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The Air Quality Standards (Wales) Regulations 2007

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PART 2Group B pollutants, polycyclic aromatic hydrocarbons and total gaseous mercury

11.  The data quality objectives set out in the table and paragraphs below are provided to guide quality assurance programmes—

Benzo(a) pyreneArsenic, cadmium and nickelPolycyclic aromatic hydrocarbons and total gaseous mercuryTotal deposition
(1)

Indicative measurement being measurements which are performed at reduced regularity but fulfil the other data quality objectives.

Uncertainty
Fixed and indicative measurements50%40%50%70%
Modelling60%60%60%60%
Minimum data capture90%90%90%90%
Minimum time coverage
Fixed measurements33%50%
Indicative measurements(1)14%14%14%33%

12.  The uncertainty (expressed at a 95 % confidence level) of the methods used for the assessment of ambient air concentrations must be evaluated in accordance with the CEN Guide to the Expression of Uncertainty in Measurement (ENV 13005-1999)(1), the methodology of ISO 5725:1994, and the guidance provided in the CEN Report, “Air quality — Approach to uncertainty estimation for ambient air reference measurement methods” (CR 14377:2002E). The percentages for uncertainty in the above table are given for individual measurements, which are averaged over typical sampling times, for a 95 % confidence interval. The uncertainty of the measurements should be interpreted as being applicable in the region of the appropriate target value. Fixed and indicative measurements must be evenly distributed over the year in order to avoid skewing of results.

13.  The requirements for minimum data capture and time coverage do not include losses of data due to regular calibration or normal maintenance of the instrumentation. Twenty-four-hour sampling is required for the measurement of benzo(a)pyrene and other polycyclic aromatic hydrocarbons. With care, individual samples taken over a period of up to one month may be combined and analysed as a composite sample, provided the method ensures that the samples are stable for that period. The three congeners benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene where they are difficult to resolve analytically. In such cases they can be reported as sum. Twenty-four hour sampling is also recommended as advisable for the measurement of arsenic, cadmium and nickel concentrations. Sampling must be spread evenly over the weekdays and the year. For the measurement of deposition rates monthly, or weekly, samples throughout the year are recommended.

14.  The National Assembly may allow for use of wet only instead of bulk sampling if it can be demonstrated to its satisfaction that the difference between them is within 10 %. Deposition rates should generally be given as μg/m2 per day.

15.  The National Assembly may apply a minimum time coverage lower than indicated in the table, but not lower than 14 % for fixed measurements and 6 % for indicative measurements provided that it is satisfied that it can be demonstrated that the 95 % expanded uncertainty for the annual mean, calculated from the data quality objectives in the table according to ISO 11222:2002 — “Determination of the uncertainty of the time average of air quality measurements” will be met.

(1)

European Committee for Standardisation (“CEN”) publication; the address of CEN is 36, Rue de Stassart, B-1050, Brussels, Belgium http://www.cenorm.be

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