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Heavy Metals' Monitoring

Published: 10th Jun 2010 in AWE International

Monitoring airborne heavy metals in rural areas

Much of the focus on measuring heavy metals in the ambient atmosphere has been to establish monitoring sites at locations close to local sources of heavy metals in industrial and urban areas. However, there is also a specific legislative requirement to monitor heavy metals in rural areas where concentrations are considerably lower. The Rural Heavy Metals Monitoring and Deposition Network determines the concentrations of heavy metals in samples of ambient air and rainwater collected at remote rural sites across the UK, which are not unduly influenced by local sources of anthropogenic emissions. This article describes the operation of this monitoring network and how the data are used to determine background levels of heavy metals in the UK (and thereby demonstrate compliance of the UK with the relevant EU legislation for rural areas), and in the quantification of heavy metal deposition from the atmosphere.

Heavy metals are released into the atmosphere from a wide range of anthroprogenic and natural sources. Trace quantities of heavy metals are found in fossil fuels and they are released into the atmosphere following combustion processes, including power generation and emissions from vehicles. Industrial processes, including the manufacture of steel and iron, and other metallurgical and chloro-alkali industries are also significant sources of heavy metals. Different industries release different metals, for example, emissions of vanadium are almost exclusively from oil combustion, whereas lead emissions, which were previously almost completely from road transport, are now dominated by processes in the iron and steel sector. The largest source of arsenic is the burning of wood which has been treated with copper-chrome arsenate. Non-combustion sources of heavy metals include demolition of buildings, corrosion and abrasion of sources such as road surfaces, tyre and brake wear. It is important to note that in addition to these anthropogenic sources, heavy metals are also released into the atmosphere from natural sources including volcanoes, forest fires, sea-spray and wind-blown soil particles. As they are chemical elements, heavy metals do not degrade. This means that any metals which are released to the environment have the potential to become re-suspended in the atmosphere, for example, wind blown particles of soil and road dust. 

The Centre for Ecology and Hydrology (CEH) has been monitoring the concentrations of a range of heavy metals in rural locations across the UK since 2004. This Defra funded monitoring network was established to measure the UK background concentration of heavy metals in collections of airborne particulate matter (the PM10 fraction, i.e. particles with an aerodynamic diameter ≤10µm), rainwater and cloudwater, which have been collected at rural locations. Each of the locations was selected to provide a network of sites across the UK which were not unduly influenced by local sources of emissions. The data are compiled to provide information of the background concentrations of these pollutants, and are used to demonstrate compliance with relevant air quality legislation and agreements, including the European Union Air Quality Framework Directive, and its First and Third Daughter directives which include limit / target values for lead, nickel, arsenic and cadmium; The Air Quality Strategy for England, Scotland, Wales and Northern Ireland; the OSPAR Convention1 which is designed to protect the North East Atlantic ecosystem; and the European Monitoring and Evaluation Programme2 (EMEP) which is designed to monitor the effects of transboundary air pollutants. In addition to these legislative requirements, the measured concentrations are also used to calculate the annual deposition of heavy metals and to produce UK maps of metal concentrations and deposition. These data are useful in reviewing the success of policies which have been introduced to reduce the emissions of heavy metals from anthropogenic sources.  

The heavy metals (and metalloids) which are monitored are aluminium (Al), arsenic (As), antimony (Sb), barium (Ba), beryllium (Be), cadmium (Cd), calcium (Ca), caesium (Cs); chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), lithium (Li), magnesium (Mg), manganese (Mn), mercury (Hg), molybdenum (Mo), nickel (Ni), potassium (K), rubidium (Rb), scandium (Sc), selenium (Se), sodium (Na), strontium (Sr), tin (Sn), titanium (Ti), tungsten (W), uranium (U), vanadium (V) and zinc (Zn).  

The network comprises a total of 16 monitoring sites across the UK. 

As the aim of the network is to establish background concentrations of heavy metals in the UK, all of the sites need to be as free from local sources of heavy metal emissions as possible, while still remaining accessible to site operators and a power supply. Samples of particulate matter (PM10) and rain water are currently collected at 11 sites. These samples are analysed for a total of 26 heavy metals. At the other sites, only samples of cloud and rain water (1 site) or just rainwater (3 sites) are collected. At the 11 particulate sites, there is an additional analysis of mercury vapour, expressed as Total Gaseous Mercury (TGM). The Rural Heavy Metals Monitoring and Deposition Network was established in 2003, although some of the monitoring sites have been in operation for longer, having previously contributed to former monitoring networks, such as the Rural Trace Elements Network and the North Sea Network. The Auchencorth site in South East Scotland is a designated EMEP supersite where a wide range of atmospheric pollutants are monitored. The Harwell site in Oxfordshire is also an EMEP supersite and this was added to the rural heavy metal monitoring network in 2008 to complement the other EMEP supersite monitoring activities which are currently undertaken there and at Auchencorth. Cloud water is now only collected at a single high altitude site, Holme Moss in the Pennines. The Scottish cloudwater site at Bowbeat, was decommissioned at the end of 2008 as the orographic enhancement in rainfall at this site was insufficient to provide estimates of the average concentration of metals in rain scavenged by cloudwater. 

Sample Collection

Heavy Metals in Particulates

The particulate samples are collected by Partisol Plus 2025 sequential samplers fitted with PM10 size selective heads, operating at a calibrated flow rate of 1m3 per hour. Samples are collected on 47mm diameter Teflon filters in FRM-style filter cassettes which are assembled in a laminar flow cabinet (ISO 5), within a dedicated clean room, taking precautions to minimise metal contamination (i.e. using acid washed equipment and clean-room powder-free gloves). Aerosol sampler deployment and changeover of filter cassettes conforms to EMEP standards and aims to minimise contamination by providing a rigorous protocol for filter changeover and laboratory preparation prior to analysis. 

The filter cassette magazines in the field samplers are changed monthly, with individual filters in the magazine being exposed for 3.5 days. The exposed filters are dispatched to CEH’s laboratories at Lancaster for analysis using UKAS accredited methods3. Upon arrival at the laboratory the exposed filters are first processed by accurately cutting the filters into two halves. One half is archived as a backup reference and the other half is extracted with 10ml of HNO3 (Baker, Ultrex II) for 12 minutes at 200°C within sealed Teflon vessels using a microwave digestion system (CEM, MARS Express). The method conforms to the temperature and time requirements for PM10 filter extraction for trace metals prescribed by EMEP, although our method uses differing equipment. The concentration of trace metals in PM10 filter digests is measured using Inductively Coupled Plasma Mass Spectrometry (ICPMS; Perkin Elmer Elan DRC II) operating under standard conditions and employing matrix matched calibration standards and internal standardisation to compensate for instrument drift. Metal concentrations in air (ng m-3) are calculated using EMEP methods4. 

Heavy metals in precipitation and cloud water

The samples of bulk precipitation (mainly rain water) are collected from all 16 monitoring sites in the network, including the 4 sites which do not have particulate samplers. The bulk collectors are of the bottle and funnel type, with a bird deterrent used to reduce perching birds fouling the sample. As the levels of metals found within precipitation are at the µg/l level or lower, it is essential that rigorous protocols are used for cleaning sampling equipment between collector deployments and to prevent contamination within the laboratory. The bulk precipitation collectors, comprising 5L polyethylene bottle, 14cm diameter polyethylene funnel and a debris filter, are cleaned by soaking for 24 hours with 0.1m hydrochloric acid followed by 24 hours with 0.1M nitric acid. Bottles and funnels are then rinsed with ultra-pure water (> 18 MΩ cm-1, Millipore) between acid soaks and after acid washing and finally dried in a filtered air drying cabinet. Bulk collectors are assembled in a dedicated laboratory, double bagged, and then sent out to field sites in flight cases along with instructions for collector changeover.

Cloud water is currently collected at a single site, (Holme Moss), using a dedicated sampler consisting of a high surface area polypropylene filament impacted droplet collector connected to a 5 litre sample bottle. Cloud water samples were also previously collected at Bowbeat, but as the level of rainfall at this high altitude site was not significantly different from that at a nearby low altitude Auchencorth site, the Bowbeat site was decommissioned at the end of 2008.  

The equipment for cloud water collection is cleaned and prepared using a similar method to that used for bulk precipitation sampler described above. However, additional cleaning steps are required to remove any soot adhered to the plastic surfaces of the collector, prior to acid washing. Firstly, all the equipment is cleaned using a mild surfactant (Decon 90) and a bottle brush to remove the majority of the soot contamination and secondly, the nylon wire cloud collector is cleaned in an ultrasonic bath by sonicating with a dilute Neutracon solution for 20 minutes. The cloud collectors are deployed at field sites using clean protocols as described above for bulk precipitation samplers.

Once they are returned to the laboratory, samples of bulk precipitation and cloud waters undergo an identical analytical process. All sample manipulations and processing is performed within a dedicated clean air laminar flow cabinet (ISO 5) to prevent contamination with background trace metals. The bulk collectors are weighed to estimate rainfall volumes then acidified with nitric acid (Baker Ultrex II) to a final strength of 1% v/v. The acidified bulk precipitation and cloud water samples are left for 24 hours to allow desorption of metals from the walls of the collector bottle and then a 50ml sub-sample is transferred to a separate acid washed bottle. Acidified and preserved samples are stored at 4° C prior to analysis by Inductively Coupled Plasma Mass Spectrometry (ICPMS; Perkin Elmer Elan DRC II) for trace metals or Inductively Coupled Plasma Optical Emission Spectrometry for Na, Ca, K and Mg (Perkin Elmer Optima DV 4300).

Mercury in air and precipitation 

As mercury is a volatile metal which is generally present at very low concentrations, special techniques are required to measure mercury concentrations in the atmosphere and rain water. Precipitation samples are collected using the EMEP protocol for sampling mercury in precipitation4. The sampling equipment and cleaning protocols are specialised for mercury because it is only present at ng/l levels in precipitation and is particularly prone to background contamination. Therefore duplicate bulk precipitation samples are collected at each of the 10 network sites at which Hg is monitored. Briefly, precipitation is collected in special precipitation samplers, based on the Iverfeldt design as used within the Swedish National Monitoring Programme. The collector is designed to prevent the diffusion of gaseous Hg into the collection vessel by the use of a 0.5m long capillary tube between collection funnel and bottle. A number of further modifications have been made in order to reduce possible contamination and improve safety of use: 

The funnel and tube is made in PTFE as per the Florida Atmospheric Monitoring Survey

The debris filter and capillary components are made of PTFE

The joints connecting individual components are close fitting and thus designed to reduce contamination from ingress of gaseous Hg

To reduce diffusion of gaseous Hgo a glass bottle is used for final collection

Glassware and reagents are rigorously tested in order to identify and reduce sources of contamination. Bottles are cleaned on a monthly basis according to EMEP protocol by soaking in 1% nitric acid followed by bromate/bromide reagent and a final rinse with ultra pure water.

Mercury samplers are deployed, in duplicate, at the 10 main network sites, pre-acidified with hydrochloric acid as a preservative and changed on a monthly basis. The field protocols are similar to those used for the deployment of bulk precipitation samplers and are designed to prevent sample contamination at the field sites and during transport. On return to the laboratory Hg collector bottles are weighed to estimate rainfall volumes and stored at 4° C prior to analysis. All analysis for Hg is completed within two weeks of the arrival within the laboratory of the first collector from the current deployment cycle. Mercury in precipitation is determined by Atomic Fluorescence Spectrometry (AFS) using a PS Analytical Galahad detector employing pre-concentration of mercury on a gold trap to increase instrument sensitivity. 

Concentrations of mercury in air, expressed as total gaseous mercury (TGM) are determined using an integrating sampler which adsorbs the mercury onto gold-coated sand which is contained within a quartz cartridge. These samplers were designed by CEH and are based upon the system of Two-Stage Gold Amalgamation5. Each sampler contains two cartridges which are connected in series to detect any breakthrough of mercury from the first cartridge. The cartridge assembly is heated during sampling to 100° C to drive off water and volatile organic compounds that would reduce capture efficiency. There are integrated mercury samplers at each of the 11 sites where particulate samplers are also located. The samplers have individually calibrated flow meters which are set to a low flow rate of 20ml per minute to prevent the cartridges becoming saturated with mercury during the two week sampling period. After exposure, the cartridges are returned to CEH’s Edinburgh laboratory where they are heated to 500° C to release the adsorbed mercury. In addition to the exposed cartridges, field blanks are used to determine contamination from handling, transport and storage of the cartridges. 

Calculation of deposition 

As the Rural Heavy Metals Monitoring Network measures concentrations in both airborne particulate matter and rainwater, the data can be used to determine the total annual background deposition of heavy metals in the UK. This is achieved by calculating the annual mean concentrations in air and rainwater at each of the measurement sites and then interpolating these data through a computer model which calculates concentration and deposition values on a 5km grid across the UK as a whole, taking into account the local orography and rainfall data which is obtained from the Met Office. Individual values of dry deposition, wet deposition and deposition from cloud water are calculated and then summed to give a value for Total Deposition for each metal. For the majority of metals analysed, the main source of deposition is via precipitation (accounting for approximately 75% of total deposition), with dry deposition responsible for the remainder. Deposition from cloud water is generally negligible with the exception of high altitude areas. Maps of interpolated concentration and deposition data are produced to provide a spatial analysis across the UK. In general, concentrations of heavy metals in the UK are highest in southern eastern areas although deposition is highest in north west areas of the UK due to the higher amounts of rainfall these upland areas receive.

The data generated by the Rural Heavy Metals Monitoring Scheme to date are summarised in a recent report6 with additional material available on the UK Pollutant Deposition Website, at www.ukpollutantdeposition.ceh.ac.uk/heavy_metals/. n

References

1. http://www.ospar.org/

2. http://www.emep.int/

3. http://www.ukas.org/testing/schedules/Actual/2506Testing%20Single_007.pdf

4. EMEP, Manual for Sampling and Analysis. EMEP Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in  Europe. 2002. p. 303

5. Fitzgerald, W. and G. Gill, Sub-nanogram determination of mercury by 2-stage gold amalgamation and gas-phase detection applied to atmosperic analysis.  Analytical Chemistry, 1979. 51(11): p. 1714-1720

6. Malcolm, H., et al., Heavy Metal Deposition Mapping: Concentrations and Deposition of Heavy Metals in Rural Areas of the UK. Draft Report for Comment. 

2010, Contract Report to the Department for Environment, Food and Rural Affairs, Centre for Ecology and Hydrology: Edinburgh. p. 94

Author

Heath Malcolm is based at CEH’s Edinburgh site. He is the project manager for CEH’s Rural Heavy Metals Monitoring Network which is funded by Defra. He has also written several international hazard assessment documents on heavy metals for the World Health Organization.

hmm@ceh.ac.uk

+44 (0)131 445 8554

Alan Crossley was a Senior Scientific Officer at CEH and is now a self employed Service Technician specialising in Environmental Monitoring and Field Exposure systems, with particular experience of Mercury in Air and Particulate samplers.

alan@acrosstech.org.uk

+44 (0)7884063061

Published: 10th Jun 2010 in AWE International

Author


Heath Malcolm and Alan Crossley


 

Heath Malcolm is based at CEH’s Edinburgh site. He is the project manager for CEH’s Rural Heavy Metals Monitoring Network which is funded by Defra. He has also written several international hazard assessment documents on heavy metals for the World Health Organization.

hmm@ceh.ac.uk

+44 (0)131 445 8554

Alan Crossley was a Senior Scientific Officer at CEH and is now a self employed Service Technician specialising in Environmental Monitoring and Field Exposure systems, with particular experience of Mercury in Air and Particulate samplers.

alan@acrosstech.org.uk

+44 (0)7884063061


Heath Malcolm and Alan Crossley

Website:
http://

Email:
hmm@ceh.ac.uk

Phone:
+44 (0)131 445 8554

hmm@ceh.ac.uk
http://
+44 (0)131 445 8554

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