Ten years of mercury measurement at urban and industrial air quality monitoring stations in the UK

Atmospheric Environment 109 (2015) 1e8

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Ten years of mercury measurement at urban and industrial air quality monitoring stations in the UK Richard J.C. Brown*, Sharon L. Goddard, David M. Butterfield, Andrew S. Brown, Chris Robins, Chantal L. Mustoe, Elizabeth A. McGhee Analytical Science Division, National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Concentrations from 10 years of gaseous mercury measurements across the UK.
Emissions and ambient concentrations of mercury have fallen over this period.
Small urban increments of about above background 0.4 ng/m3 concentrations.
Total gaseous mercury to particulate phase mercury ratio was large across the UK.
Higher particulate phase mercury concentrations close to emissions point sources.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 December 2014 Received in revised form 23 February 2015 Accepted 4 March 2015 Available online 6 March 2015

Concentrations and trends from a decade of measurements of total gaseous mercury and particulate phase mercury at a number of monitoring stations across the UK are presented. Both emissions and ambient concentrations of mercury in the UK have continued to fall slightly during the measurement period despite already being at historically low levels. The median UK concentration of total gaseous mercury recorded in recent years was around 2.0 ng/m3. Small urban increments of about 0.4 ng/m3 above background concentrations were noted, with larger increments above the background only observed close to industrial point sources. The total gaseous mercury to particulate phase mercury ratio was large across the UK e indicating the dominance of the gaseous mercury in the atmosphere e and was observed to be larger at background and urban locations than at industrial sites, as a result of higher relative particulate phase mercury concentrations close to primary emissions point sources. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Air quality Emissions Particulate matter Mercury vapour Long term trends

1. Introduction Mercury's toxicity and ability to bio-accumulate make it one of

* Corresponding author. E-mail address: [email protected] (R.J.C. Brown). http://dx.doi.org/10.1016/j.atmosenv.2015.03.003 1352-2310/© 2015 Elsevier Ltd. All rights reserved.

the most important global environmental pollutants. As a direct result its emissions to, and presence in, aquatic and terrestrial biosystems are regulated and assessed by national and international legislation, most notably the UNEP Minimata Convention on Mercury (United Nations Environment Programme, 2013) and the UNECE Protocol on Heavy Metals which targets three particularly harmful elements: lead, cadmium and mercury (United Nations

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Economic Commission for Europe, 2012). In Europe, air quality Directive 2004/107/EC (Directive 2004/107/EC, 2005) requires the measurement of total gaseous mercury at background stations commensurate with the size of the EU Member State. For the UK this represents a minimum of two background stations. Further monitoring of transboundary pollution and long range transport of mercury species is delivered by the European Monitoring and Evaluation Programme (EMEP) (United Nations Economic Commission for Europe, 2014). The main UK emissions sources of mercury are from iron and steel production processes, public electricity and heat production, waste incineration, crematoria, the manufacture of chlorine in mercury cells, coal and other forms of industrial combustion. Emissions have declined in recent years as a result of improved controls on mercury cells and their replacement by diaphragm or membrane cells, the decline of coal use and its replacement with gas, and tighter abatement controls on waste incineration and crematoria, in particular during the 1990s. The decrease in UK emissions of mercury since 1970 has been dramatic (National Atmospheric Emissions Inventory, 2014) and is shown in Fig. 1. Despite these reductions and the emphasis on the transboundary nature of mercury pollution, it is increasingly recognised that emissions can also effect local environments. Because the current legislation is aimed only at background and transboundary measurement there is generally a lack of a requirement for measurement in urban and industrial environments to gauge these effects, or of a target or limit value for the concentration of mercury in ambient air for the protection of human health (although exposure to ambient mercury is still minor compared to exposure from eating food in which mercury accumulates, such as fish). In part to address this deficiency, and in part to assess likely total gaseous mercury (TGM) concentrations at relevant locations across the UK in preparation for achieving compliance with Directive 2004/107/EC, the UK government made measurements of total gaseous mercury and particulate phase mercury at a number of urban and industrial monitoring stations between 2003 and the end of 2013 (Defra, 2014). According to Directive 2004/107/EC ‘total gaseous mercury’ is defined as “elemental mercury vapour (Hg0) and reactive gaseous mercury, i.e. water-soluble mercury species with sufficiently high vapour pressure to exist in the gas phase.” Directive 2004/107/EC states that the “Measurement of particulate and gaseous divalent mercury [reactive gaseous mercury] is also recommended.” Using the methods described in this paper, measurements of total gaseous mercury (gaseous elemental mercury and reactive gaseous mercury) and particulate mercury have been

Fig. 1. Estimated UK emissions of mercury by year from 1970 to 2012 (inclusive) showing contributions from each industry sector.

made. Other measurements specifically for reactive gaseous mercury are made elsewhere in the UK (Cape, 2009). From the start of 2014 TGM measurements at urban and industrial stations will only be made at Runcorn Weston Point and London Westminster and no measurements of mercury in PM10 will be made. It is timely therefore that this paper reviews the results of this measurement campaign over the last ten years, which represents the first UK assessment of total gaseous mercury concentrations in urban and industrial locations, and provides critical analysis of the concentrations and trends observed over this decade of measurement. Similar studies have been made at urban and background locations in other territories (Cole et al., 2014; Han et al., 2014; Parsons et al., 2013; Ebinghaus et al., 2011; Cole et al., 2013; Slemr et al., 2010; Cole and Steffen, 2010), and recently summarised (Pirrone et al., 2013; Tørseth et al., 2012). Similarly, a study of mercury concentrations at rural UK stations has been made previously (Kentisbeer et al., 2011), but no nationwide study of mercury in urban UK locations has been performed previously. 2. Experimental 2.1. Overview The history of the UK Heavy Metals Monitoring Network has been described previously (Brown et al., 2008a, 2013). Since 2004 NPL have operated the network on behalf of the UK Department for Environmental, Food and Rural Affairs, producing annual data that is publically available (Defra, 2014). 2.2. Sampling Sampling for TGM took place at (on average) 13 of the Network sites using a low-volume pump (calibrated annually by NPL). Ambient air was pumped through ‘Amasil’ (gold-coated silica) tubes (PS Analytical, UK) at a calibrated flow rate of 100 ml min1 for either one week or four weeks, depending on the specific site and the required resolution of data. The mercury vapour sampling equipment is housed in a specially designed box on the side of the Partisol 2000B samplers, which keep the sampling line sufficiently warm that no additional adsorption tube heating was required (as has been used elsewhere where the tubes are part of standalone TGM sampling (Kentisbeer et al., 2011)). At these low flow rates and at ambient concentrations no sample breakthrough is observed (Brown et al., 2008b). A schematic diagram of the mercury vapour sampling equipment is given in Fig. 2. This set-up is designed to sample both elemental gaseous mercury and reactive gaseous mercury (considered together as TGM) as defined by the EN 15852 European reference method. At the vast majority of urban and industrial locations the gaseous elemental component of TGM is likely to be in excess of 98%, and TGM is likely to be in excess of 99% of the total mercury in ambient air (as discussed below). For this reason, even if the reactive gaseous mercury phase is absorbed by the particulate pre-filter and not by the sorbent material upon which the analytical measurement is made, which has previously been observed (Finley et al., 2013), such a bias would be insignificant compared with the overall uncertainty of the method.

Fig. 2. Schematic diagram of the total gaseous mercury sampling apparatus, where: 1 e direction of air flow, 2 e inlet particulate filter, 3 e PTFE tubing (with length minimised between the inlet particulate filter and ‘Amasil’ adsorption tube to fewer than 10 cm), 4 e ‘Amasil’ adsorption tube, 5 e flow restrictor, 6 e pump.

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Particulate mercury samples were taken at all sites in the Network using Partisol 2000B instruments (fitted with PM10 heads) operating at a calibrated flow rate, nominally of 1 m3/h, in accordance with EN 12341 (European Committee for Standardisation, 2014). Samples were taken for a period of one week onto 47 mm diameter GN Metricel membrane filters. During 2013 there were 25 stations on UK Heavy Metals Monitoring Network taking particulate samples, although not all of these measured TGM e see Brown et al. (2013) for details and station locations. (Sheffield Tinsley replaced the Sheffield Brinsworth station during 2013, and to ensure consistency the stations monitored in parallel for 6 months, meaning that for this period there were 26 stations operational on the network (Brown et al., 2014)). Positive and negative artifacts are known during sampling for particulate phase mercury (Lynam and Keeler, 2005) and this is especially important to consider here since the EN 12341 sampling method is not specifically designed for particulate phase mercury. Therefore, for this reason the absolute values obtained must be treated with caution. However the approach used has been consistent over the ten-year monitoring period and so any measurement bias should not adversely affect trend analyses (Rutter et al., 2008). In addition, the particulate phase mercury concentrations and the TGM to particulate phase mercury concentrations ratios are highly consistent with metaanalysis in the literature (Pandey et al., 2011; Hu et al., 2014). We also expect any sampling artefacts to be small compared to the uncertainty of the overall measurement. 2.3. Analysis Analysis of total gaseous mercury samples took place at NPL using a PS Analytical Sir Galahad II analyser with a fluorescence detector, using NPL's procedure, accredited by UKAS to ISO 17025 (Brown et al., 2008b, 2008c), which is in accordance with the published reference method EN 15852 (European Committee for Standardisation, 2010). (The manual variant of EN 15852 used on the Network has been recently shown to be equivalent to the automatic reference method within the uncertainty of the analytical determination (Brown et al., 2012).) The instrument was 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 were placed in the remote port of the instrument and heated to 500  C, desorbing the mercury onto a permanent trap. Subsequent heating of this trap then desorbed the mercury onto the detector. Analysis for mercury in the particulate-phase metals took place at NPL, as previously described (Goddard and Brown, 2014), using PerkinElmer Elan DRC II and Elan 9000 ICP-MSs, following NPL's procedure, accredited by UKAS to ISO 17025, which is fully compliant with the requirements of EN 14902 (European Committee for Standardisation, 2005). The method was validated with NIST SRM 2583 from which recoveries consistent with 100% within the uncertainty of the measurement were obtained. The filters were digested at temperatures up to 220  C using an Anton Parr Multiwave 3000 microwave. The digestion mixture used was 5 ml of 0.69 g/g aqueous nitric acid solution and 5 ml of 0.37 g/g aqueous hydrochloric acid solution. ICP-MS analysis of the digested solutions took place using at least four gravimetrically-prepared calibration solutions. A quality assurance (QA) standard was repeatedly analysed (after every two solutions), and the change in response of the QA standard was mathematically modelled to a polynomial function in order to correct for the long-term drift of the instrument. The short-term drift of the ICP-MS was corrected for by use of an internal standards mixture (containing In & Rh) continuously added to all the samples via a mixing block. Each sample was analysed in triplicate, each analysis consisting of five replicates.

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The amount of each analyte in solution (and its uncertainty) was subsequently determined by a method of generalised least squares using XLGENLINE (an NPL-developed program (Smith and Onakunle, 2007)) to construct a calibration curve. All the data produced by the UK Metals Network is publically available on-line (Defra, 2014). The expanded uncertainties at the 95% confidence interval for the measurement of total gaseous mercury and particulate phase mercury in ambient air were 15e20 % and 20e40 % respectively. Emissions data for mercury was obtained from the UK National Atmospheric Emissions Inventory (National Atmospheric Emissions Inventory, 2014) and has an uncertainty of 30% to þ40% (Passant, 2003). 3. Results and discussion 3.1. Measured concentrations Table 1 shows the annual average TGM concentrations at monitoring stations measuring during the period of the study. The station type classifications for these locations has been recently reviewed (Ricardo-AEA, 2013) against the guidance in the relevant Directive (Directive 2004/107/EC, 2005). Fig. 3 shows the distribution of measured values recorded at these stations in histogram form. It is clear that the concentrations measured at the Runcorn station are significantly higher than those measured at other stations. Excluding Runcorn, very few concentrations below 1 or above 3 ng/m3 are recorded. At Runcorn over 86% of concentrations occur between 5 and 50 ng/m3. This is not surprising since the station is near to an industrial facility including a chlor-alkali works. The other stations show much lower annual average concentrations, between about 1.5 and 3.5 ng/m3. On occasions, higher annual average values have been observed at the two Walsall stations because of high concentration episodes during the year, possibly resulting from their proximity to a metal smelting works. Taking averages and standard errors of the mean from stations making measurements continuously between 2008 and 2013 and from Sheffield Centre (which started in 2009) we can observe increments attributable to different station types. The rural background concentration at Eskdalemuir was (1.67 ± 0.07) ng/m3 e this is very similar to the overall average of (1.53 ± 0.05) ng/m3 measured by Kentisbeer et al. (2011) between 2005 and 2008 at 10 rural background stations in the UK (not including Eskdalemuir) and with other measurements of background levels of mercury in the atmosphere of between 1.5 and 1.7 ng/m3 (Ebinghaus et al., 2011). The average at urban background stations (London Westminster, Motherwell, Sheffield Centre, Swansea Morriston and Belfast) was (2.07 ± 0.03) ng/m3 and was not significantly different from the average of (2.01 ± 0.12) ng/m3 at the three urban traffic stations at London Cromwell Road, Cardiff Llandaff and Manchester. We should note that the first two of these urban traffic stations have subsequently been deemed not to be compliant with the siting requirements for urban traffic stations under the relevant Directive (Directive 2004/107/EC, 2005) since they are too close to major roads or road junctions. This notwithstanding, the urban (including urban traffic) increment over and above the rural background is approximately 0.4 ng/m3. The additional increment for urban industrial stations close to metal refining works (Walsall Centre and Walsall Bilston Lane, which averaged (3.74 ± 0.70) ng/ m3) was approximately 1.7 ng/m3. Of course the industrial increment depends largely on the source being considered e the increment from the urban background to the mostly industrially impacted station close to the chlor-alkali plant at Runcorn Weston Point, which recorded an average concentration of (24.5 ± 2.1 ng/ m3), is of the order of 22e23 ng/m3.

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Table 1 Annual average TGM concentrations at monitoring stations measuring during the period of the study (with the standard deviations of these values given in brackets in italics; no standard deviation data is available for 2003). Sampling was continuous and data coverages were in excess of 90% unless otherwise stated. The symbol ‘e’ indicates that the station did not measure TGM, whilst ‘*’ indicates an average covering less than the calendar year owing to mid-year installation or closure of the station. Monitoring station

London Brent Leeds Glasgow Newcastle London Westminster London Cromwell Road Motherwell Eskdalemuir Manchester Cardiff Llandaff Walsall Bilston Lane Walsall Centre Runcorn Weston Point Bristol Avonmouth Port Talbot Sheffield Centre Swansea Morriston Belfast Centre Sheffield Devonshire Green

Annual average TGM concentration/(ng/m3) 2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

e e e 1.66* () e e e e e e 4.44 () 3.54 () 36.1 () e e e e e e

2.49 1.83 2.53 2.33 2.34 1.84 1.00 2.40 1.81 2.34 1.86 2.44 23.5 e e e e e e

1.16 1.28 1.67 1.78 2.37 1.71 1.06 1.39 1.38 1.46 1.85 1.79 40.2 e e e e e e

2.54 2.61 2.04 3.01 2.02 3.13 2.50 1.69 1.96 2.61 1.96 2.60 31.1 e e e e e e

1.86* (0.60) 1.41* (0.78) 2.15* (0.45) 2.28* (0.96) 2.15 (0.83) 2.59 (2.01) 2.34 (2.13) 1.69 (1.27) 1.82 (0.96) 1.64 (0.45) 2.44 (0.79) 2.65 (1.59) 41.5 (17.0) e e e e e e

e e e e 2.08 (0.93) 2.05 (0.49) 2.23 (1.02) 1.61 (0.22) 1.74 (0.58) 1.91 (0.25) 3.19 (0.98) 4.31 (3.73) 32.0 (18.3) e e e 1.58* (0.04) 2.41* (0.78) e

e e e e 2.17 (0.92) 1.92 (0.56) 2.62 (1.24) 1.56 (0.64) 2.49 (0.60) 1.93 (0.57) 6.82 (12.7) 4.54 (5.07) 28.9 (11.2) 1.47* (0.60) e 1.64* (0.50) 1.73 (0.48) 1.77 (0.34) e

e e e e 2.21 1.94 2.94 1.69 2.13 1.35 2.46 2.28 17.9 2.13 e 1.72 1.49 1.91 e

e e e e 2.88 (2.33) 2.01 (1.16) 1.52 (0.37) 1.47 (0.56) 2.14 (0.38) 1.72 (0.73) 6.94 (6.57) 5.07 (6.58) 21.1 (14.7) e 1.83* (1.93) 1.53 (0.47) 1.94 (0.74) 2.29 (1.59) e

e e e e 3.68 2.00 1.88 1.71 1.56 1.67 2.52 2.56 23.5 e 1.91 1.62 2.05 1.58 e

e e e e 3.28 (1.92) 3.02 (1.85) 2.10 (1.03) 1.96 (0.51) 2.05 (0.88) 2.55 (2.54) 2.27 (1.94) 1.94 (1.11) 23.5 (21.0) e 2.10 (0.72) 2.05* (0.86) 1.65 (0.66) 1.47 (0.27) 2.07* (1.12)

(1.29) (0.69) (4.42) (0.36) (0.57) (0.34) (0.51) (0.84) (0.70) (1.18) (0.76) (1.19) (10.1)

(0.78) (0.72) (1.55) (1.24) (1.47) (0.94) (0.70) (0.49) (0.50) (0.56) (0.84) (0.66) (20.1)

(3.15) (1.49) (1.70) (1.34) (0.84) (2.51) (1.66) (0.42) (0.96) (1.60) (0.56) (0.99) (16.8)

(0.50) (0.46) (1.26) (0.95) (0.78) (0.34) (0.58) (0.95) (10.0) (0.78) (0.59) (0.30) (0.88)

(1.97) (0.72) (0.51) (0.38) (0.70) (0.42) (2.51) (2.90) (14.5) (0.67) (0.60) (1.36) (0.98)

Despite the significant concentrations measured at Runcorn Weston Point these are still less than half of the target value of 50 ng/m3 for the protection of human health, which was considered in the EC position paper on mercury prior to the drafting of the Fourth Air Quality Daughter Directive (European Commission, 2011). Furthermore, there is some evidence to suggest that the concentration increment measured at Runcorn is not simply a measurement of contemporaneous primary pollution to air but may be in part the result of historical industrial contamination of local soils, as has been found in other studies (Vane et al., 2009). Fig. 4 shows the monthly concentrations of TGM measured at the Runcorn Weston Point station in 2010 plotted against the previous

month's average maximum temperature (to allow for a lag in the warming and cooling of the ground) (Kim et al., 2012). The coefficient of determination is R2 ¼ 0.80, providing some evidence that some mercury contamination is in the ground and its vaporisation increases with ambient temperature. The coefficient of determination using the same month's average maximum temperature data is a less convincing R2 ¼ 0.56. This notwithstanding, the concentrations at Runcorn, even at low temperatures (in 2010 an average of 8.9 ng/m3 at temperatures below 10  C) still show a significant increment above urban background stations. There are no other obvious causes of this effect, such as seasonal changes in wind direction or industrial emissions. Furthermore, no other

Fig. 3. Histograms showing the occurrence of individual (monthly and weekly) TGM concentrations recorded at all UK Heavy Metals Network stations listed in Table 1, except Runcorn, between 2003 and 2013, inclusive (blue bars). The inset shows the occurrence of individual (weekly) TGM concentrations at the Runcorn station between 2003 and 2013, inclusive (orange bars). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. The average maximum monthly temperature (offset by one month) against the average monthly total gaseous mercury concentration recorded at the UK Heavy Metals Monitoring Air Quality Network station at Runcorn during 2010. The best linear fit to the data is shown (R2 ¼ 0.80) as the dotted line. The errors bars represent the expanded uncertainty at the 95% confidence interval in the total gaseous mercury concentrations.

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m3 (i.e. the concentration over and above the global background that UK emission impact upon) this would constitute a decrease of 21%, very similar to the decrease in emissions over this period. This gives more confidence that the overall trend in TGM concentrations reported above is real. Dividing the measured urban increment by the predicted emissions, gives a sensitivity of the ambient concentrations to local emissions of TGM over and above the global background level of 0.06 (ng/m3)/Mg. This is comparable with previous studies that have determined similar sensitive for pollutants released from diffuse, traffic and area sources (e.g. 0.033, 0.077, 0.020, 0.015 and 0.058 (ng/m3)/Mg for Cd, Cu, Ni, V and Zn, respectively) (Brown, 2010). 3.3. Trends at Runcorn Fig. 5. The UK median TGM concentration recorded across the UK Heavy Metals Monitoring Network (excluding Runcorn) with error bars representing the median absolute deviation across all stations, and the total UK mercury emissions from 2003 onwards with the error bars representing the uncertainty quoted by the NAEI (Passant, 2003).

obvious intra-year trends are observed in the measured mercury concentrations at other stations, such as have sometimes been observed in other studies at rural stations and attributed to meteorological conditions (Ebinghaus et al., 2011). For the urban and industrial stations considered here the source emission, sampling and analytical variability is clearly larger than any possible underlying seasonal variation. 3.2. Trends in concentration The trends in UK median TGM concentrations measured by the network (excluding at Runcorn) and in declared UK emissions are shown in Fig. 5. Both quantities have shown a tendency to decline during this period. If we perform an weighted least squares regression for the concentrations and emissions data sets, using as uncertainties the robust standard deviations across all stations (except Runcorn) each year and the declared NAEI uncertainties of 30% to þ40%, respectively, the significance of any trends may be calculated. For the concentrations recorded across the UK the trend observed is very small in relative terms and not significantly different from zero: (0.012 ± 0.030) (ng/m3)/a. In particular this is because the 2003 concentration, which one might suppose would cause a larger slope, has a large robust standard deviation associated with it because of the small number of contributing stations in that year (3 as compared to at least 10, and usually 12 or 13, in other years). The trend observed in emissions is much larger in relative terms: (0.19 ± 0.22) Mg/a. The uncertainty in the emissions trend is still larger than the trend itself, although unlike the uncertainty contributions to the ambient concentrations which were predominantly random, the uncertainty estimate of emissions is likely to be dominated by systematic contributions, i.e. the same offset is present year after year, and as such should not affect the certainty of the trend. This is highlighted by a robust regression using Thiel's complete method (AMC, 2012), which uses only the spread of the data as an input parameter. This yields a significantly negative gradient: 0.21 Mg/a, with a 95% confidence interval in this gradient of between 0.26 and 0.12 Mg/a. On closer inspection these gradients seems consistent with the observed data. In the period from 2006 to 2012, emissions have shown a monotonic decrease and have reduced by 24% in total. Using the rate of decrease in ambient content of 0.012 (ng/m3)/a we would expect a total decrease of ambient concentration of 0.084 ng/m3. If we consider the urban increment previously calculated of about 0.4 ng/

Runcorn has been examined as a separate case because of the significantly higher concentrations observed during monitoring. The profile of measured concentrations at the Runcorn station is shown in Fig. 6 against the permitted emissions mercury emission to air of the nearby chemical industry site according to permits AL7294 (in 2003 and 2004) and BS5428IP (from 2005 onwards) (Environment Agency, 2014). Emissions from these permits accounted for an average of 12.6% of the UK's Hg emissions to air between 2003 and 2012. A similar trend analysis has been performed for these data as for the UK wide concentrations. For the concentration data an uncertainty in the annual average of 15% has been assumed for the weighted least squares regression (Brown et al., 2008b). Since no uncertainty data is associated with the permitted emissions values an un-weighted regression has been performed in this case. For the concentration data a gradient significantly different from zero was obtained: (1.36 ± 0.43) (ng/ m3)/a. The emissions data produced a gradient of 0.063 Mg/a. Considering these gradients and the period between 2003 and 2012 we might expect a total reduction in permitted emissions of 0.57 Mg, representing a reduction from 2003 levels of about 54%. A similar percentage reduction in ambient concentrations that is consistent with the calculated trend in ambient concentrations would require the presence of a significant additional background concentration at Runcorn, not owing to direct emissions or long range transport but from other local sources such as contaminated soil, as suggested in Fig. 4, of approximately 10e12 ng/m3. The presence of such a background is consistent with the change in

Fig. 6. The TGM concentration recorded at the UK Heavy Metals Monitoring Network station at Runcorn with the error bars representing the estimated expanded uncertainty in these annual averages at the 95% confidence interval, and the permitted emissions of mercury to air according to permits AL7294 (in 2003 and 2004) and BS5428IP (from 2005 onwards) since 2003.

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Fig. 7. A weighted least squares extrapolation of annual average TGM concentrations at the Runcorn monitoring station against permitted emissions of mercury to air according to permits AL7294 (in 2003 and 2004) and BS5428IP (from 2005 onwards) from 2003 to 2012, inclusive.

concentrations observed with ambient temperatures as shown in Fig. 4. Such an analysis would suggest that for the annual average concentrations of 23.5 ng/m3, 6% comes from the global background, 47% from primary industrial emissions and 47% from historical contamination. This analysis is corroborated by a weighted regression of concentrations against emissions (see Fig. 7), which predicts an ambient concentration in the absence of permitted emissions of (13.6 ± 4.3) ng/m3. This is consistent with the predictions of the contributions of historical contamination made above. The very high gradient of the extrapolation, (12.5 ± 5.1) (ng/ m3)/Mg, is also noteworthy. This is two orders of magnitude higher than the sensitivities observed when whole UK sensitivities of emissions to ambient concentrations were considered (Brown, 2010) and is consistent with a point source emitter having a significant impact on the concentrations measured locally. 3.4. TGM to particulate phase mercury ratio

low concentrations meant that many of the recorded levels were below the detection limits in the earlier years, but following a change in procedures during 2008, allowing the bulking together of filters for mercury analyses, the method detection limit was improved and the majority of measurements were subsequently above the detection limit and of much greater use for the interpretation of concentrations and trends. The annual median particulate phase mercury concentration measured across all UK stations (excluding Runcorn) together with the annual median recorded at Runcorn between 2009 and 2013 (inclusive) is shown in Fig. 8. Not only are these concentrations fairly stable over this period, but the concentrations are extremely low when compared to the TGM measurements. Fig. 9 plots the TGM and particulate phase mercury concentrations recorded between 2009 and 2013 at the stations where monthly measurements of these two components were made concurrently. Considering in Fig. 9 the TGM to particulate phase mercury ratios of the 661 points shown: 3% have a ratio less than 10, 48% are between 10 and 100, 47% are between 100 and 1000, and 2% are greater than 1000. The median ratio observed was 97. This is consistent with the observation in many other studies that TGM is by far the dominant mercury phase in ambient air. Previous work on the speciation of mercury emissions in the UK has estimated that TGM emissions (elemental mercury and reactive gaseous mercury combined) make-up 96% of total emissions, whilst particulate phase emissions constitute only 4% (AEA, 2011). It was also noted that 64% of emissions come from point sources whilst 36% derive from area sources. Despite the large uncertainties in these estimates, they are very consistent with the field observations. This is especially so if we consider that these estimates relate to primary emissions and as these propagate away from the emission sources so the ratio of TGM to particulate phase mercury will increase, matching even more closely the observed measurements. This is because as time after emission increases the particulate phase mercury is removed by deposition or volatilisation to TGM and the reactive gaseous mercury is removed by reaction and subsequent deposition, whilst the elemental mercury within the TGM remains generally unaffected. Since the distance that a pollutant moves

From 2003 measurements were also made of mercury in particulate at a number of UK Heavy Metals Monitoring stations. The

Fig. 8. The annual median particulate phase mercury concentration e denoted Hg(p) e measured across all the UK stations excluding Runcorn (solid blue circles, left hand axis, error bars represent the estimated expanded uncertainty in these values at the 95% confidence interval), together with the annual median recorded at Runcorn between 2009 and 2013, inclusive (empty red squares, right hand axis, error bars represent the estimated expanded uncertainty in these values at the 95% confidence interval). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. TGM and particulate phase mercury concentrations e denoted Hg(p) e recorded between 2009 and 2013 at the UK stations (Runcorn, orange squares; all other stations, blue circles) where monthly measurements of these two components were made concurrently. Loci corresponding to TGM to particulate phase mercury ratios of 10, 100 and 1000 are shown as dotted lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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from its source is correlated with the time after emission, with the dispersion conditions governing this relationship, we might expect to observe larger TGM to particulate phase mercury ratios the further from significant sources of emissions the monitoring stations are located. Fig. 8 displays the average TGM to particulate phase mercury ratios measured between 2009 and 2013, inclusive, at stations where these measurements were made together. This supports the hypothesis that stations close to primary point sources of industrial emissions have the lowest TGM to particulate phase mercury ratios (e.g. Runcorn Weston Point, Bristol Avonmouth, Walsall Bilston Lane, and Sheffield Centre e albeit as a upwind station close to a point source) whilst those furthest from point sources show the highest (Eskdalemuir is a rural background station; Belfast and Motherwell are urban background stations away from significant traffic sources and not close to any significant point sources). The intermediate ratios correspond to stations further away from, or upwind of, known point sources (Swansea Morriston, Port Talbot, Walsall Centre) or those stations at heavily trafficked locations (London Cromwell Road, London Westminster and Cardiff Llandaff at urban traffic locations, and Manchester next to the hard shoulder of a motorway). Fig. 10 also shows the average particulate phase mercury concentration at these stations between 2009 and 2013. These show the opposite trend to the TGM to particulate phase mercury ratios, demonstrating that, in general, this ratio is driven by particulate phase mercury concentrations and providing corroborating evidence that the particulate phase mercury concentrations decrease with distance from the nearest point source. 4. Conclusions The results of an extensive monitoring programme for total gaseous mercury and particulate phase mercury across monitoring stations representative of UK urban areas has been presented. Both emissions and ambient concentrations of mercury have fallen slightly during the measurement period despite already being at

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historically low levels. This mirrors similar trends observed elsewhere (Slemr et al., 2010; Cole and Steffen, 2010) where regulation has been successful in reducing local controls. However, this must be considered in the context of global emissions that are not decreasing and are even rising slightly (UNEP, 2013) e for instance, there is no evidence that rural background concentrations (where long range transport is a significant contributor to measured levels) are decreasing. Nonetheless, it has still been possible to detect small urban increments of about 0.4 ng/m3 over and above the background concentrations of 1.5e1.7 ng/m3 observed at the rural Eskdalemuir station and on the UK's rural metals network. Significant industrial increments were observed near to a metal refining works and a chlor-alkali works of approximately 1.7 ng/m3 and 22 ng/m3, respectively. In the latter case there is some evidence to suggest that a portion of the concentration observed is due to mercury volatilisation from contaminated land, especially at elevated ambient temperatures. No significant correlation between UK wide emissions and concentrations was observed, most probably because of the low dynamic range of these values over the last ten years, but some evidence of local correlation at Runcorn was observed where there is a much greater sensitivity of ambient concentrations to emissions. The additional measurement of mercury in the particulate phase has allowed calculation of total gaseous mercury to particulate phase mercury ratios. These are very high at most locations, confirming the dominance of the gaseous mercury fraction in ambient air. These ratios are dominated by changes in particulate phase concentrations, which seem to decrease as the distance from primary emissions sources increases. From the start of 2014 two stations (in Runcorn and London Westminster) will continue to monitor for total gaseous mercury, although there will no longer be routine monitoring for particulate bound mercury. Some additional UK urban monitoring would be useful in future to gauge urban increments and public exposure owing to emissions sources such as crematoria, which are becoming more significant as emissions from other sources continue to decrease. Acknowledgements The identification by Prof John Dearden (Liverpool John Moores University) of the relationship between mercury concentrations and temperature at Runcorn is gratefully acknowledged. The previous contributions from Dharsheni Muhunthan and Rachel Yardley to the operation of the UK Heavy Metals Monitoring Network are gratefully recognised. The funding of the National Measurement System's Chemical and Biological Metrology Programme by the UK Department for Business Innovation and Skills and of the UK Heavy Metals Monitoring Network by the UK Department of Environment, Food and Rural Affairs is gratefully acknowledged. The research leading to these results has also received funding from the European Union on the basis of Decision No. 912/2009/EC.

Fig. 10. The average TGM to particulate phase mercury concentration, in (ng/m3)/(ng/ m3), measured between 2009 and 2013, inclusive, at monitoring stations where these measurements were made together (blue solid bars, left hand axis). The error bars represent the standard error of the mean across the five contributing annual values. The top of the Eskdalemuir error bar is off-scale at 498 (ng/m3)/(ng/m3). The average particulate phase mercury concentration recorded at each station between 2009 and 2013 is also shown (empty, red squares, right hand axis, error bars represent the estimated expanded uncertainty in these values at the 95% confidence interval). As described in the text, the darkest four bars on the left represent stations close to primary point sources of industrial emissions; the lightest three bars on the right represent rural and urban stations far from significant traffic or industrial sources; the seven bars in the middle represent stations at heavily trafficked locations or those upwind of industrial point sources. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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