Monitoring discrepancies spell trouble for PM10 standard

A monitoring technique used to measure fine particulates (PM10) in UK cities under-reports pollution levels by up to 30%, a study for the European Commission has found. A proposed EC air quality standard is based on a different monitoring method - and may therefore be much harder to meet than the current UK standard.

PM10 levels are measured at 43 automated monitoring sites run by the Department of the Environment, Transport and the Regions (DETR). All these sites use the tapered element oscillating microbalance (TEOM) technique.

The UK's air quality objective for PM10 is to ensure that a rolling 24-hour mean standard of 50µg/m3 is met at the 99th percentile by 2005. The objective is based on data gathered by TEOM monitors. In contrast, a proposed EC air quality standard uses the alternative gravimetric monitoring technique as a reference method.

A major discrepancy between the two techniques has been uncovered by a recent study by the EC's Joint Research Centre. TEOM monitors in Berlin and Birmingham recorded PM10 levels 35% and 20-30% lower, respectively, than gravimetric instruments at the same locations. In contrast, good agreement was observed between the two techniques at a site in Madrid.

Air quality experts believe that under-reporting by TEOM methods occurs because heating of the probe to 50°C volatilises some aerosol particles. This effect is most important during winter pollution episodes.

Last year, the European Commission proposed a Directive which would set a new 50µg/m3, 24-hour standard for PM10 (ENDS Report 273, p 42 ). On paper, this is less stringent than the UK standard because it is not expressed as a rolling mean and because a relatively large number of exceedences would be permitted in 2005 and, to a lesser extent, 2010.

But the new findings suggest that the EC proposal may in fact be more demanding than the UK objective - and could require sharper reductions in PM10 emissions.

Discussions on the draft Directive are at a delicate stage. Environment Minister Michael Meacher has highlighted it as a priority for the UK Presidency - but the new findings may dent prospects of an agreement at the Environment Council in June.

Air quality consultant Duncan Laxen says that the EC standard "is going to cause enormous problems in the UK, where we've based our whole monitoring network on a different method." The National Society for Clean Air has asked the DETR to clarify the implications for UK practice.

The DETR remains hopeful that a robust statistical relationship between the two techniques can be developed to permit continued use of the existing network. It points out that the automated TEOM method has considerable advantages over gravimetric methods - not least in rapid provision of public information and data to support research into the behaviour and sources of air pollutants.

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