Heavy Metals Monitoring Network aids UK meet 'target values'

Dr Richard J C Brown, Corresponding Author at the Analytical Science Group, National Physical Laboratory, Teddington, describes the role of the UK Heavy Metals Monitoring Network which comprises 17 monitoring sites across the UK. The Network was created in 2001 from existing 'Lead in petrol', 'Industrial Metals'
and 'Multi-element' Network sites, and has been managed and operated by NPL since October 2004.


The Network operates in support of the Air Quality Strategy for England, Scotland, Wales and Northern Ireland (1), and the UK's obligations under the European Commission's First (2), and Fourth (3), Air Quality Daughter Directives (DDs).
As shown in Table 1, the measured annual mean concentrations of the relevant 1st and 4th DD metals across the UK are well below the LATs, in all cases.

Operation
At each Network site Partisol 2000 instruments collect weekly samples of ambient PM10 particulate matter on to air filters. The Network site at Weston Point, near Runcorn, shown in Figure 2.

These filters are returned to NPL by the local site operators where they are logged in a filter database and stored in a fridge until analysis.
Analysis involves the microwave-assisted digestion of the filters in a strongly acidic solution, followed by measurement of the resulting digest for total metals content
using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). NPL's measurement procedure is UKAS accredited and fully compliant with the forthcoming European standard for the measurement of metals in the PM10 fraction of ambient air, prEN14902, which has been developed by CEN/TC264/WG14, and is in the final stages of being approved by European Member States.

Metals measured
In addition to the metals that the 1st and 4th DDs require to be measured, the Network also measures particulate phase chromium, copper iron, manganese, platinum, vanadium and zinc. These extra particulate phase metals were originally only measured at 11 Network sites, however, since NPL took over operation of the Network in October 2004, these extra metals have been measured at all 17 sites. Additionally, vapour phase mercury measurements are made at 13 sites.

Sampling for vapour-phase mercury is performed by pulling ambient air through gold-coated silica absorption tubes at a rate of 100 ml.min-1, using a low-volume pump, for either one week or four weeks, depending on the specific site and ambient concentration levels. This sampling apparatus is usually housed in a small box attached to the side of the Partisol sampler. Analysis of the absorption tubes is performed using an instrument that thermally desorbs mercury vapour from the gold-coated silica and then measures its concentration by an atomic fluorescence technique.
In general the measured ambient concentrations of all the metals measured on the Network, and its predecessors, have fallen steadily over the last 25 years. Exemplar concentration trends over this period for lead, iron and cadmium are shown in Figure 3.
Despite decreasing concentration trends, the monitoring of ambient heavy metal concentrations is still of great importance:

q To ensure that heavy metal concentrations remain low and the downward trends continue, for the benefit of human health and the environment.
q To maintain a robust database for long-term trend analysis.
q To provide data in support of the UK's obligations under European Commission air quality legislation.
q To provide an infrastructure for monitoring other heavy metal pollutants as they become more relevant. For instance in 2004 the Network began monitoring for platinum owing to more widespread use of the metal in catalytic converters.

Data from the UK Heavy Metals Monitoring Network (including historical data from the Lead in Petrol, Trace Elements and Industrial Metals Networks) can be accessed at the Air Quality Archive: www.airquality.co.uk/archive/data/metals/
metals_data.xls
For further information about the UK Heavy Metals Monitoring Network please contact
Dr Richard Brown,
e-mail: richard.brown@npl.co.uk,
T: 020 8943 6409

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