| BOX 2: Gravimetric vs TEOM measurement data Monitoring of PM10 levels in the UK has, to date, been largely based upon the use of TEOM analysers. A principal concern with the TEOM instrument is that the filter is held at an elevated temperature (50°C) in order to minimise errors associated with the evaporation and condensation of water vapour. This can lead to the loss of the more volatile species (some hydrocarbons, nitrates etc) and has led to the identification of differences between TEOM and gravimetric measurements at co-located sites. Gravimetric instruments also have the potential to lose some volatile particles, especially ammonium nitrate, the proportion of which is dependent upon the history of the sample. The recently published Airborne Particles Expert Group (APEG) report concluded that at concentrations around 50 mg/m3 the TEOM tends to under-read compared with a gravimetric sampler by between 15 and 30%. However, this effect is not constant, and varies depending upon the mass concentration, the distance from a specific source, and the environmental conditions. Further studies have been commissioned by DETR to investigate these effects, and to provide a more robust relationship between the TEOM and the European transfer gravimetric reference method. In the meantime, it should be borne in mind that the existing objective, which was based upon the air quality standard recommended by the Expert Panel on Air Quality Standards (EPAQS), has been largely based upon epidemiological data derived from measurements made using the TEOM instrument. There is therefore a direct relationship between TEOM measurements and the existing objective. The proposed EU Stage 1 Limit Values however have been derived from epidemiological studies based upon measurements made using a range of different samplers, and the Directive specifies a gravimetric reference method. There is therefore a potential inconsistency between measurements of PM10 concentrations made using a TEOM analyser and the proposed objectives - for example, a daily mean concentration of 45 mg/m3 measured using a TEOM analyser could be underestimating the 'gravimetric' concentration by 15 mg/m3 or more. It is therefore necessary to apply a 'correction factor' when assessing TEOM measured concentrations against the proposed objectives. For the purpose of this note, a constant factor of 1.3 has been used (consistent with the APEG report), and is applied to all TEOM measured concentrations. For example, a TEOM concentration of 20 mg/m3 would be expressed as 20 x 1.3 = 26 mg/m3, gravimetric. To make this clear, all data are expressed as [µg/m3, TEOM] or [µg/m3, gravimetric] as appropriate. To avoid confusion, it is important that authorities follow a similar approach and clearly identify the units of PM10 concentrations in all reports that are produced. |