“Preliminary
Assessment (PA)”
This is the term used
in the EC air quality Directives for the air quality assessment that is to be
done in each Member State as a basis for dividing the territory into zones and
agglomerations, which are to be the air quality management areas, in which the
MS needs to do annual assessments. Guidance for methods that can be used to
carry out this PA is found in the web link section below.
Regular (annual) assessments
Assessment
regimes
Please refer to the
Topic Terms in the EC Air Quality Directives: What do they mean?, especially its Annex 1 on how the air pollution concentration
levels in a zone compared to the Limit Value (LV) (assessed through the PA or
some other means) determine the methods to be used in the assessment:
·
Concentrations above
the LV: high quality measurements, may be supplemented by
Supplementary assessment methods;
·
Concentrations between
Upper and Lower Assessment Thresholds (UAT and LAT): quality measurements are
mandatory, but fewer, less intensive measurements may be needed, provided it is
supplemented by other information;
·
Concentrations below
LAT: modelling, indicative monitoring and objective estimation is sufficient.
In the following, we
will concentrate on the two first regimes. Regarding the lowest regime (below
LAT), methods like those described in the Preliminary Assessment Guidance (see
the web link section below) may be used.
Monitoring
The minimum
requirements to the monitoring network in zones are given in Annex of the Daughter
Directives, as well as specifications of reference methods. The use of other
methods require that their equivalence to the reference methods has been proven
(see e.g. Guidance
for the Demonstration of Equivalency of Ambient Air Monitoring Methods"
(pdf 950Kb), as well as the topic descriptions on how to
monitor various pollutants).
The monitoring network
shall include stations that represent both hot spot exposure as well as the
more typical exposure situation in the zone/agglomeration (see the AQ
Directives’ annexes).
Supplementary
assessment
Monitoring alone will
normally not be sufficient to satisfy the requirements in the Directives that
the population exposure should be assessed. The needs for quantification of the
spatial distribution of air pollution concentrations cannot be satisfied by
monitoring only. Also, if the LVs are exceeded, an
action plan shall be developed, which requires that the contributions from
various sources of emissions be quantified. Normally, monitoring alone is not
sufficient to assess these contributions quantitatively.
Supplementary
assessment methods include emissions inventorying and air pollution modelling.
Emissions
inventorying: This is dealt with in the
Topic How to develop urban Emission Inventories?.
Air Pollution
modelling: This is dealt with in the topic
descriptions related to air pollution modelling, such as “How to use modelling
techniques in air quality assessment and projections?” on air pollution
modelling in the context of the EC Air Quality Directives, as well as the other
topics on modelling. The model to be used should be validated for use in the
urban area in question.
Assessments in
different types of zones
The zones of the
national territory that the Member States have defined are of different types.
Typically they are either:
1. Urban areas /agglomerations (so called if having more than
250,000 inhabitants), usually limited by administrative borders, or by
topographical features such as valleys, bowls, ridges which define a natural
air-shed. They are usually defined as zones, if the PA or other assessment has
estimated that the concentrations or a regulated pollutant is higher than the
UAT.
2. Larger regions, composed of rural
as well as urban areas.
3. Regions/areas affected by specific industrial sources.
Assessments in area
types 1 and 3 above will typically follow the process as described below.
Assessments in the larger regions of type 2 above can be more complicated as
these region areas have sometimes been defined as separate zones because the
air pollution levels are mostly low, but smaller areas within the cities in
these zones may have levels above the UAT. Most Member States will rely on
monitoring to assess the air quality in these zones and still comply with the
requirements in the directives, if in addition to monitoring simpler forms of
modelling and estimation methods are used to back up the monitoring.
Necessary input data
for air pollution modelling
4. GIS digital map for
the zone / agglomeration,
5. Activities and
emissions inventory, its spatial and temporal distribution (see Topic How to develop urban Emission Inventories?),
6. Population
distribution, in km2 or better,
7. Meteorological and
topographical data,
8. Scenarios for future
development (see Topic How to develop scenarios for Air Quality in the future?).
Procedure for
the AQ assessments
9. The results of the
monitoring can be plotted on the GIS map, its various statistics values (e.g.
annual average, percentiles corresponding to the LVs,
etc).
10. The results of the
modelling results in iso-lines of concentration
values (for the same types of statistics as the monitoring data, for instance
annual average), which are also plotted on the GIS map.
11. Data assimilation
techniques could be used to get the most possible information out of the
combined monitoring and modelling results (see e.g. http://www.ecmwf.int/newsevents/training/rcourse_notes/
)
12. Estimation of the
distribution of population exposure to concentrations of various levels, by
combining the concentration distribution with the population distribution. This
can be done using the GIS mapping system.
There are various types
of air pollution models available (see e.g. the Model Documentation System of
the ETC-ACC http://air-climate.eionet.eu.int/databases/MDS/index_html,
and the topic descriptions on modelling).
Some models only deal with concentration
assessments in a grid system in the area (e.g. 1km2 grids), and some
models include in addition so-called sub-grid models, which deal with
concentrations close to specific sources (hot-spots) such as near streets and
roads, and near industrial stacks. The latter models will come closer to an
estimate of the real exposure situation, which includes also the higher end of
the exposure distribution.
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