Guidance on air quality assessment around
point sources under the
EU Air Quality Directive 2008/50/EC
European Commission,
DG Environment
2010
Guidance on assessment around point sources under the EU Air Quality Directive 2008/50/EC
Abstract
This short guidance gives recommendations on the assessment of air quality around point sources
under Air Quality Directive 2008/50/EC. It gives an overview of recommendations given by former
EU working groups and discusses the advantages and drawbacks of assessment by measuring and by
mathematical modelling.
For sources of low to medium elevation a recommendation on the location of monitoring sites is
given. For high sources it is recommended to base the assessment on mathematical modelling,
possibly complemented by monitoring.
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Guidance on assessment around point sources under the EU Air Quality Directive 2008/50/EC
Contents
1
1.1
1.2
Introduction .................................................................................................................................. 4
General ....................................................................................................................................... 4
Legal provisions ......................................................................................................................... 4
2
Information on monitoring and assessment in existing publications (position papers and
guidance on assessment) ....................................................................................................................... 4
3
Assessment by monitoring ........................................................................................................... 5
4
Assessment by modelling.............................................................................................................. 5
5
5.1
5.2
6
Recommendations ......................................................................................................................... 6
Fugitive emissions and point sources of low to medium elevation ............................................ 6
Elevated point sources ................................................................................................................ 7
References...................................................................................................................................... 8
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Guidance on assessment around point sources under the EU Air Quality Directive 2008/50/EC
1
1.1
Introduction
General
This guidance aims to give recommendations on how to assess air quality around point sources under
the EU Air Quality Directive 2008/50/EC1. It is intended for professionals that are responsible for the
assessment of air quality in the framework of this directive.
The guidance is based on Note 2003/3 of the former CAFE Working Group on Implementation. It has
been updated to the new legislation, and references to the authorship of the working group have been
removed.
1.2
Legal provisions
The Air Quality Directive requires that, once limit values and alert thresholds have been set, ambient
air quality shall be assessed throughout the territory of Member States. This assessment can be done
using monitoring data and supplementary assessment like modelling. Since assessment has to be done
throughout the territory, even small-scale hot spot situations should in principle be tackled.
In the directive some considerations concerning the siting of fixed monitoring stations are given. In
Annex III it is stated that sampling points directed at the protection of human health should be sited to
provide data on the following:
(i)
the areas within zones and agglomerations where the highest concentrations occur to which
the population is likely to be directly or indirectly exposed for a period which is significant in relation
to the averaging period of the limit value(s),
(ii)
levels in other areas within the zones and agglomerations which are representative of the
exposure of the general population.
The annex further states:
Sampling points shall in general be sited in such a way as to avoid measuring very small microenvironments in their immediate vicinity, which means that a sampling point must be sited in such a
way that the air sampled is representative of air quality for a street segment no less than 100 m length
at traffic-orientated sites and at least 250 m × 250 m at industrial sites, where feasible.
Concerning the determination of the minimum number of sampling points it is stated in Annex V that:
for the assessment of pollution in the vicinity of point sources, the number of sampling points for fixed
measurement shall be calculated taking into account emission densities, the likely distribution patterns
of ambient-air pollution and the potential exposure of the population.
It should be noted that in most cases the point sources are installations that require permits to operate.
These permits are issued on the basis on national legislation, some of it transposing also EU
requirements such as IPPC Directive 2008/1/EC., and may require air quality assessment as one of the
permitting conditions.
2
Information on monitoring and assessment in existing publications
(position papers and guidance on assessment)
Some indications on a proper monitoring and assessment strategy at industrial hot spots are given in
the position papers on pollutants in the former Daughter Directives and in the ‘Guidance on
Assessment’http://ec.europa.eu/environment/air/pdf/guidanceunderairquality.pdf . However, it has to
1
Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality
and cleaner air for Europe, http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2008:152:0001:0044:EN:PDF
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Guidance on assessment around point sources under the EU Air Quality Directive 2008/50/EC
be emphasised that the information in the position papers and the guidance report are only
recommendations, and that they can be complemented by more specific and mandatory requirements
set at the national level or in the individual industrial permit.
The most detailed information is given in the position paper on heavy metals. Summing up, the
Working Group on Heavy Metals http://ec.europa.eu/environment/air/pdf/guidanceunderairquality.pdf
recommended:
that the monitoring site should be representative at least of an area of 250 m x 250 m. Consequently,
sampling points should keep a minimum distance of 100 - 200 m from the fences of installation areas.
It is recommended that a measuring station to check compliance in an industrial hot spot should be
placed at a distance of 150 +/-50 m from the border of the installation, if protective goods can be
exposed there.
A
similar
recommendation
is
contained
in
the
Position
paper
on
PAH
http://ec.europa.eu/environment/air/pdf/pp_pah.pdfIn
the
Position
paper
on
SO2
http://ec.europa.eu/environment/air/pdf/pp_so2.pdf it is stated that:
an averaging area of 10 000 m2 is proposed for compliance analysis. In terms of monitoring siting, this
means that the station should not be sited to measure local peaks with a spatial extent smaller than
100 m. This spatial extent seems also reasonable for the protection of cultural heritage.
In practical implementation these recommendations may have to be reconsidered where for example
the exposure of the population is likely at closer vicinity to the fence.
3
Assessment by monitoring
Monitoring the impact of point sources on the ambient air quality is a difficult task, in particular for
elevated sources. This difficulty is linked to the complexity and unpredictability of the meteorological
parameters that control the dispersion of air pollutants from elevated sources.
Elevated point sources emit well above the ground, and, in the occurrence of a temperature inversion,
frequently above the inversion layer. Most of the time, the meteorological conditions observed during
“normal” situations, will guarantee a good dispersion of the pollutants. Local impacts on the air quality
from elevated sources are therefore seldom, and the effect is usually felt at longer range.
In the case of point sources of medium elevation and under very turbulent atmospheric conditions, the
plume may reach the ground in the vicinity of the source, giving rise to exceedances of short-term
limit values. These events generally occur during periods characterised by thermal instability of the
atmosphere associated with strong insolation and weak winds. The areas of maximum concentrations
are usually not foreseeable, and do not necessarily coincide with the prevailing wind directions.
Exceptional meteorological conditions characterised by strong atmospheric stability may cause the
plume from elevated point sources to stagnate in the vicinity of the emission source and to cause
exceedances of the air quality thresholds at local level. This will happen when the emissions occur
under the temperature inversion layer in association with low or still winds. Such adverse
meteorological conditions are in particular observed during anti-cyclonic weather situations. It should
be noted that because of the high variability of the wind direction during such episodes, the sites of
maximum pollution are not foreseeable. Nor do these sites coincide with the prevailing wind directions
that occur under “normal” meteorological conditions.
Because of this, controlling the impact of elevated point sources by monitoring alone, would require
the installation of a dense measurement network around the emission source. For reasons of efficiency
and cost-effectiveness, this solution would not be acceptable.
4
Assessment by modelling
On the other hand, the use of mathematical models would offer a number of advantages:
 models give more complete information on the spatial distribution of air pollutants than a
monitoring network;
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Guidance on assessment around point sources under the EU Air Quality Directive 2008/50/EC
 the use of models is by far more cost-effective than measurements;
 models can be used for the analysis and forecasts of air pollution episodes.
The exceptional meteorological situations identified in Chapter 3 are at least for the moment not well
covered by modelling either. The main difficulty with the use of models however resides with the
accuracy of the emission data. The existing emission inventories rarely reflect the real emissions:
 accidents are unpredictable;
 special episodes (maintenance, cleaning of installation) may not be appropriately taken into
account;
 fugitive emissions are often not controlled and considered in the emission inventory.
However, for air pollutants for which long-term limit values are considered (yearly average),
accidental or episodic releases will not modify the emission budget importantly. In this case, and
provided there are no fugitive emissions, (validated) models offer an alternative solution to monitoring
for checking compliance with limit values.
In all other cases, a combination of measurements and mathematical models would in this case offer
clear advantages with respect to monitoring alone.
5
Recommendations
It seems that general rules for the assessment of air pollutants in the vicinity of point sources cannot be
derived easily. It is important to develop and document the assessment strategy on assessment of point
sources, taking account "…emission densities, the likely distribution patterns of ambient air pollution
and the potential exposure of the population…", as described by Annex V of the AQ Directive, then
apply it consistently for all types of point sources.
Such strategy may consist of combination of different assessment approaches (fixed monitoring,
modelling, combination and indicative monitoring…). Specific consideration should be given to
assessment around new or modified sources, and installations with derogations or record of failure to
respect provisions related the environmental protection. In case of several different point sources in
close vicinity the strategy should consider the possibility to differentiate between different
contributions, in particular where the permits or the short term action plan for the area (Art. 24 of
Ambient Air Quality Directive) establish direct link the ambient concentration levels and the activity
of the installation. To the extent appropriate, the air quality assessment provisions of the permits
should be integrated with the assessment strategy under the Ambient Air Quality Directive.
As regards the practical implementation, FAIRMODE (http://fairmode.ew.eea.europa.eu/) has
developed the Guidance on the use of models that includes recommendation on the selection and
validation of models to be fit for purpose of assessment around point sources. When models are
applied, most appropriate input has to be used (e.g. multi-annual meteorology, or actual meteorology
when appropriate). Emission inventory has to reflect real emissions to the maximum extent.
Depending on the emitted pollutant(s), automated continuous monitoring of a relevant subset of
pollutants may be appropriate to catch transient effects and enable proper model validation where
applied.
Some more specific recommendations are given below.
5.1
Fugitive emissions and point sources of low to medium elevation
The focus of the assessment would be on residential areas most adjacent of the installation.
The prevailing wind direction would be considered; for pollutants where the limit/target value is one
year, a potential sampling point could be located where the highest concentration caused by the
installation is expected.
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Guidance on assessment around point sources under the EU Air Quality Directive 2008/50/EC
5.2
Elevated point sources
The assessment would be based on mathematical models, simulating meteorological conditions
observed for a period of several years, preferably at least 5 years. The modelling can be based either
on a time series of meteorological data, or on a number of meteorological scenarios combined with
information on their frequency of occurrence. Averaging periods of interest are short-, medium- and
long-term (1h, 24h, 1 year).
Complementary to the mathematical model simulations, the assessment could be completed by a
limited number of measurement stations (one or maximum two), focusing on locations with potential
problems. Due to some inherent uncertainty in modelling (see above) these stations also provide
important assurance that the model behaves correctly. These stations would also serve the purpose of
informing the public, as required by the Air Quality Directive, and would be sited in residential areas
most representative of the population exposure.
5.3
Situations with high background concentrations
The simplified discussion in Chapter 3 regarding the link between emissions and the ambient
concentration levels assumes that major contribution to the elevated concentrations comes from the
point source, and therefore short term standards are most likely to be exceeded. In case of additional
sources in the are or high background concentrations (example NO2) these assumptions may not hold
anymore; even relatively small contributions from the point source might lead to exceedances
including of yearly averages.
These situations approach more closely the 'standard' assessment situation addressed by the 'diffusive
sources'
assessment
strategies
described
in
Guidance
on
Assessment
http://ec.europa.eu/environment/air/pdf/guidanceunderairquality.pdf . Specific regime (location of
sampling points etc.) addressing the point source should still follow the recommendations above, in
particular as the points sources usually emit several pollutants that need to be monitored, and some can
be very specific to the activity. High background or important nearby sources should however be
considered in model selection, validation and input data, ensuring that the assessment is still fit for
purpose and can be coherently integrated with the assessment of diffuse sources.
5.4
Supplementary information
It is recommended that the competent authority collects additional information on the operation of the
plant and local meteorological information so this can be looked at in conjunction with the assessment
results. It can help separate contributions of different sources and also identify what operations on the
site are leading to poor air quality.
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6
References
More information on the selection and application of models can be found in:
Denby et al. (2010) Guidance on the use of models for the European Air Quality Directive. ETC/ACC
report.http://fairmode.ew.eea.europa.eu/fol429189/forumsguidance/model_guidance_document_v6_2.pdf]
Model Documentation System of the European Topic Centre on Air and Climate Change (http://airclimate.eionet.europa.eu/databases/MDS/index_html).
Website of FAIRMODE (Forum for AIR quality MODelling) http://fairmode.ew.eea.europa.eu/
Proceedings of the conferences on the Harmonisation within Atmospheric Dispersion Modelling for
Regulatory Purposes (http://www.harmo.org/default.asp).
Recommendations of the AIR4EU project http://www.air4eu.nl/, in particular R.S. Sokhi et al.
(2006) ‘First recommendations for best methods and techniques for conducting air quality assessment
at local and hotspot scales’ http://www.air4eu.nl/PDF/Reports_products/D3.2.pdf
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