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The following upper and lower assessment thresholds will apply:aSULPHUR DIOXIDE

	Health protection	Ecosystem protection
Upper assessment threshold	60% of 24-hour limit value (75 μg/m3) not to be exceeded more than 3 times in any calendar year	60% of winter limit value (12 μg/m3)
Lower assessment threshold	40% of 24-hour limit value (50 μg/m3) not to be exceeded more than 3 times in any calendar year	40% of winter limit value (8 μg/m3)

bNITROGEN DIOXIDE AND OXIDES OF NITROGEN

	Hourly limit value for the protection of human health (NO2)	Annual limit value for the protection of human health (NO2)	Annual limit value for the protection of vegetation (NOx)
Upper assessment threshold	70% of limit value (140 μg/m3) not to be exceeded more than 18 times in any calendar year	80% of limit value (32 μg/m3)	80% of limit value (24 μg/m3)
Lower assessment threshold	50% of limit value (100 μg/m3) not to be exceeded more than 18 times in any calendar year	65% of limit value (26 μg/m3)	65% of limit value (19.5 μg/m3)

cPARTICULATE MATTER

	24-hour average	Annual average
Upper assessment threshold	60% of limit value (30 μg/m3) not to be exceeded more than 7 times in any calendar year	70% of limit value (14 μg/m3)
Lower assessment threshold	40% of limit value (20 μg/m3) not to be exceeded more than 7 times in any calendar year	50% of limit value (10 μg/m3)

	Averaging Period	Limit Value	Margin of tolerance	Date by which limit value is to be met
124-hour limit value for the protection of human health	24 hours	50mg/m3 PM10 not to be exceeded more than 7 times a calendar year	To be derived from data and to be equivalent to Stage 1 limit value.	1st January 2010
2Annual limit value for the protection of human health	Calendar year	20 mg/m3 PM10	50% on 1st January 2005 reducing every 12 months thereafter by equal annual percentages to reach 0% by 1st January 2010	1st January 2010

dLEAD

	Annual average
Upper assessment threshold	70% of limit value (0.35 μg/m3)
Lower assessment threshold	50% of limit value (0.25 μg/m3)

Exceedances of upper and lower assessment thresholds must be determined on the basis of concentrations during the previous five years where sufficient data are available. An assessment threshold will be deemed to have been exceeded if it has been exceeded during at least three separate years out of those previous five years.

Where fewer than five years' data are available, measurement campaigns of short duration during the period of the year and at locations likely to be typical of the highest pollution levels may be combined with results obtained from emission inventories and modelling to determine exceedances of the upper and lower assessment thresholds.

aSampling points directed at the protection of human health should be sited:ito provide data on 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);iito provide data on levels in other areas within the zones and agglomerations which are representative of the exposure of the general population.Sampling points should in general be sited to avoid measuring very small micro-environments in their immediate vicinity. As a guideline, a sampling point should be sited to be representative of air quality in a surrounding area of no less than 200 m2 at traffic-orientated sites and of several square kilometres at urban-background sites.Sampling points should also, where possible, be representative of similar locations not in their immediate vicinity.Account should be taken of the need to locate sampling points on islands, where that is necessary for the protection of human health.bSampling points targeted at the protection of ecosystems or vegetation should be sited more than 20 km from agglomerations or more than 5 km from other built-up areas, industrial installations or motorways. As a guideline, a sampling point should be sited to be representative of air quality in a surrounding area of at least 1,000 km2. A sampling point may be sited at a lesser distance or to be representative of air quality in a less extended area, taking account of geographical conditions.Account should be taken of the need to assess air quality on islands.

The following guidelines should be met as far as practicable:the flow around the inlet sampling probe should be unrestricted without any obstructions affecting the airflow in the vicinity of the sampler (normally some metres away from buildings, balconies, trees, and other obstacles and at least 0.5 m from the nearest building in the case of sampling points representing air quality at the building line);in general, the inlet sampling point should be between 1.5 m (the breathing zone) and 4 m above the ground. Higher positions (up to 8 m) may be necessary in some circumstances. Higher siting may also be appropriate if the station is representative of a large area;the inlet probe should not be positioned in the immediate vicinity of sources in order to avoid the direct intake of emissions unmixed with ambient air;the sampler’s exhaust outlet should be positioned so that recirculation of exhaust air to the sampler inlet is avoided;location of traffic-oriented samplers:for all pollutants, such sampling points should be at least 25 m from the edge of major junctions and at least 4 m from the centre of the nearest traffic lane,for nitrogen dioxide, inlets should be no more than 5 m from the kerbside,for particulate matter and lead, inlets should be sited so as to be representative of air quality near to the building line.

The following factors may also be taken into account:interfering sources;security;access;availability of electrical power and telephone communications;visibility of the site in relation to its surroundings;safety of public and operators;the desirability of co-locating sampling points for different pollutants;planning requirements.

The site-selection procedures should be fully documented at the classification stage by such means as compass-point photographs of the surrounding area and a detailed map. Sites should be reviewed at regular intervals with repeated documentation to ensure that selection criteria remain valid over time.

a

Population of agglomeration or zone (thousands)	If concentrations exceed the upper assessment threshold	If maximum concentrations are between the upper and lower assessment thresholds	For SO2 and NO2 in agglomerations where maximum concentrations are below the lower assessment thresholds
0–250	1	1	not applicable
250–499	2	1	1
500–749	2	1	1
750–999	3	1	1
1 000–1 499	4	2	1
1 500–1 999	5	2	1
2 000–2 749	6	3	2
2 750–3 749	7	3	2
3 750–4 749	8	4	2
4 750–5 999	9	4	2
> 6 000	10	5	3
For NO2 and particulate matter: to include at least one urban-background station and one traffic-orientated station

bFor the assessment of pollution in the vicinity of point sources, the number of sampling points for fixed measurement should be calculated taking into account emission densities, the likely distribution patterns of ambient-air pollution and the potential exposure of the population.

The following data-quality objectives for the required accuracy of assessment methods, of minimum time coverage and of data capture of measurement are laid down to guide quality-assurance programmes.

	Sulphur dioxide, nitrogen dioxide and oxides of nitrogen	Particulate matter and lead
Continuous measurement
Accuracy	15%	25%
Minimum data capture	90%	90%
Indicative measurement
Accuracy	25%	50%
Minimum data capture	90%	90%
Minimum time coverage	14% (One measurement a week at random, evenly distributed over the year, or eight weeks evenly distributed over the year)	14% (One measurement a week at random, evenly distributed over the year, or eight weeks evenly distributed over the year)
Modelling Accuracy:
Hourly averages	50%–60%
Daily averages	50%
Annual averages	30%	50%
Objective estimation
Accuracy:	75%	100%

The accuracy of the measurement is defined as laid down in the “Guide to the Expression of Uncertainty of Measurements” (ISO 1993) or in ISO 5725-1 “Accuracy (trueness and precision) of measurement methods and results” (ISO 1994). The percentages in the table are given for individual measurements averaged, over the period considered, by the limit value, for a 95% confidence interval (bias + two times the standard deviation). The accuracy for continuous measurements should be interpreted as being applicable in the region of the appropriate limit value.

The accuracy for modelling and objective estimation is defined as the maximum deviation of the measured and calculated concentration levels, over the period considered by the limit value, without taking account of the timing of the events.

The requirements for minimum data capture and time coverage do not include losses of data due to the regular calibration or the normal maintenance of the instrumentation.

The Department may allow for random measurements to be made instead of continuous measurements for particulate matter and lead by methods for which accuracy within the 95% confidence interval with respect to continuous monitoring has been demonstrated to be within 10%. Random sampling must be spread evenly over the year.

The following information should be compiled for zones or agglomerations within which sources other than measurement are employed to supplement information from measurement or as the sole means of air quality assessment:a description of assessment activities carried out;the specific methods used, with references to descriptions of the method;the sources of data and information;a description of results, including accuracies and, in particular, the extent of any area or, if relevant, the length of road within the zone or agglomeration over which concentrations exceed limit value(s) or, as may be, limit value(s) plus applicable margin(s) of tolerance and of any area within which concentrations exceed the upper assessment threshold or the lower assessment threshold;for limit values the object of which is the protection of human health, the population potentially exposed to concentrations in excess of the limit value.

Where possible maps shall be compiled showing concentration distributions within each zone and agglomeration.

ISO/FDIS 10498 (Standard in draft) Ambient air—determination of sulphur dioxide—ultraviolet fluorescence method.

ISO 7996: 1985 Ambient air—determination of the mass concentrations of nitrogen oxides—chemiluminescence method.

The reference method for the sampling of lead will be that described in the Annex to Directive 82/884/EEC until such time as the limit value in Schedule 1 to these Regulations is to be met, when the reference method will be that for PM10 specified in Part IV of this Schedule.

ISO 9855: 1993 Ambient air—Determination of the particulate lead content of aerosols collected in filters. Atomic absorption spectroscopy method.

The reference method for the sampling and measurement of PM10 will be that described in EN 12341 “Air Quality—Field Test Procedure to Demonstrate Reference Equivalence of Sampling Methods for the PM10 fraction of particulate matter”. The measurement principle is based on the collection on a filter of the PM10 fraction of ambient particulate matter and the gravimetric mass determination.