Monitoring Heavy Metals

Recent changes to the monitoring of heavy metals in ambient air

The UK Heavy Metals Monitoring Network measures the concentration of metals in ambient air in order to assess the UK’s compliance with European air quality legislation and the exposure of the general population to potentially harmful compounds. This article describes the operation of the Network, and how its constituent monitoring sites around the UK have recently been reorganised in order to ensure compliance with European legislation.

Introduction

Air pollution continues to be an issue of great concern to the scientific and medical communities, as well as to members of the general public, the media, and environmental pressure groups. Several requirements drive the need for air quality measurements, including: measuring the exposure of the general population to a variety of toxic compounds; assessing compliance with legislative limits or similar target values; informing policy development and assessing the effectiveness of abatement strategies. In addition there is a need to provide air quality information for the general public and to inform other scientific endeavours (for example, climate change research), and to provide an infrastructure that can readily respond to new and rapidly changing requirements, such as the specification of new pollutants requiring measurement, or assessment of episodes, such as local, regional or trans-boundary pollution events.

The determination of the total concentrations of metals in ambient air is of great importance within this framework. The general public and the environment can be exposed to several classes of hazardous compounds containing metallic elements, which occur naturally or are released by domestic or industrial processes. Until relatively recently, the presence of metals in ambient air had not been a primary topic of concern to air quality research. This has changed over the last couple of decades, as the toxic effect of metals on human health and the environment has become more completely understood. As a result, the total concentration levels of Pb, Ni, As and Cd allowable in the PM 10 fraction of ambient air (particles with an aerodynamic diameter of 10 μm or less), are now limited by European legislation at 500, 20, 6 and 5 ng.m -3 respectively 1,2 . The 2007 Air Quality Strategy for England, Scotland, Wales and Northern Ireland specifies a more challenging limit of 250 ng.m -3 for Pb 3 .

In order to enforce legislation, measure human and environmental exposure, and show compliance with limit and target values, the total concentration levels of ambient metals must to be measured at multiple sites on nationwide air quality monitoring networks. These measurements need to employ rigorously validated, comparable and quality assured analytical measurement methodologies with traceable uncertainties in the results. To this end nationwide networks for the measurement of a wide range of particulate-borne and gaseous pollutants are now well-established in many developed countries around the world.

The UK Government’s Department for the Environment, Food and Rural Affairs (Defra) runs a series of nationwide networks to assess the levels of pollutants, including metals, in ambient air to assess compliance against European and UK air quality objectives, and to determine ecological and environmental impacts. Important trace metals have been measured in UK ambient air since 1976. However, data was not made widely available until 1980; this is thought to be partly because they were not fully part of the established national networks infrastructure at that time, and partly because of low data capture rates during the early years of operation.

The current UK Heavy Metals Monitoring Network (UKHMMN) is based on sites that were set up to measure: 1) lead emissions from road vehicles; 2) emissions from specific industrial processes, and; 3) other important trace elements in background locations.

The disparate nature of these historic monitoring networks for heavy metals in the UK, which have in their time individually responded to the needs of specific air quality legislation, have sometimes resulted in differences in operating methodologies. That was not conducive to consistent nationwide reporting of the type required for assessment against the relevant European directives, so in 2003, in order to address this problem, all monitoring was rationalised into a single integrated network, thereby realising the current UKHMMN. The history of metals monitoring in the UK has recently been described in detail elsewhere 4 .

The present UKHMMN is operated on behalf of Defra by the National Physical Laboratory (NPL). The concentration of total gaseous mercury is also measured by the UKHMMN at a subset of the Network sites as this is also required by the European legislation, although no limit or target value currently exists for this pollutant.

Network operation

Particulate samples are taken at all sites in the UKHMMN using Partisol 2000 instruments, fitted with PM 10 size selective heads, operating at a calibrated flow rate of 1m 3 .h -1 Samples are collected for a period of one week onto 0.8 μm pore size GN Metricel membrane filters. Filters are dispatched monthly by NPL to local site operators (LSOs), who then return the sampled filters to NPL, also on a monthly basis. After sampling the filters are cut in half accurately, and each portion digested at temperatures up to 220°C using a CEM Mars X microwave. The digestion mixtures used are: Hg & Pt: 5 ml of nitric acid and 5 ml hydrochloric acid; all other metals: 8 ml of nitric acid and 2 ml hydrogen peroxide.

Analysis for particulate-phase metals takes place using a PerkinElmer Elan DRC II ICP-MS and the standard procedure detailed in BS EN 14902 5 . ICP-MS analysis of the digested (and subsequently diluted) solutions takes place using at least four gravimetrically prepared calibration solutions. A quality assurance (QA) standard is repeatedly analysed, and the change in response of the QA standard is mathematically modelled to correct for the long-term drift of the instrument. The short-term drift of the ICP-MS is corrected for by use of an internal standards mixture continuously added to all samples by a mixing block. Each sample is analysed in triplicate with each analysis consisting of five replicates. The amount of each metal in solution (and its uncertainty) is then determined by a method of generalised least squares using XLGenline (an NPL-developed program) to construct a calibration curve.

Sampling for vapour-phase mercury takes using low volume pumps to draw air through ‘Amasil’ (gold-coated silica-containing absorption) tubes at a rate of 100 ml.min-1 for either one week or four weeks, depending on the specific site and the expected ambient concentration. The short sampling line is preceded by a 0.8 μm pore size filter to remove particulates. Cleaned Amasil tubes are dispatched monthly by NPL to LSOs, who then return the sampled tubes to NPL, also on a monthly basis.

Analysis of vapour-phase mercury samples takes place using a PS Analytical Sir Galahad II analyser with an atomic fluorescence detector. The instrument is calibrated by use of a gas-tight syringe, making multiple injections of known amounts of mercury vapour onto the permanent trap of the analyser. Sampled adsorption tubes are placed in the remote port of the instrument and heated to 900°C, desorbing the mercury onto a permanent trap. Subsequent heating of this trap then desorbs the mercury onto the detector.

Total expanded measurement uncertainties for all analytes, except particulate phase mercury, are usually significantly below ±25 % expressed at a level of confidence of approximately 95 %. The uncertainty for particulate phase mercury measurements is ±40 %. Uncertainties for the more abundant analytes are about ±15 %. Following ratification by statistical analysis and an independent quality circle, the data is sent to the National Air Quality Archive for publication 6 . Results are expressed as mass concentrations in terms of ng (of elemental metal) per m 3 (of ambient air). The term ‘concentration’ has been used here to refer to ‘mass concentration’

Network reorganisation

In 2007 the UKHMMN underwent a reorganisation 7 . This principally involved movements in the locations of monitoring sites. The drivers for these changes were several fold:

• To ensure compliance with the relevant EU air quality legislation, particularly at locations identified as having high emissions levels
• To make sure that the requirement in legislation for upwind and downwind sites at industrial point source emitters was satisfied
• To respond to the UK’s changing emissions profile: some industrial processes have closed, others have changed their emissions levels
• To ensure the optimum cost-effectiveness of the Network with respect to using sites to cover identified point sources and representative population exposure
• To ensure the optimum positioning of Network sites with respect to the processes they are measuring

In response to these requirements the following actions were taken in the second half of 2007 and the first half of 2008:

• Four monitoring sites were closed because the point sources they were monitoring had shut down, or because they were in urban areas where measured concentrations were so low as not to justify continued monitoring
• Ten new sites were installed where the need for additional monitoring was identified for the reasons given above
• In the Swansea conurbation the existing Network site was devolved into local authority control whilst two local authority sites were assimilated into the Network
• Extra mercury vapour monitoring was installed at two of the new Network sites

A summary of the changes to the Network are displayed in Figure 1. The sites that closed were located in Glasgow, Leeds, London Brent, and Newcastle. The new sites that opened were in located in Belfast (to provide urban background measurement in Northern Ireland), Redcar x2 (to monitor up and downwind of an industrial point source), Scunthorpe x2 (to monitor up and downwind of an industrial point source), the Thames Estuary x2 (to monitor up and downwind of an industrial point source), Sheffield (to provide an upwind site to compliment an existing downwind site monitoring an industrial process), Cardiff (to provide a downwind site to compliment an existing upwind site), and Port Talbot (to monitor an industrial point source). The new installation at Port Talbot is shown in Figure 2.

In order to gauge the effect of a point source on the concentrations measured locally it is important that the relevant monitoring sites are correctly positioned. The requirement is for the concentrations measured at the upwind site to have little or no contribution from the point source in question and that the downwind site is positioned so as to measure the maximum ground concentrations resulting from the emissions of the point source. These conditions are not usually possible to meet entirely, given the variability of meteorological conditions, especially wind direction.

However, sites should be placed so that, given the prevailing weather conditions, these requirements are best satisfied. Equally, it is not always possible to place monitoring sites in the most desirable locations for a number of reasons such as: lack of easily accessible power supply, the property owner not giving consent, lack of planning permission, or poor accessibility of sampler for maintenance and filter changing (e.g. in a secure compound, or location not continuously occupied). The site reorganisation in Swansea was performed in collaboration with an air quality modelling study carried out by NPL to ensure that the new sites were located in the correct positions to best meet the requirements for upwind and downwind sampling locations mentioned above.

The results of this exercise are shown in Figure 3. This shows that the new Network sites are located at ideal upwind and downwind locations to assess the effect of the point source in the Swansea area. Moreover, the new locations are a substantial improvement on the previous Network site in Swansea (the easternmost of the two local authority sites).

It is also desirable following reorganisation of Network sites to ensure that the annual average values do not exhibit a step-change owing to the different composition of the Network and the processes that are being monitored. This is especially important for the metals that are limited by European legislation (Pb, Ni, As, Cd). This is not particularly easy to assess, as annual average concentrations are not stable even when the composition of the Network is invariant. Most metals have showed a steady decrease in measured concentration over the last 25 years.

Therefore the best way to gauge whether the changing composition of the Network has had any affect on measured values is to look not at absolute concentrations, but to assess whether there has been any significant change in the rate of decrease of measured concentrations following reorganisation. The results of this analysis are displayed in Figure 4, which displays the change in the annual average concentration of the metals measured by the UKHMMN (excluding Hg and Pt which are regularly below the detection limit) between 2004 and 2006 (when the Network composition was unchanged), and between 2006 and 2008 (during which time period the Network underwent reorganisation).

As can be seen, there is no significant variation in the rate of decrease of metals concentrations, with the exception of Fe and Mn. The reason for this is that a subset of the new monitoring sites is targeted specifically at emissions from steel works. This notwithstanding, Figure 4 demonstrates that the Network reorganisation has not significantly affected the measured annual average values and the trends in these annual averages, especially for the metals that are limited by European legislation.

Ratified data from the UKHMMN continues to be available at: http:// www.airquality.co.uk/archive/data/metals/metals_data.xls

References

1 Council Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe, Official Journal of the European Union, 2008, vol. L152, pp 1-44.

2 Council Directive 2004/107/EC of the European Parliament and of the Council of 15 December 2004 relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air, Official Journal of the European Union, 2005, vol. L023, pp 3-16.

3 The Air Quality Strategy for England, Scotland, Wales and Northern Ireland, Department for Environment, Food and Rural Affairs in partnership with the Scottish Executive, Welsh Assembly Government and Department of the Environment Northern Ireland, Cm 7169 NIA 61/06-07, 2007.

4 Twenty-five years of nationwide ambient metals measurement in the United Kingdom: concentration levels and trends, Brown, R J C, Yardley, R E, Muhunthan, D, Butterfield, D M, Williams, M, Woods, P T, Brown, A S, Goddard S L, Environmental Monitoring and Assessment, 2008, vol. 142, pp 127-140.

5 BS EN 14902:2005 Ambient air quality – Standard method for the measurement of Pb, Cd, As and Ni in the PM10 fraction of suspended particulate matter, CEN, Brussels, 2005.

6 www.airquality.co.uk (assessed January 2009).

7 NPL Report AS 28 – UK Heavy Metals Network Expansion Report, Butterfield, D, Lipscombe, B, Brown, R J C, Williams, M, NPL, November 2008.

Published: 01st Jun 2009 in AWE International