representatives
from
the
Devolved
Administrations, and the Regulatory and Conservation Agencies in the UK.
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The report has been produced following a series of meetings which were held
in London and Edinburgh. Each meeting was dedicated to a specific chapter of
the RoTAP report. Core Members and Advisors were invited to attend all
meetings. Associate members were invited to attend the meetings of the
chapters which covered their areas of expertise.
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Aims of RoTAP
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The review focuses on the main chemicals causing acid deposition,
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eutrophication, ground level ozone and heavy metal pollution in the UK,
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namely sulphur dioxide (SO2), nitrogen oxides (NOx), ammonia (NH3),
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aerosols (particulate matter), heavy metals, nitric acid (HNO3) and ozone
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(O3). The RoTAP report reviews the current data available to the group on
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these pollutants and their contribution to acidification, eutrophication,
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ground level ozone and heavy metal deposition in the UK. The report
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covers:
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1.
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atmospheric pollutants.
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2. Changes in the atmospheric processing of pollutants and their
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significance for policy.
Trends
in
emission,
concentration
and
deposition
Review of Transboundary Air Pollution (RoTAP) Consultation Draft V1.02
of
the
Page i
main
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3. Changes in the exceedance of critical loads and levels for all pollutants
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and UK ecosystems for which this methodology applies
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4. Evidence of; chemical and biological recovery from the effects of acid
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deposition, changes in the eutrophication of UK ecosystems, timescales to
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improve the heavy metal status of soils and changes in the effects of
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ground level ozone on UK ecosystems.
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5. The use of long range transport models to assess the prospects for
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further change in concentrations, deposition and ecosystem impacts
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6. The European and Global perspective on transboundary air pollution
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Details of the RoTAP Review are available at
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http://www.RoTAP.ceh.ac.uk
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Review Questions
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The review on the RoTAP report is focussed around the following 7 questions:
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Q1) Does the RoTAP report evaluate the current situation of transboundary
air pollution in the UK and predict what may happen to UK air quality
(including its impacts on the environment) in the future in response to current
legislation?
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Q2) Are the data presented in the RoTAP report valid?
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Q3) Are there additional data which if included in the report would change the
conclusions of the RoTAP report? If so, please supply details.
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Q4) Do you agree with the recommendations of the RoTAP report?
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Q5)Are there important aspects of transboundary air pollution which have not
been included in the RoTAP report?
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Q6) Do you agree with the content of the RoTAP Summary For Policy Makers
report, including its Key Messages ?
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Q7) Do you have any additional comments ?
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If you have any comments on this report, please send them to the
RoTAP Secretary, Heath Malcolm (hmm@ceh.ac.uk) by 5pm on
Monday 11th January 2010.
Review of Transboundary Air Pollution (RoTAP) Consultation Draft V1.02
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Summary for Policy Makers
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Purpose of the RoTAP report
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The key purpose of the RoTAP report is to provide a scientific review of the available evidence
4
on emissions, concentrations, deposition and effects of transboundary air pollutants in the
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UK. The report considers impacts on ecosystems but it does not cover direct impacts on
6
human health. This Summary for Policy Makers draws together all of the key policy-related
7
findings of the RoTAP report.
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The RoTAP report provides a detailed overview of the current status of transboundary air
10
pollution in the UK. It reviews the current state of air pollution issues, evaluates the
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extensive measurements of atmospheric pollutants and their effects, and provides a
12
synthesis of current understanding.
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deposition1, eutrophication2, ground level ozone and heavy metal pollution in the UK, namely
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emissions of sulphur dioxide (SO2), nitrogen oxides (NOx), ammonia (NH3), volatile organic
15
compounds (VOC), and heavy metals. The fate of these emissions and their effects on
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ecosystems are a particular focus in the report, as it is the potential for effects that drives the
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policy interest. A summary of the current status at European and Global scales is included to
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provide a wider perspective for the assessment. The report reviews the temporal and spatial
19
trends in transboundary air pollution and effects in the UK since the introduction of control
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measures to limit the emissions of atmospheric pollutants, with particular focus on data from
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the last twenty years. It also identifies the main future threats to ecosystems from
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transboundary air pollution. The relationships between emissions of pollutants, their
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concentrations and deposition and their subsequent effects on ecosystems, including
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ecosystem recovery are evaluated.
The review focuses on the main causes of acid
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This Summary for Policy Makers identifies the policy issues covered in the RoTAP report,
27
reviews the success of policies implemented to date and identifies areas where further policy
28
action is required. It reviews whether the UK is on target to meet the agreed reductions in
29
emissions of atmospheric pollutants, whether there has been a decrease in concentrations of
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these pollutants in the environment, and a subsequent reversal of any impacts, both of which
31
represent aspects of recovery.
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current policies on future transboundary air pollution policies and their interactions with
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climate change.
The policy makers’ summary also identifies the effects of
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1
The deposition of acidic compounds from the atmosphere, causing changes to the pH of the
receiving soil or water.
2
An increase in the concentration of chemical nutrients in an ecosystem causing adverse
environmental effects.
Review of Transboundary Air Pollution (RoTAP) Consultation Draft V1.02
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2
Key messages
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Transboundary air pollution has been an issue of national and international concern over the
4
last 30 years, especially since the identification of the problems caused by acid rain in the
5
1970s. More recent transboundary air pollution problems identified include eutrophication,
6
and ground level ozone.
7
substantially by a reduction in the emissions of pollutants, and a subsequent reduction in
8
their concentrations in air, rain, freshwaters and soil, and there is now evidence that
9
ecosystems are in the process of recovery. However, other concerns including eutrophication,
10
ground level ozone and heavy metal pollution have yet to be addressed as successfully, with
11
further policy action required.
12
In addition to data on observed effects of pollutants on the environment, an indication of the
13
potential for effects to occur is provided via the critical load concept. Critical loads are defined
14
as a quantitative estimate of an exposure to one or more pollutants below which significant
15
harmful effects on specified sensitive elements of the environment do not occur according to
16
present knowledge. If the amount of deposition is greater than the critical load for that
17
habitat, an exceedance is reported. Exceeding steady-state critical loads does not necessarily
18
indicate that damage has occurred, but it indicates the potential for damage to occur.
Some of these concerns such as acidification have been reduced
19
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Acidification
21
Acidification of soils and waters can be caused by deposition of sulphur, nitrogen or
22
hydrochloric acid (or a combination of these pollutants).
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reduced sulphur dioxide emissions by over 90% from their peak value in the 1950s,
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resulting in reduced concentrations of sulphur and levels of acidity in the atmosphere,
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soils and freshwater. Ecological recovery of these habitats is underway but soils and
26
freshwaters in some regions remain acidified and the legacy of previous emissions,
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land use and climate change may limit the extent of the recovery. Further reductions
28
in the emissions of sulphur may be required to aid ecological improvement. Currently
29
58% of all habitat areas sensitive to acidification exceed the Critical Load for acidity,
30
predominately due to the deposition of nitrogen which also has the potential to acidify.
31
Typically, most of the deposited nitrogen is accumulating in soils and vegetation, and
32
relatively little is currently contributing directly to acidification, but significant leaching
33
to surface waters is occurring in higher deposition areas, and in particularly in
34
catchments with sparse soils. The Critical Load Exceedance is predicted to decrease to
35
40% of habitat areas by 2020 in the absence of further measures.
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
Emission controls have
Eutrophication
37
Eutrophication from atmospheric deposition in the UK is caused by the emissions of
38
nitrogen oxides and ammonia. Emissions of nitrogen oxides have decreased by 50%
39
since 1970, with a corresponding 50% reduction in air concentrations of nitrogen
40
oxides. Emissions of ammonia are only reliable from 1990 onwards, since when they
41
have decreased by 24%, although there is large annual variability, masking any
Review of Transboundary Air Pollution (RoTAP) Consultation Draft V1.02
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1
overall trend. Concentrations of ammonia have changed little over the last decade.
2
The proportion of ammonia to total nitrogen deposition has increased over the last
3
twenty years from 45 to 50%.
4
5
Despite the large reduction in emissions, total deposition of nitrogen (oxidised and
6
reduced forms) has changed little. This surprising result is due to changes in the
7
chemistry and removal of nitrogen compounds accelerating the removal from the
8
atmosphere over twenty years of monitoring. Thus a larger proportion of nitrogen
9
emissions are deposited in the UK than occurred twenty years ago. The main
10
consequence of the emission reductions has been a reduction in export. At sites in the
11
UK where nitrogen deposition exceeds the capacity of the vegetation and soil to
12
sequester inputs, nitrate is leaching into surface waters and has the potential to
13
stimulate algal growth and affect species composition. In the terrestrial environment,
14
changes in species composition have been observed close to sources of ammonia, and
15
in the national surveys (such as the Countryside Survey). Controlled experiments also
16
show reductions in species composition with nitrogen addition, especially for ammonia
17
treatments.
18
Currently, 60% of all sensitive habitat areas sensitive to eutrophication from nitrogen
19
deposition exceed the critical load for nutrient nitrogen. This figure is predicted to
20
decrease to 49% (i.e. by only 11%) by 2020. Reducing ammonia concentrations is a
21
high priority and further action is required to reduce nitrogen deposition. Remedial
22
action may also be required to allow full recovery of damaged plant communities.
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24

Ground level ozone
25
Important progress has been made in reducing ozone precursor 3 emissions in the UK
26
and more widely throughout Europe, and these have reduced peak ground level
27
concentrations by almost 30% since the 1980s. However, emissions throughout the
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northern hemisphere have led to background ozone concentrations increasing by
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~10% between 1987 and 2007. Current ozone exposures exceed critical thresholds
30
for effects on crops, forests and semi-natural vegetation over substantial areas of the
31
UK. It is therefore necessary to increase the scale at which emissions of ozone
32
precursors are controlled, with the hemispheric scale being the most appropriate.
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34

Metals
35
Emissions of heavy metals have decreased in the UK, although there are considerable
36
uncertainties in the primary sources of metals in the UK atmosphere as the amount of
37
metals deposited from the atmosphere greatly exceeds the amount of metals reported
3
Ozone is not emitted, but is produced in the atmosphere by chemical reactions between
precursors including the oxides of nitrogen (NOx), and volatile organic compounds (VOC) in
the presence of sunlight.
Review of Transboundary Air Pollution (RoTAP) Consultation Draft V1.02
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1
to be emitted from anthropogenic sources. Specific sites and some areas of the UK
2
contain soils with large and potentially ecologically-damaging concentrations of a
3
range of metals. However, the contributions from atmospheric and local metal
4
industrial sources, eg the legacy from mining and metal processing waste are unclear.
5
Concentrations of metals in these contaminated soils change very slowly; in some
6
cases the timescale for recovery is centuries.
8
What changes have there been to trends in transboundary air
pollution in the UK over the last twenty years?
9
Current Trends in Emissions
7
10
Emissions of primary pollutants contributing to transboundary air pollution have declined
11
substantially during the last 20 years, mainly in response to domestic and international
12
control measures such as the Environmental Protection Act (1990), the EC Directive on
13
Integrated Pollution Prevention and Control (IPPC), (Directive 96/61/EC), the Gothenburg
14
Protocol (UN/ECE Executive Body Decision 30/11/1999), the Large Combustion Plant
15
Directive (National Emission Reduction Plan Regulations, SI2007, No 2325) and the MARPOL
16
Protocol Annex VI: Regulations for the Prevention of Air Pollution from Ships. . In 2001 the
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National Emissions Ceilings Directive (NECD Directive 2001/81/EC) was agreed, setting
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ceilings for the emissions of key atmospheric pollutants, namely sulphur dioxide (SO 2), oxides
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of nitrogen (NOx), ammonia (NH3) and volatile organic compounds (VOC) for European
20
Community member states including the UK to reach by 2010 (see Table i).
Chemical
SO2
NOx
NH3
1970
1990
2007
Estimated
Reduction
emissions
emissions
emissions
uncertainty
Emissions
(kt)
(kt)
(kt)
(kt)
6,370
3,717
590
±4%
91% (1970-2007)
585
84% (1990-2007)
(on course to meet target)
50% (1970 -2007)
1,167
46% (1990-2007)
(target unlikely to be met)
24% (1990-2007)
297
3,125
Not
2,968
383
1,490
289
±10%
±20%
in
available
VOC
1,925
NECD target for 2010
(on course to meet target)
2,388
940
±10%
52% (1970-2007)
1,200
64% (1990-2007)
(on course to meet target)
21
22
Table i
Changes in UK Emissions of Atmospheric Pollutants Covered by the National
Emissions Ceilings Directive (NECD).
23
Sulphur Dioxide
24
Sulphur dioxide is emitted to the atmosphere from combustion of fossil fuels, with the
25
combustion of coal being the dominant source across Europe. The changes in sulphur dioxide
Review of Transboundary Air Pollution (RoTAP) Consultation Draft V1.02
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1
emissions have arisen following a decline in heavy industry in the UK (with a move to a
2
service-based economy), a substantial reduction in the use of coal as a fuel for electricity
3
production and an increased use of sulphur abatement technology.
4
domestic sector have also decreased due to the introduction of Smoke Control Areas,
5
increased availability and affordability of natural gas, and increased energy efficiency.
6
Nitrogen Oxides
7
Nitrogen oxides are also emitted from the combustion of fossil fuels, the dominant source in
8
the UK being road transport and the generation of electricity and heat. Although emissions
9
from both road transport and electricity generation have decreased, the overall emissions of
10
nitrogen oxides have not been reduced in recent years by as much as originally anticipated.
11
The UK is one of 17 Member States of the European Union unlikely to meet the 2010
12
Emissions Target for nitrogen oxides. One of the causes of this is that emissions from new
13
cars in the real world are now considered to be higher than the predicted values which
14
manufacturers have to provide from laboratory dynamometer studies. Emissions of nitrogen
15
dioxide (NO2) (the component of nitrogen oxides which has impacts on health) from transport
16
have not decreased in line with overall emissions of nitrogen oxides, and the UK is currently
17
projected to exceed European Union Limit Values for ambient air quality in a large number of
18
urban locations.
19
Ammonia
20
Ammonia is primarily emitted to the atmosphere from the agricultural sector, particularly
21
from manure management.
22
1990 onwards, and from this time the reduction in emissions is modest (24%).
23
revisions to agricultural projections based upon decreases in livestock numbers indicate that
24
the UK is expected to meet the NECD ammonia emissions target by 2010.
25
Volatile Organic Compounds
26
Volatile Organic Compounds (VOC) are released to the atmosphere primarily from solvent
27
and industrial processes and road transport in the UK. Emissions of almost all individual VOC
28
have decreased following the introduction of three-way catalytic convertors and evaporative
29
emission controls in vehicles, and measures taken by industry to reduce emissions. UK
30
emissions of VOC are expected to meet the 2010 NECD Target Values.
31
Heavy Metals
32
UK emissions of lead, cadmium and mercury have significantly decreased since 1990,
33
achieving compliance with the 1998 UNECE Protocol on Heavy Metals, mainly due to
34
reductions in the consumption of coal, the decline of heavy industry in the UK and the
35
withdrawal of leaded petrol.
Emissions from the
Data on ammonia emissions in the UK are reliable only from
Review of Transboundary Air Pollution (RoTAP) Consultation Draft V1.02
Page vii
Recent
1
Current Trends in Concentrations and Deposition
2
Ambient concentrations of sulphur dioxide and nitrogen dioxide (the main atmospheric
3
component of nitrogen oxides) in air declined by an average of 91% and 50%, respectively
4
between 1980 and 2007. Concentrations of ammonia remain high over extensive areas of the
5
UK, and have changed little over the last decade.
6
decreases in the rainwater concentrations of acidity (85%), sulphate (SO 4) (75%), and non-
7
marine chloride, from mainly coal combustion (95%) have been reported.
8
Pollutants in the atmosphere are transported to the Earth’s surface either by the dry
9
deposition of particles and gases or by wet deposition by being removed from the
10
atmosphere by precipitation such as rain and snow. Total deposition is the sum of dry and
11
wet deposition. Dry and wet deposition of sulphur in the UK have decreased by 93% and
12
57% respectively, over the last twenty years. The total deposition of nitrogen in the UK
13
(which is fairly evenly split between the oxidised forms such as NO X and reduced forms such
14
as ammonia) has changed little over the same period, remaining close to 400kt nitrogen per
15
year, despite large reductions in the emissions of nitrogen oxides (46%) and smaller
16
reductions in the emissions of ammonia (24%). The available evidence implies that most of
17
the deposited nitrogen is accumulating in terrestrial ecosystems, and may be released at a
18
future date, although ecologically significant levels of nitrate (with respect to both
19
acidification and eutrophication) are leaching into upland waters in higher deposition regions.
20
Peak ozone concentrations in rural areas of the UK have declined by about 30% in the last 20
21
years. However, background ozone concentrations have increased by approximately 0.2ppb
22
per year (an increase of approximately 10% over twenty years), in many regions of the UK.
23
Current Trends in Exceedances of Critical Loads and Levels
24
The Critical Loads and Levels concept is an effects-based risk assessment tool used in the
25
development of pollutant abatement strategies. Critical Levels for vegetation are defined as
26
the “concentration, cumulative exposure or cumulative stomatal flux of atmospheric
27
pollutants above which direct adverse effects on sensitive vegetation may occur according to
28
present knowledge”. Critical Levels are exceeded when concentrations (or exposure or flux)
29
are above the set Critical Level. The general definition of a Critical Load is “a quantitative
30
estimate of an exposure to one or more pollutants below which significant harmful effects on
31
specified sensitive elements of the environment do not occur according to present
32
knowledge”.
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observed changes in the structure and function of ecosystems (e.g., empirical Critical Loads
34
of nutrient nitrogen) or using a model-based approach (e.g., steady-state mass balance
35
models or dynamic models).
36
specified “critical limit” is not violated, for example, to prevent the acid neutralising capacity
37
(ANC) in surface water falling below a specified threshold value, or a concentration of metal
38
in soil increasing above a specified critical limit value. Critical Load exceedance is the amount
39
by which pollutant deposition exceeds this Critical Load. Exceedance of steady-state Critical
40
Loads represents the potential for harmful effects from pollutant deposition to systems in
41
steady-state conditions; thus current exceedance of Critical Loads does not necessarily
Over the last twenty years, large
Critical Loads may be derived empirically, where the values are based on
The model-based Critical Loads are calculated so that a
Review of Transboundary Air Pollution (RoTAP) Consultation Draft V1.02
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1
equate with current damage.
Similarly, achievement of non-exceedance of Critical Loads
2
does not necessarily mean that affected systems have recovered.
3
A Critical Load Function for acid deposition has been developed which quantifies the
4
combined potential acidification from both sulphur and nitrogen deposition. Using deposition
5
data for 1996-98, 71% of broad habitat areas exceeded the Critical Loads for acidity, and
6
using data for 2004-06 Critical Load exceedance had decreased to 58% of the area of UK
7
broad habitats. When exceedance of the Critical Load of acidity is broken down into separate
8
contributions from sulphur and nitrogen, only 16% of the broad habitat areas are exceeded
9
by sulphur deposition alone, compared to 41% for nitrogen deposition alone.
This
10
demonstrates that nitrogen is now a far greater contributor to exceedance of the Critical Load
11
of acidity than sulphur.
12
Critical Loads for eutrophication by nitrogen deposition were exceeded across 62% of the
13
sensitive broad habitat area using deposition data for 1986-88, and this figure remains
14
virtually unchanged (at 61%) 20 years later using the deposition data for 2004-06. By 2020
15
the area exceeded is predicted to decline to 48%.
16
eutrophication for freshwater habitats (except softwater lakes and dune slack pools) and
17
these should be developed.
18
Earlier assessments of ozone effects on vegetation have been based on cumulative
19
exceedance over a growing season of a concentration of 40ppb (the AOT40 index), and this
20
was the basis on which critical levels of ozone were defined. There is increasing evidence
21
from across Europe that the definition of critical levels, and evaluation of ozone impacts,
22
should be based on the cumulative flux of ozone through the stomata into leaves over a
23
defined exposure period. Flux-based risk critical levels for effects on growth of sensitive
24
forest trees are exceeded throughout the UK, and the application of flux-response
25
relationships for wheat suggested that, in 2000, yield losses exceeded 10% over much of
26
southern Britain, and that total national yield loss was about 2 million tonnes. For semi-
27
natural vegetation, AOT40-based critical levels are exceeded over a significant proportion of
28
the UK.
29
Critical loads for ecosystem effects have been calculated for a range of metals, including
30
cadmium (Cd), lead (Pb), copper (Cu), nickel (Ni) and zinc (Zn) for 6 UK habitats. Over 50%
31
of areas of managed broad-leafed woodland and unmanaged woodland exceed the Critical
32
Loads for Cu, Pb and Zn. Exceedance of the Critical Limit, indicating that the current soil
33
concentration may be having an impact, was reported for zinc in 25% of all UK broad habitats
34
and for copper in 50% of broadleaved woodland. It should be noted that exceedance of the
35
critical limit is not due entirely to heavy metals deposited from the atmosphere, as metals
36
may also be present in soils as a result of geological processes or via the application of
37
wastes.
There are no Critical Loads for
38
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2
What are the future trends in Emissions of Transboundary Air
Pollution ?
3
Future emission projections of key atmospheric pollutants have been compiled up to 2020.
4
These projections take into account various forecasts, including future fuel consumption (by
5
fuel type and sector), economic growth forecasts (with projections of changing activity in
6
some sectors), traffic growth, expert judgement in technological and operating efficiencies,
7
and Government policies and legislation, including air quality and climate change. The
8
assumptions upon which these forecasts are based are constantly updated as new
9
information becomes available and the projected emission values are therefore revised
10
regularly. The projections for 2020 included in the RoTAP report are based upon forecasts
11
made during 2008. Both the Gothenburg Protocol and the NECD have recently been
12
reviewed, and new Emission Ceilings for 2020 are expected.
13
Following the large reductions in sulphur emissions from the major source sectors, the
14
relative contribution from other sectors such as shipping is increasing. Annex VI of the
15
MARPOL convention set limits on the sulphur content of fuel oil for shipping with additional
16
provision of “SOx Emission Control Areas” (SECAs) to be established where the sulphur
17
content of the fuel oil is restricted further unless the ships employ emissions abatement
18
equipment. The North Sea and English Channel have been designated as SECAs, and ships
19
using these lanes have been required to use low sulphur fuel since 22nd November 2007. This
20
legislation is expected to produce an 85% reduction of emissions of sulphur dioxide from
21
shipping in the vicinity of the UK by 2020.
22
Emissions of nitrogen oxides from road vehicles are likely to decrease as older vehicles are
23
replaced with more modern ones with lower emissions, although emissions from vehicles are
24
also governed by other socioeconomic factors.
25
Emissions of ammonia are expected to decline, but only by a relatively small amount and are
26
difficult to predict accurately, as they depend on agricultural livestock numbers and fertiliser
27
use. As large pig and poultry farms come under the Integrated Pollution Prevention and
28
Control (IPPC) legislation, an increased uptake of mitigation methods is predicted.
1
29
30
Despite controls on the emissions of ozone precursor gases across Europe, emissions from
31
shipping, aircraft, vehicles and industrial sources are increasing in other parts of the Northern
32
Hemisphere, especially in rapidly developing countries such as India, China and South Korea.
33
Without concerted policy action at the hemispheric scale to control precursor emissions, the
34
background ozone concentration arriving into the UK from these larger scale emissions will
35
continue to increase, eroding the benefits of European control measures.
36
37
Future trends in emissions of heavy metals are uncertain, as we are currently unable to
38
account for most of the metals present in the air or deposited in the UK. The emissions
39
estimates therefore need to be substantially improved to include currently unaccounted
40
sources.
41
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2
Have the reductions in emissions of pollutants resulted in a similar
reduction in measured concentrations and deposition ?
3
The reductions in emissions of sulphur dioxide and nitrogen oxides have led to major
4
improvements in air quality, including removal of ~90% of the sulphur and nearly half of the
5
nitrogen oxides from the UK atmosphere. The decline in sulphur in particular has greatly
6
reduced the main cause of historic acidification, and evidence of a recovery from the effects
7
of acidification is widespread. However, deposited nitrogen remains a potential source of
8
terrestrial and freshwater acidification.
9
The reductions in emissions of nitrogen compounds, and in particular nitrogen oxides have
10
reduced nitrogen dioxide concentrations but have not substantially reduced the deposition of
11
nitrogen to the UK landscape. In fact the 40% reduction in emissions of nitrogen oxides
12
between 1990 and 2006 has resulted in a reduction of only 10% in oxidized nitrogen
13
deposition. The cause of this non-linearity appears to be an increase in the oxidation of
14
nitrogen oxides to nitric acid (HNO3) and aerosol nitrate (NO3) over the UK with time, but is
15
not fully understood. The main consequence and benefit of the 46% reduction in emissions
16
of nitrogen oxides, has been a 49% reduction in exports of nitrogen oxides from the UK to
17
the wider European atmosphere.
1
18
19
Reductions in the emissions of ozone precursor gases across Europe have resulted in a
20
decrease in peak ozone concentrations in the UK over the last twenty years. However due to
21
increasing emissions of precursors from northern hemisphere sources outside Europe,
22
especially from South East Asia and international shipping, the background and mean ozone
23
concentrations in the UK have been rising.
24
benefits of the European reductions in precursor emissions have been eroded by the growing
25
hemispheric emissions of ozone precursors from outside Europe. It is therefore important
26
that future policy development occurs at a much larger preferably hemispheric scale, includes
27
international shipping and aviation and has reference to both background and peak ozone
28
concentrations.
The consequence of these trends is that the
29
30
Although emissions of most heavy metals, especially lead, cadmium and mercury, have
31
decreased significantly since 1990, and atmospheric concentrations are not considered a
32
major threat to ecosystem health, there is concern over the accuracy of heavy metal
33
inventories which are only able to explain 20% of the observed concentrations and
34
depositions in the UK for many of the metals. This discrepancy is thought to be due to a
35
variety of factors. In particular, the resuspension of material into the atmosphere from
36
previously deposited sources is not included in the national emission inventory. There are
37
also uncertainties associated with the sources in the emissions inventory. It is suspected that
38
fugitive emissions are underestimated, some source strengths are thought to be more
39
variable than the available data indicate, and unaccounted sources may be significant.
40
41
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2
What are the impacts of the pollutants on the environment and is
there any evidence of recovery from these impacts?
4
Effects of acidity (sulphur and nitrogen concentrations and deposition) on
terrestrial mosses and lichens
5
Historically the greatest impacts of air pollution in the UK were caused by large toxic
6
concentrations of sulphur dioxide, smoke and nitrogen dioxide in air. In urban areas these
7
resulted in the widespread loss of sensitive species, especially mosses and lichens. This era of
8
the mid 20th century ended in the 1970s and 1980s as controls on emissions in urban areas
3
9
10
began to take effect.
11
Acidification of UK soils and waters and its impact on biota
12
Scientific and policy interest in acid rain was at a peak between 1970 and 1990. During this
13
time widespread terrestrial effects of the deposited acidity, mainly from sulphur, (as sulphur
14
dioxide) but also from chlorine emissions (as hydrochloric acid, HCl, which is also released to
15
the atmosphere from the combustion of fossil fuels such as coal) were detected. For example,
16
the growth of crops and semi-natural vegetation was shown to be substantially reduced
17
during the 1970s and 1980s in the areas of the UK with the greatest pollution from sulphur
18
dioxide. Concomitantly the problem and spatial extent of freshwater acidification was also
19
identified and quantified. The species composition of diatoms (a type of alga) preserved in
20
sediment cores revealed that lakes in many upland areas had been acidifying since the onset
21
of the industrial revolution. Acidity linked to both sulphate and nitrate was shown to be
22
responsible for a reduction in acid neutralising capacity (ANC) of lakes and streams, resulting
23
in water of low pH and elevated levels of biologically toxic aluminium, and the consequent
24
reduction in health, abundance and loss of sensitive species from aquatic plants to freshwater
25
macroinvertebrates (e.g. mayflies) and fish including Atlantic salmon and brown trout.
26
27
There is widespread evidence of recovery from acidifying air pollutants in the UK.
This
28
comprises chemical recovery, where the concentrations of acidifying pollutants in the air, soil
29
and freshwater and their acidity have declined, and biological recovery where ecosystems
30
have responded to the changes in the chemical climate.
31
32
Atmospheric concentrations of sulphur dioxide and nitrogen oxides have declined to levels
33
which do not pose a direct toxic threat to UK terrestrial vegetation except close to major
34
roads and in urban areas. A subsequent re-appearance of acid-sensitive biota has been
35
reported, including sensitive lichen and bryophyte species, In acidified freshwaters the
36
reduction in inputs of sulphate has resulted in recent increases in ANC, water pH and
37
decreases in biologically toxic aluminium and this has begun to stimulate the return of some
38
acid-sensitive species and changes in species composition toward communities characteristic
39
of less acidic waters.
40
41
This recovery is an ongoing process and further chemical and biological improvement is
42
expected with further anticipated reductions in acid pollutant loads. However, the process is
43
slow. Some landscapes are so depleted in base minerals that full recovery to pre-acidification
44
conditions might not be achieved in the short term, while a number of physical, chemical and
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1
biological factors may limit the rate at which acid sensitive organisms are able to re-
2
establish. Furthermore, changes in temperature, nutrient nitrogen levels and land use in
3
recent decades may result in ecosystems that differ in structure from those that existed prior
4
to acidification.
5
6
Deposited nitrogen compounds also contribute to acidification and are now a major
7
contributor to the exceedances of acidity critical loads in the UK. Nitrate concentrations have
8
risen in remote sensitive waters in some regions in recent decades, and as sulphate
9
concentrations decline nitrate represents an increasing proportion of incoming acidity. The
10
future fate of the bulk of deposited nitrogen that is thought to have been accumulating is
11
soils remains unclear but potential acidification from this source remains.
12
13
Effects of eutrophication on plants and species diversity.
14
The deposition of nitrogen compounds also cause eutrophication. There is strong evidence
15
from the Countryside Survey data reported in 2001 in the NEGTAP report covering surveys
16
back to the 1970s that nitrogen deposition has significantly reduced the diversity of plant
17
species in a range of habitats of conservation value over large areas of the UK. The link
18
between nitrogen deposition and plant species composition in the UK landscape remains clear
19
in the most recent Countryside Survey (2007) but there is no countrywide evidence of further
20
declines in plant diversity over the last decade in areas of high nitrogen deposition. Studies of
21
phytoplankton growth rate in UK upland lakes demonstrate the potential role of atmospheric
22
N deposition in changing algal productivity and the balance of nutrients required for plant
23
growth. Some lakes may be undergoing changes considered a deviation from the good
24
ecological status required under the EU Water Framework Directive.
25
26
Despite the reductions in emissions of nitrogen oxides, there is no UK evidence of recovery
27
from the eutrophication caused by nitrogen deposition. This is due to several contributing
28
reasons, total nitrogen deposition has not declined significantly; the deposited nitrogen is
29
accumulating in vegetation and soils; and the most damaging contributor to the effects,
30
gaseous ammonia concentrations, have not been reduced in the UK.
31
32
To address the problems of eutrophication further substantial reductions in emissions of
33
nitrogen compounds are therefore needed. The available evidence of plant effects shows that
34
nitrogen deposited as gaseous ammonia is more damaging to sensitive plant communities
35
than nitrogen deposited in rain. Reductions in emissions of ammonia are therefore the
36
37
priority target to reduce effects of eutrophication.
38
Effects of ozone on vegetation
39
Biological indicators of ground level ozone show that concentrations continue to exceed
40
thresholds for effects on sensitive species. Background concentrations of ozone in the
41
northern hemisphere have now increased to a level where exposure to ozone may cause
42
adverse effects on vegetation. These effects are most likely to occur in crops grown in the
43
southern areas of the UK during spring and summer. It has been estimated that ozone
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reduces the yield of wheat grown in southern Britain by 5% to 15%, equivalent to a reduction
2
in national yield in 2000 of approximately 2 million tonnes.
3
4
Impacts of ozone on semi-natural vegetation have also been demonstrated, causing problems
5
for some areas of the UK in meeting the requirements of Biodiversity legislation. EU Directive
6
92/43/EEC on the Conservation of Natural Habitats and of Wild Fauna and Flora (Habitats
7
Directive) requires member states to maintain or restore protected natural habitats and wild
8
species at a favourable conservation status.
9
10
Ground level ozone also reduces carbon sequestration by vegetation, and thus ozone
11
contributes to global warming indirectly by reducing carbon sequestration. As ozone is a
12
greenhouse gas in its own right (i.e. by absorbing the outgoing thermal wavelengths in the
13
radiation budget of the earth), and is third in importance behind carbon dioxide and
14
methane, the effects of ozone on vegetation increase the importance of controlling ground
15
16
17
level ozone .
18
Impacts of metals on vegetation, soils and water
19
20
Emissions of heavy metals have had impacts on terrestrial and freshwater species, although
21
there is often insufficient data to quantify the contributions of metals deposited from the
22
atmosphere relative to other local pollution sources.
23
24
Chemical recovery of soils and sediments contaminated with heavy metals may take
25
centuries, especially for metals which are strongly bound to the soil and sediment.
26
Concentrations of metals such as nickel, zinc and cadmium may return to background levels
27
in some areas during the present century, but this is not the case for copper and lead.
28
Despite reductions in metal deposition, inputs of lead and mercury to remote upland lakes in
29
the UK have remained relatively constant, indicating that the erosion of contaminated soils
30
within their catchments might now be an important source.
31
33
How do assessments of the impacts of transboundary air pollution
on the environment vary between policy arenas?
34
Transboundary air pollution and its impacts on the environment have been considered in
35
RoTAP largely within the policy arena directed towards directives and protocols specifically on
36
air pollution emissions (e.g. the Gothenburg protocol, the EU Large Combustion Plant
37
Directive and the National Emission Ceilings Directive). The environmental issues covered in
38
the report however are also relevant to other environmental protection policy arenas
39
including the Water Framework Directive and the EU Habitats Directive. The extent to which
40
the changes in emissions and deposition observed in the report deliver benefits in the
41
different policy arenas is an important policy question and is difficult to quantify with helpful
42
precision. For example, the EU Water Framework Directive is concerned with maintaining
43
aquatic ecosystems in, or restoring them to, good (or better) ecological health. In contrast to
32
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earlier water directives, which were based around chemical targets, the Water Framework
2
Directive places emphasis on the ecological structure and function of aquatic ecosystems with
3
biological elements at the centre of the status assessments, and hydromorphology and
4
physico-chemistry as supporting elements. Ecological quality is judged by the degree to
5
which present-day conditions deviate from those in the absence of anthropogenic influence,
6
termed ‘reference’ conditions. Sites in which the various elements correspond totally or
7
almost totally to undisturbed (reference) conditions are classed as High status. Four further
8
categories of Good, Moderate, Poor and Bad status refer to the degree of deviation from the
9
reference state.
10
Although the objectives of the different directives and protocols with respect to acid waters
11
are the same, i.e. improvement in conditions with respect to a target, the concepts and
12
procedures in assessing the extent of recovery, are essentially different. The UNECE protocols
13
involve targets that are derived using a chemical methodology and the target of an ANC4 of
14
20 µeq/l is set for nearly all sites, i.e. irrespective of the starting or reference ANC of any
15
individual site, the exception being the naturally very acidic sites mentioned above where the
16
Critical Limit has been set as 0 µeq/l. In other words the 20 ANC target can be very
17
undemanding for sites with significantly higher pre-acidification ANC values. Eliminating
18
Critical Loads exceedance at such sites satisfies the UNECE standards but nevertheless allows
19
considerable damage, that, in the phraseology of the WFD, may maintain the site below the
20
“good to moderate” boundary. In the Water Framework Directive, on the other hand, the
21
target is defined ecologically rather than chemically, and recovery is measured against the
22
yardstick of the reference state, as described above, rather than a standard national target.
23
In theory this is a more appropriate approach as it takes into account the extent of deviation
24
from the pristine or high status condition and uses the reference state as a target for
25
recovery. There are two problems, however. First, the reference state needs to be defined,
26
and second, the WFD typology places all acid waters into a single (alkalinity = < 200 µeq/l)
27
class. There are a range of approaches that can be used to address the former problem. The
28
latter is of great concern as Environmental Protection Agencies may not treat all low alkalinity
29
waters in the same way and may be tempted to adopt the same standard target for all
30
acidified waters.
31
32
Critical Loads and Levels were originally developed for use as the basis for evidence-based
33
discussion of emissions reductions at an international level through CLRTAP. Over the last
34
decade it has been realised they are also very useful tools for assessing environmental
35
impacts at a national and local scale, although they need to be adapted for specific
36
applications. This report describes how Critical Loads are used nationally and internationally
37
under CLRTAP.
38
policy areas, such as the NEC Directive, the Convention on Biological Diversity (CBD), COST
39
729 and INI. Within the UK, Defra have used Critical Load exceedance data to inform the UK
4
In addition, the Critical Loads concept has been adopted for use in other
ANC refers to the Acid Neutralising Capacity is the overall capacity of a freshwater or soil
solution to buffer against acidification.
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Air Quality Strategy and as a Sustainable Development indicator5. The UK environment
2
agencies have used national Critical Loads and exceedance data in the review of the
3
Electricity Supply Industry Permits.
4
Although Critical Loads provide an effects-based framework they were not aimed at the
5
Habitats
6
incorporation as an assessment tool have been developed. In collaboration with JNCC “Site-
7
Relevant Critical Loads” (SRCL), based on the national data and methods, are being used to
8
help inform the Common Standards Monitoring of SSSIs and reporting of Favourable
9
Conservation Status of Annex I habitats of the Habitats Directive.
Directive
or
Water
Framework
Directive;
nevertheless,
methods
for
their
In addition, other
10
techniques have been developed to adapt Critical Load methods for application to
11
Conservation Sites (e.g., NATURA 2000).
12
The Habitats Directive requires Member States to take measures to maintain at, or restore to,
13
favourable conservation status Habitats and Species of Community importance.
14
States are required to report on conservation status (range, area, structure and function,
15
future prospects, pressures and threats) every six years. The Directive applies to the whole
16
range of habitat and species and not just those within designated sites. The UK 2007 report
17
on conservation status included an assessment that acid deposition and nutrient nitrogen are
18
a “current threat” and “pressure to future viability” for a substantial proportion of sensitive
19
habitats in the UK. A common approach for developing Europe-wide assessments is being
20
discussed under COST 729 and INI.
21
Reducing emissions of N (and S) deposition will help to achieve the objectives of the Habitats
22
Directive, by providing more favourable conditions to promote chemical and biological
23
conditions and ultimately reduce the areas of habitats at risk. This will not only help promote
24
recovery but prevent further decline at sensitive habitats.
25
Thus, while the direction of travel towards an improving environment is consistent in these
26
three areas, the methodology and philosophical basis underpinning each differ substantially
27
and preclude a quantitative comparison.
Member
28
30
What are the International perspectives on Transboundary Air
Pollution ?
31
Sources of pollutants outside the UK, but within Europe have declined during the last two
32
decades. However, the contribution of external sources to measured concentrations in the UK
33
remains significant, both from European and shipping sources of sulphur, nitrogen and
34
metals. In the case of ground-level ozone, most of the ozone over the UK is imported as the
35
hemispheric background with contributions from all countries in the northern hemisphere.
29
36
37
Changes in the emissions of sulphur dioxide, nitrogen oxides , and ammonia at the European
38
scale are similar to those observed in the UK, with large decreases in emissions of sulphur
5
See (http://www.defra.gov.uk/sustainable/government/progress/national/28.htm)
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dioxide and nitrogen oxides, and small reductions in emissions of ammonia. The changes in
2
emissions across Europe have resulted in changes to the atmospheric budget for sulphur over
3
the UK between 1987 and 2006, with imports decreasing from 60 to 20 kt-sulphur, and
4
exports from the UK decreasing from 1498 to 273 kt-sulphur. Corresponding changes in the
5
UK oxidised nitrogen budget were imports decreasing from 60 to 40 kt-nitrogen, and exports
6
decreasing from 789 to 423 kt-nitrogen.
7
8
The transboundary transport of oxidised sulphur and nitrogen compounds was the main
9
driver for the CLRTAP convention and a substantial fraction of emitted sulphur and nitrogen is
10
exchanged between countries. The terrestrial and freshwater effects of deposited acidity
11
observed widely in northern Europe, especially in Nordic countries, are similar to those
12
observed in the UK, and were detected much earlier in the 1960s, leading to the discovery of
13
the acid rain problem. Reductions in acid deposition, especially sulphur, are leading to
14
recovery of these damaged ecosystems. Even though emissions have been reduced
15
substantially over the last 20 years, the UK still contributes to transboundary exchange.
16
Approximately half of the UK emissions of sulphur and nitrogen are deposited within the UK.
17
Ammonia is perceived to act locally as a gaseous air pollutant, however, its role in forming
18
aerosol particles permits much longer-range transport, and may contribute to the observed
19
non-linearity between nitrogen emissions and deposition in the UK. It is therefore an
20
important component for future control strategies at a European scale.
21
22
Controls of emissions of nitrogen oxides and volatile organic compounds across Europe have
23
reduced the concentrations of ozone precursors, with a subsequent decrease in peak ground-
24
level ozone concentrations which is an important step in the process of achieving clean air.
25
However, substantial problems due to ozone remain throughout Europe. To address the
26
continuing ground-level ozone problems, further controls on the emissions of ozone
27
precursors throughout Europe are required and to be fully effective in protecting human
28
health and ecosystems, emission reductions need to be made at hemispheric scales.
29
31
How do Transboundary Air Pollution issues interact with Climate
Change?
32
Little research is available to quantify the feedbacks between acidification and climate
33
change, although in principle substantial feedbacks are possible since many of the underlying
34
processes are known to be climate sensitive. For example the atmospheric processing of the
35
pollutants is very sensitive to the supply of oxidants linked to ozone and related reactive
36
radical species found in the atmosphere, and the chemistry of these oxidants is sensitive to
37
climate. Many of the biological responses to pollutants are also very climate sensitive. For
38
example in a future, warmer climate, an increase in storminess and consequent increase in
39
sea-salt deposition could threaten further recovery in freshwaters through the recurrence of
40
strongly acidic episodes. There are therefore likely to be important feedbacks which need to
41
be explored and the policy relevance identified. Policies need to be developed in the light of
42
these feedbacks. Discussions of the potential to develop policies which address both issues
43
are a current priority, although there is no progress to report. These interactions are likely to
30
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1
have
wide-ranging
effects
on
transboundary
air
2
atmospheric transport and transformation to effects.
pollution,
ranging
from
emissions,
3
4
There is increasing interest in the effects of nitrogen deposition on the emissions of
5
greenhouse gases and carbon sequestration by plants and soil. For example, deposited
6
nitrogen can act as a nutrient which increases plant growth, with a subsequent increase in
7
photosynthesis which absorbs carbon dioxide. The fact that the global cycles of carbon and
8
nitrogen are coupled lies at the heart of the debate.
9
10
There is no doubt that policies for climate change present important opportunities to reduce
11
the scale and effects of long range transport of air pollutants. It will be important in the
12
policy context to take full advantage of these co-benefits for climate and long range transport
13
of air pollutants in the development of control measures.
14
15
The impacts of ozone are sensitive to climate change. Ozone significantly reduces carbon
16
sequestration in soils and plants. Ozone may also interfere with a plant’s ability to respond to
17
drought conditions.
18
conditions as predicted to occur in during UK summers as a consequence of climate change.
19
However, elevated concentrations of carbon dioxide may reduce the effects of ozone, by
20
reducing ozone uptake. The role of ozone as a contributor to the direct radiative forcing of
21
global climate has grown in importance, and ozone is now the third most important
22
greenhouse gas after carbon dioxide and methane.
Ozone will therefore exacerbate the impacts of increased drought
23
24
If climate change causes a reduction in soil organic matter, there is a risk that heavy metals
25
which were previously bound to the organic matter will become bioavailable, increasing the
26
risk of toxicity to organisms.
27
28
Conclusion
29
Since the problems of transboundary air pollution were identified in the 1970s, UK and
30
International policy action has achieved considerable success in managing the acid deposition
31
problem. The acid deposition resulting from emissions of sulphur dioxide has been greatly
32
reduced and the acidity of soils and acidified surface waters has declined, although many of
33
these ecosystems remain in an acidified condition, and recovery is slow.
34
problems are being addressed, and while there has been a reduction in emissions and
35
concentrations of nitrogen oxides, further policy action is required to reduce nitrogen
36
emissions, and especially ammonia to prevent further damage and allow recovery of some
37
plant communities to begin.
38
gases which form ozone have successfully reduced peak ozone concentrations in the UK.
39
However, emissions throughout the Northern Hemisphere are causing increasing background
40
concentrations of ozone in the UK, with ozone exposures in some areas sufficient to damage
41
crops and semi-natural vegetation. Control of emissions of ozone precursors at the
42
hemispheric scale is therefore required.
43
concern, although due to the legacy of historical emissions that remains in soils, some areas
44
of the UK may take centuries to recover to background levels.
Eutrophication
Policy action to reduce European emissions of the precursor
Emissions of heavy metals are not a major policy
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