2. General outline of the procedure

Central to the procedure recommended in this report is the production, for each zone, of one or more maps of the entire zone. On these maps, all areas of exceedance or near-exceedance in the zone for quantities, for which limit values have been set in the Directives, should be clearly indicated. Such quantities may be annual average concentrations, percentiles, or other statistical concentration or deposition quantities. In some cases, Member States may find it more practical to develop a statistical overview of occurrence of exceedances and near-exceedances in the entire zone as an alternative to a detailed map. The requirement of the FWD (Annex IV) for maps to be provided as part of formal action plans for improving air quality should however be borne in mind.

Although this report does not deal directly with the location of measuring stations for ongoing assessment, maps will be useful also for this purpose.

Unlike previous EC legislation on air quality the FWD envisages the use of tools other than measurement to provide the full picture needed to underpin successful air quality management. Article 2 defines "assessment" as "any method used to measure, calculate, predict or estimate the level of a pollutant in ambient air". Three main assessment methods or tools can be used singly or in combination for preliminary air quality assessment:

• preliminary air quality measurements;
• air emission inventories;
• air pollution modelling.

Preliminary air quality measurements (preliminary in the sense that these measurements do not serve to demonstrate compliance with Article 6 of the FWD) are used to explore air quality, particularly at those places where exceedances are to be expected, and/or emission information is inadequate.

Air emissions inventories provide comprehensive information on sources and their emissions and emission fluxes in the entire zone. This enables a first estimate of areas at risk of exceeding limit and target values.

Air pollution modelling serves to relate air quality to emissions in a quantitative sense, and provides a better basis for describing areas of exceedance in the entire zone. It also provides additional essential information for the management of the air quality in the zone, as required under the FWD.

All three methods or tools provide information with inherent uncertainty. By uncertainty we mean a quantitative measure of the most likely possible deviation of the value from the "true" value. See the text box below which defines some relevant concepts in this context.

Measurements - apart from sampling and analysis errors - may introduce major uncertainties if stations are not representative, meaning that air quality in the surroundings differs substantially from air quality at the station, or that concentrations vary appreciably in time while the measurements have only limited time coverage.

Emission inventories can be incomplete or may be based on inaccurate or inappropriate emission factors or activity figures.

Models may produce uncertain results either due to uncertainty in input such as meteorological quantities or emission data, or because of improper description or calculation of atmospheric processes and the resulting concentrations.

These assessment uncertainties can be reduced if information from all three methods is available. In order to judge the overall uncertainty in the assessment result, it is necessary to quantify and document uncertainties for each of the assessment methods.

As the FWD and Daughter Directives are implemented, the air quality is expected to change over the years for a variety of reasons. These include changes in human activity patterns (city growth, traffic increase, industrial production), technological developments (penetration of three-way catalyst, industrial abatement technology, etc.), and air quality management. Therefore, it is important to re-assess air quality on a regular basis, or if specific reasons suggest that this should be done. In chapter 7, recommendations are made for the updating procedure.

Some definitions of the concepts of Uncertainty, Verification and Validation (from the EMEP/CORINAIR Atmospheric Emissions Inventories Guidebook)

The following definitions of key terms are provided to promote common usage in the context of this guidance.

Accuracy

Accuracy is a measure of the truth of a measurement or estimate. The term accuracy is often used to describe data quality objectives for inventory data, however, accuracy is hard to establish in inventory development efforts since the truth for any specific emission rate or emissions magnitude is rarely known.

Precision

The term precision is used to express the repeatability of multiple measurements of the same event. In experimental applications a measurement or measurement technique could have high precision but low accuracy. The term precision is also used to describe the exactness of a measurement. The term precision is not well suited for use in emissions inventory development.

Confidence

The term confidence is used to represent trust in a measurement or estimate. Many of the activities discussed in this chapter are designed to increase the confidence that inventory developers and inventory users have in the databases. Having confidence in inventory estimates does not make those estimates accurate or precise, but will help to develop a consensus that the data can be applied to problem solving.

Reliability

Reliability is trustworthiness, authenticity or consistency. In the context of emissions inventories reliability and confidence are closely linked. If the approaches and data sources used in an inventory development project are considered reliable, then users will have an acceptable degree of confidence in the emissions data developed from those techniques.

Uncertainty

Uncertainty is a statistical term that is used to represent the degree of accuracy and precision of data. It often expresses the range of possible values of a parameter or a measurement around a mean or preferred value.

Validation

Validation is the establishment of sound approach and foundation. The legal use of validation is to give an official confirmation or approval of an act or product. Validation is an alternate term for the concept of verification as used in this context.

Verification

The term verification is used to indicate truth or to confirm accuracy and is used in this chapter to represent the ultimate reliability, and credibility of the data reported.