The review highlights that ambient concentrations of particulate-bound arsenic have been declining over the last two decades. This downward trend reflects the decrease in emissions of arsenic in the UK, notably as a consequence of changes in the power generation sector where switches have been made from traditional coal-burning facilities to those that now burn natural gas as a cheaper alternative. In addition, advances in clean technology and a greater regulation of industrial processes, both through Integrated Pollution Control (IPC) and Local Authority Air Pollution Control (LAAPC), have facilitated this decline.

Currently, annual mean concentrations of arsenic in the UK rural environment are typically in the range of 1 - 4 ng m^-3, whilst annual mean urban concentrations are higher, in the range of 5 - 7 ng m^-3. Typically, the highest concentrations of arsenic in the UK are found at sites located in the immediate vicinity of industrial processes such as smelters, incinerators and cement works. For example, the highest reported concentrations of arsenic in the UK have been found in the vicinity of a smelter in Walsall, West Midlands. For the monitoring period covering 1975 - 1990, the highest annual mean concentration of arsenic reported at the site was 223 ng m^-3, whilst for other urban sites, during the same period, annual mean values were found to vary from 1 - 18 ng m^-3. The highest reported concentration (quarterly mean) of arsenic found at the Walsall site was 573 ng m^-3.

Current gaps in the knowledge of arsenic occurrence have been identified in the current work. Notably, there is a lack of information surrounding monitored levels of arsenic in Scotland and Wales – concentrations reported here concern mostly the Midlands area in England. Moreover, there is a lack of information surrounding the identification of different arsenic species in air - to date, only total arsenic concentrations have been reported - and the reporting of vapour-phase concentrations (as arsenic trioxide (AsO₃). In the case of the latter it is envisaged that this is only a problem in respect of occupational exposure or in the vicinity of industrial processes where methylated forms (monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) of arsenic may occur. For population exposure, it has been shown that it is the ultrafine fraction of particulates (PM_2.5) for which arsenic compounds are mostly bound.

The review highlights that there is currently a paucity in monitoring of arsenic in the UK. Specifically, the inclusion of various industrial sectors that may be emitting relatively high levels of arsenic at a local scale. A recent programme of monitoring has begun at 30 sites within the vicinity of various industrial processes to determine ambient concentrations of lead and heavy metals (of which arsenic is included) in the UK. The programme, funded by the Department of the Environment, Transport and the Regions and carried out by consultants, Stanger Science and Environment, has been initiated in response to the recent Review of the UK National Air Quality Strategy (NAQS), and in anticipation of the future EC Daughter Directive which will aim to set limit values for a number of pollutants, including heavy metals. The programme is scheduled to run for a period of 12 months. It is anticipated that a full set of results will be made available in December 2000.

A technical annex is included in this review which gives a detailed overview of the properties of arsenic and its various compounds and the various issues relating to its presence in air. These issues are dealt with only broadly in the following section.
 

Elemental arsenic (As) is a silver-grey crystalline metallic solid that exhibits low thermal conductivity. Although arsenic is often referred to as a metal, it is classified chemically as a non-metal or metalloid belonging to Group 15 (VA) of the periodic table. The principle valances of arsenic are +3, +5 and -3. The main physical properties of arsenic are presented in Table 2.1 below.

Table 2.1: Physical Properties of Arsenic

Property	Value
Atomic weight	74.92
Melting point	816 oC
Boiling point	615 oC
Specific gravity (26° C)	5,778 kg/m3
Specific heat	24.6 J/(mol.K)
Latent heat of fusion	27,740 cal/g
Latent heat of sublimation	31,974 cal/mol
Linear coefficient of thermal expansion (20° C)	5.6 m m/m° C
Electrical resistivity (0° C)	26 mW /cm
Crystal system	hexagonal (rhombohedral)
Lattice constants (26° C, mm)	a = 0.376 e = 1.0548

‘Metallic’ arsenic remains stable in dry air, but its surface will oxidise when exposed to humid air, creating a superficial bronze tarnish that turns black upon prolonged exposure. It typically exists in two forms; the ‘alpha’ crystalline metallic form which is steel grey in appearance and brittle in nature, and the ‘beta’ form; a dark grey amorphous solid.

2.2 Occurrence of Arsenic

Arsenic is found widely in nature, most often combined with oxygen, chlorine and sulphur. It is found in trace quantities in all living things, the atmosphere, water and geological formations. It is usually found in ores containing gold, silver, cobalt, nickel and antimony. There are over 150 known arsenic-bearing minerals, the most common of which are summarised in Table 2.2 below.

Organic compounds of arsenic occur due to its affinity as an element to combine easily with carbon to form a wide variety of organic compounds with one or more As-C bonds. Organic species of arsenic in air are considered to be negligible, although the most commonly occurring organic forms are monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA).

The main inorganic and organic compounds of arsenic are dealt with in more detail in Annex 2.
 

In the United Kingdom, estimated total annual arsenic emissions in 1996 were 51 tonnes (NAEI, 1997), although these have been decreasing steadily to levels of around 8 tonnes per annum in 1998 (Pollution Inventory, 1998) as a result of an increase in the use of natural gas for power generation in favour of coal burning.

The use of organic arsenic compounds in pesticides and herbicides used to be an important source of arsenic emissions to the atmosphere. However, as arsenic-based pesticides, specifically insecticides, have been gradually replaced by alternatives, emissions have significantly decreased in certain parts of the world (WHO (in preparation); Becher and Wahrendorf, 1992).

3.1 Particulate Arsenic  
Arsenic in air primarily exists in the form of various compounds adsorbed onto the surface of fine particles (mostly in particles less than 2 m m in diameter) and is usually a mix of arsenite and arsenate (see Annex 2).

These particles can be transported by wind and air currents until they are brought back to earth by wet or dry deposition. The residence time of particulates depends not only on the size of the particles and the prevailing meteorological conditions, but also the operational conditions of the industrial process. For example, levels of arsenic in air vary according to the distance from the source; the height of the stack; the exit velocity of the flue gas and the prevailing wind speed to name just a few parameters. Areas in close proximity to non-ferrous metal smelters have reported high concentrations of arsenic in air (Lee et al., 1994).

In general, larger urban conurbations have higher arsenic-in-air concentrations than smaller ones. Historically, this has been due to the contribution of emissions from domestic coal burning although recent shifts in the number of households burning coal in the UK mean that this is no longer necessarily the case.

Due to the accumulative nature of arsenic, the long-term effects rather than the short-term effects are those that are of most concern in the UK and other countries. Deposition studies facilitate the understanding of accumulation and likely occurrence of damaging effects of metals and metalloid species in the environment. The effective dry deposition rate of the average arsenic-containing aerosol is about 0.2 cm s^-1 in the vicinity of emission sources. During transport in the atmosphere, the deposition rate decreases due to preferential removal of larger particles. A representative deposition rate of 0.1 cm s^-1 is found for arsenic in outdoor air (Slooff et al, 1990). For the Netherlands, the removal through dry deposition takes place at an average rate of 0.5% per hour and through wet deposition at 1.2-1.5% per hour. From this, a mean lifetime of atmospheric arsenic aerosol of about 2.5 days can be calculated, allowing for arsenic aerosol to be transported over distances of 1000 km or more (Slooff et al.,1990).

For the Netherlands, during the period 1978-1982, dry deposition has been estimated at 70-220 m g m^-2 yr^-1 and wet deposition i.e., the wash-out of from the atmosphere in rainwater, has been estimated at 0.75-1.5 m g l^-1 yr^-1 - equivalent to about 400-680 m g m^-2 yr^-1 (Slooff et al.,1990). Elsewhere in Europe, deposition values have been determined for a number of discreet areas. For example, in Germany, the dry deposition rates for arsenic vary from about 0.5 m g m^-2 day^-1 in remote areas; below 3 m g m^-2 day^-1 in rural areas; 3-9 m g m^-2 day^-1 in urban areas, and >15 m^-2 day^-1 in the vicinity of emission sources (Kuhling and Peters, 1994).

For the UK, there is a distinct lack of available information on the dry and wet deposition of arsenic, both at the local level and at the regional scale.

3.2 Vapour-phase Arsenic

Lahmann et al (1986) highlight in their consideration of heavy metal pollution across Europe, that there exists the possibility that a number of metals may occur in the gaseous state as a result of high temperature volatilization. For example, metallic mercury, as well as organic and inorganic mercury compounds, are known to occur in the vapour phase, as well as being bound to particulates.

In general, Lahmann et al report that vapour phase concentrations of metals and their compounds are unlikely to be a problem in the atmosphere and pose little in the way of danger to health. In particular, the issue of lead alkyl compounds, which have previously made a significant contribution to air pollution owing to their volatility, are no longer regarded as a problem since their use in fuel additives in large quantities is now restricted. However, it is possible that methylated species of arsenic (e.g. monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) may occur as a consequence of chemical transformations in the atmosphere around certain industrial processes.

For arsenic, arsine (AsH₃) and arsenic trioxide (As₂O₃) are known to occur in the vapour phase. Whether these compounds occur at such high concentrations to warrant extra attention is uncertain. In any case, it is likely that problems will only exist in the vicinity of industrial processes in which these compounds are used. To date, there has been no reporting of vapour phase concentrations of arsenic in the UK, or anywhere else in Europe.

By far the most significant emission sources for arsenic in the UK (ca. 87%) are found in the ‘combustion source’ category. This occurs primarily as a result of the concentration of arsenic in coal which are known to vary from trace to 1-2%.

Emissions of arsenic from stationary combustion sources are only relevant for processes where fuel burning results in a considerable ash residue. For example, in coal-fired units, the fineness of grinding is the controlling parameter - the level of ‘fly ash’ carried by the stack emissions and, consequently the quantities of particulate-bound metals, is dependent upon this parameter. In order that the residual products are dealt with accordingly, it is necessary for the units to use dust separators. The quantities of heavy metals released into the atmosphere is therefore also dependent upon the efficiency of these dust separators. In contrast, where fuel oils are burnt, the level of particulates in the emissions is dependent mainly on the efficiency of combustion.

Commercial and residential thermal generators have similar characteristics to larger power generation units although, in general, lack the dust separators employed by the larger units. Thus, where a large number of commercial or residential thermal generators are present, the cumulative emissions of particulates can be significant.

In all combustion processes, arsenic is generally present in the flue gas as compounds (e.g. oxides, chlorides, etc.) condensed onto the surface of particles. In addition, it has been estimated that arsenic is emitted to a small extent in the vapour phase - calculated as 0.5% (wt) from the arsenic content of coal (USEPA, 1998).

Results for the latest estimates of emissions of arsenic in the UK obtained from the National Atmospheric Emissions Inventory (NAEI, 1997) indicate that, of the top ten companies emitting arsenic to the atmosphere, eight of the top ten positions are held by companies belonging to the power generation sector (Table 4.1). The remaining two positions are held by companies belonging to the mineral and metal production industrial sectors.

Table 4.1: Top Ten Emitters of Arsenic in the UK, 1997 (Pollution Inventory, 1999)

4.2 Production Processes

Commercial arsenic is primarily produced as a by-product in the smelting of non-ferrous metal ores containing gold, silver, lead, nickel and cobalt. The amount of arsenic found in lead and copper ores may range from a trace to 2 - 3 %.

The primary uses of arsenic containing compounds in the UK today are as wood preservatives and in the manufacture of chemical compounds (with arsenic trioxide (AsO₃) being the sole base material). In recent years, the use of refined arsenic trioxide, used as a decoloriser and finishing agent in the manufacturing of bottle glass and other types of glassware, has been replaced by arsenic acid. In addition, there is also a limited demand for high-purity arsenic (99.9% and greater) for use in the semiconductor and electronics industry where it is used together with gallium or indium for producing light emitting diodes (LED), infrared detectors, and lasers.

The following sections consider some of the more relevant processes in more detail :
 
4.2.1 Non-ferrous Metal Industry

Smelting processes are carried out at high temperatures and large quantities of dust and metal oxides fumes can be generated as a result. It is known that a large number of trace elements present in ores, and in concentrates, are volatilized by the high temperatures used in processes such as roasting, syntering, smelting and converting operations (Lahmann et al, 1986).

Small amounts (around 0.5%) of arsenic are added to lead-antimony grid alloys, used in lead acid batteries, to increase endurance and corrosion resistance. Additions of the same order (0.02% to 0.5%) to copper alloys raise the re-crystallisation temperature and improve high temperature stability and corrosion resistance of the alloys. Other uses of metallic arsenic include the addition (up to 2%) to lead in shot to improve the sphericity of lead ammunition.

Arsenic trioxide is easily volatilized during the smelting of copper and lead concentrates, and is known to become concentrated in the flue dust. Crude flue dust may contain up to 30% arsenic trioxide. This crude flue dust is subsequently upgraded by mixing with a small quantity of pyrite or galena and roasting the subsequent mixture. During the roasting process the gases and vapours are allowed to pass through a cooling flue which consists of a series of brick chambers or rooms called ‘kitchens’. The arsenic vapour which condenses in these chambers is of varying purity (from 90 to 95%). Higher purity products can be obtained by resubliming the crude trioxide, an operation typically carried out in a reverberatory furnace.
         

4.2.2 Iron & Steel Industry  

Iron ores contain a number of arsenic compounds that can be released as particle-bound pollutants during different production processes from the iron and steel industry. The exact profile of emitted metal compounds is dependent upon the composition of the ores used and the individual production steps in the iron and steel industry make varying contributions to arsenic levels in the atmosphere. Most emissions of arsenic from the iron and steel industry result from blast furnace charging where coarse dust (with particle sizes > 10 m m) is found. Other relevant emissions occur in connection with melting processes of arsenic-polluted scrap in electric arc furnaces.

4.2.3 Sinter Plants

Another relevant source of emissions of arsenic in the UK is the sintering process, an ore pre-treatment step in primary metals production where fine particles of metal ores are agglomerated to briquettes, sinter or pellets. The whole sintering process consists of several steps (e.g. mixing, crushing, sieving and sintering) where emissions of metal-containing dusts can occur. Hutton & Symon (1986) estimate that sinter production in the UK produced atmospheric emissions of 7.5 tonnes arsenic per year in 1986.

4.2.4 Non-ferrous Metal Mining

Hutton and Symon (1986) highlight that a large number of disused mines in the UK could represent a potentially significant source of contamination of arsenic at the local level through wind dispersal of material from unstable spoil heaps at these sites. However, despite the widespread occurrence of such heaps, it is currently not possible to estimate the quantities of elements released into the atmosphere due to uncertainties in emissions and the reliance on local meteorological conditions.

4.2.5 Waste Treatment and Disposal  

Emissions of arsenic from waste incinerators are wholly dependent upon the composition of the waste, the combustion conditions and the clean technology applied to the stack. Incineration is a complex combustion process involving several key steps: pyrolitic decomposition; surface and gas combustion; conductive, convective and radiative heat transference, and gas flow through beds of materials whose quality, size and shape are continuously changing. Due to the complex nature of the waste incineration process, it is difficult for operators to define optimum conditions for operation and thus environmental problems can result as a consequence of emissions. It has been shown that urban incinerators can be an important source of airborne heavy metals such as arsenic (Law & Gordon, 1979). In addition, waste sites have been shown to be sources of methylated gaseous compounds (Feldmann et al., 1994).

4.2.6 Timber Industry

The manufacture of copper chromium arsenic (CCA) wood preservatives can lead to significant emissions of arsenic to the UK atmosphere. For example, the disposal of treated timber after use results in an estimated environmental discharge of approximately 9 tonnes arsenic per year as a result of burning (Hutton and Symon, 1986). Currently, a large quantity of CCA-treated timber is still in service making it likely to remain a potentially significant emission source in the future. This fact has been emphasised in a recent report by WS Atkins in which the relevant risks associated with the use arsenic-based wood preservatives, covering the manufacture of CCA and the production, use and disposal of CCA-impregnated wood is discussed. In its appraisal of the report, the Scientific Committee on Toxicity, Ecotoxicity and the Environment (CSTEE), broadly accept the risk assessment methodology which highlights that, in general, the risks most associated with the use of CCA-impregnated wood are those resulting from the leaching of CCA into water and soil. For emissions to air, the report indicates that it is the disposal of CCA-impregnated wood that is likely to represent the largest risk. In particular, uncontrolled burning of CCA-treated wood in homes or on open ground.
 
4.3 Emission Inventory Estimates

The UK National Atmospheric Emissions Inventory (NAEI) currently reports emissions of arsenic on an annual basis. The reported figures cited in Table 4.2 and Figure 4.1 are quoted from the latest (1997) report. Results show that emissions have declined by 78% since 1970.

In the UK, the main source of arsenic emissions has been the combustion of fossil fuel (notably coal), other sources being small in comparison. In part, the reduction in UK emissions of arsenic over the past two decades has resulted as a consequence of a decline in coal use, in favour of natural gas use as a cheaper alternative.

In addition to the NAEI emissions estimates of arsenic, the Environment Agency have recently disseminated information regarding a wide variety of industrial processes in England and Wales through the Pollution Inventory. In Scotland, this duty falls to the Scottish Environment Protection Agency for which currently no similar database of emitters is available. Thus, without detailed scrutiny of the Public Registers, there is a lack of available information reported in the current context.

The Pollution Inventory has been developed to provide information on annual mass releases of specified substances to air, water land or produced as waste which arise from any large industrial sites (i.e. those authorised by the Environment Agency under Integrated Pollution Control (IPC)). The reporting requirements for the Pollution Inventory encompass emissions from the whole of the IPC authorisation. That is, it includes, along with the specific release points (point sources, e.g. chimneys), non-point sources and fugitive emissions (e.g. leaks or spillages).

All authorisations for large industrial sites (i.e. those under IPC) have a condition requiring the annual reporting of releases of a "core" list of substances and groups of substances These are substances considered to be important in relation to environmental protection. By ensuring that all sites report, the Environment Agency can be more confident that they are getting a complete picture of what is being released nationally from large industrial sites.

The latest (1998) Pollution Inventory estimates of arsenic emissions for large industrial sites in England and Wales are summarised in Annex 1.
 
Figure 4.1 : Annual UK Emission of Arsenic (1970 - 1996) (NAEI)

Results of the NAEI confirm Pollution Inventory estimates on arsenic emissions in the UK which indicate that the largest emission sources of arsenic in the UK are associated with combustion processes related to the power generation sector.

PowerGen, National Power, and Eastern Merchant Generation power generating plants dominate the top ten rankings. In addition to these power generators, other large emission sources of arsenic in England and Wales include the Rugby Group plc. plant on Humberside, and the Britannia Zinc Ltd. plant in Avonmouth, Bristol. In 1998, arsenic emissions from these plants were estimated at 1.000 tonnes and 0.7359 tonnes, respectively.

Table 4.2 : UK Emissions of Arsenic by UNECE Category (tonnes) (NAEI, 1997).

The lack of ambient air quality guidelines in Europe for arsenic is, in part, due to the fact that the likely levels of arsenic in air are, in general, low. This fact highlights that exposure via air is less significant that other routes, e.g. the consumption of foodstuffs and drinking water.

Currently, the unit risk value cited in the update of the WHO Air Quality Guidelines (WHO, in preparation) is 1.5 x 10^-3 per mg m^-3 of pollutant, based upon observations of lung cancer in the population at ambient concentrations of 1 - 30 mg m^-3 x 10^-3.

The unit risk value has been given an International Agency for Research on Cancer (IARC) classification of ‘1’ - that is, arsenic is a proven carcinogen.
 
  6 Measurement Techniques  
This section gives a general overview of methods employed for the measurement of arsenic concentrations in air in the UK. It outlines analytical methods employed for the analysis of total suspended particulates. In addition, consideration is given to the determination of vapour-phase arsenic.
 
6.1 Sampling

Conventional measurement methods for arsenic typically involve a two-stage process whereby samples are collected and then analysed at a later date.

A variety of sampling and analytical techniques have been used to measure trace metal concentrations in air which include impingers, electrostatic precipitators and filters. Common features to these sampling and analysis techniques employed in the UK over the last 15 - 20 years are:

• total atmospheric particulate samples are collected and analysed with no distinction between particle sizes;
• the duration of sampling varies but typically spans one week (in the UK). The temporal resolution thus tends to be less than for many other pollutants measured in the UK with the consequence that ‘peak’ short-term concentrations may pass unrecorded.

Typically, a number of different filter media have been considered for the determination of trace metal concentrations in air. These include glass fibre filters (borosilicate glass), quartz fibre filters, membrane filters and teflon filters.

An important consideration in selecting the most appropriate filter medium for the collection of trace elements in particulate matter is the background metal concentration of the filter material (the blank value). For example, Harrison (1986) determined that the blank value of arsenic in glass fibre filters was 80 ng As cm^-2. Teflon filters are generally considered to be the best filter collecting medium for the determination of trace metal concentrations due to the very low blank levels. Table 6.1 shows typical properties of the filter types including blank values.

Table 6.1 : Properties of different filters types used for the collection of particulate matter in air.

Filter Type	Blank value ng As cm-2 filter area	Properties
Glass Fibre	40.0 - 60.0	Borosilicate glass, blank values can be variable, low flow resistance.
Quartz Fibre	0.5 - 5.0	Quartz glass, lower blank values for many elements compared to glass fibre filters, similar flow resistance to glass fibre filters. Total digestion is easier than glass fibre filters. Relatively expensive.
Membrane	0.1 - 4.0	Cellulose acetate or cellulose nitrate, low blank values, high flow resistance (increased under high humidity conditions). Prone to electrostatic charge.
Teflon	ca. 0.3	Very low blank values. High flow resistance. Relatively expensive.

For quality assurance, field filter blanks should be included in any monitoring programme. These filters should be handled in the same way as ‘normal’ filters but without sucking ambient air through them. In addition, laboratory filter blanks should be analysed.
 
6.3 Sample Preparation and analysis

In dust analysis, the analytical step is generally preceeded by an appropriate preparation of the sample in order to dissolve the collected particulate matter. This dissolution can be achieved by either wet or dry procedures.

After dissolution, analysis is carried out using a number of different techniques which vary in their procedures and detection limits. They include; atomic absorption spectrometry (AAS); inductively coupled plasma spectrometry (ICP); or, X-ray fluorescence spectrometry (XRF).

Further details of the sample preparation and the analytical techniques are given below:
 
6.3.1 Sample Preparation

In wet digestion procedures, the sample is treated with a mixture of different acids, e.g., HCL, HNO₃, HClO₄ or HF, at normal or elevated pressure. The choice of acid and digestion conditions depends upon the filter material used to collect the sample and the element to be determined. In many cases, digestion with hot nitric acid (with the addition of perchloric acid if the sample has been collected on a membrane filter) has proved satisfactory.

Recent cross-comparisons between sample dissolution methods (ultrasonic agitation, hot-plate digestion and micro-wave assisted digestion procedures) indicate that variable results can be obtained. For example, Butler & Howe (1999) report that ultrasonic agitation digestion methods were satisfactory for most of the elements with the exception of chromium and nickel which showed up to 30% lower recovery values than micro-wave assisted digestion. For laboratories carrying out digestions utilising hot-plates, Butler & Howe report the largest amount of variation. Errors with this method were attributed to the fact that the analyst is required to heat the samples to a ‘fuming’ end-point which was open to interpretation. Insufficient or excessive ‘fuming’ were likely to introduce errors through acid-matrix mismatches.

Small filters (width up to 50 mm diameter) can be digested whole. For filters that are larger than 50 mm, it is recommended that these filters be partitioned into smaller pieces in order to reduce the amount of digestion solution involved and to improve the homogeneity of the collection efficiency over the surface of the complete filter area. If partitioning of filters is used, it is necessary to ensure that sub-samples are representative of the entire filter.

In dry pre-analysis treatment, the sample is dry-ashed, preferably at low temperature (within the range 100 - 150 ^oC). An advantage of this type of sample preparation is that no reagents are required and hence, blank values can be kept low. However, care is required in order to minimise losses of volatile elements from the sample.

6.3.2 Commonly Used Analytical Methods

The following section describes briefly the most commonly used analytical methods for the determination of trace metal concentrations. It does not attempt to produce a detailed review of the techniques which has been carried out previously by a number of workers (e.g. see Harrison & Perry, 1977).

Atomic Absorption Spectrometry (AAS)

AAS is probably the most frequently used analytical method for the determination of particle-bound heavy metals concentrations in air. In this technique, the analyte is aspirated into an ionising flame that causes a chemical reduction of metal ions to ground state atoms, the light absorption of which is measured. The atomic absorptions are discrete lines of narrow band-width at wavelengths which are characteristic of the given element. The wavelength range of AAS is limited by light absorption by the flame that is used for the analysis. For arsenic, an argon-hydrogen-entrained air flame is used with a typical atomic absorption spectrophotometer such as a Perkin-Elmer. Typically, the detection limits for arsenic quoted for the Perkin-Elmer spectrophotometer are 0.05 m g ml^-1 (double-beam instrument).

Inductively Coupled Plasma / Mass Spectrometry (ICP-MS)

This technique includes the following general procedures; the analyte solution is first transferred into a pneumatic nebulizer to produce an aerosol. A stream of argon carries this aerosol into the inductively coupled plasma formed by ionising and exciting the inert argon gas. De-solvation, atomisation and ionisation of the analyte occurs in the plasma with the resultant ions separated in a vacuum on the basis of their mass-to-charge ratio by the mass spectrometer. The ions transmitted through the mass spectrometer are quantified by a pulse counting detector. Calibration with reference solutions, based on the linear relationship between concentration and measured pulse rate, allows a quantitative analysis of the sample analyte.

Analysis of X-Ray Fluorescence (XRF)

This technique relies on the principal that a metal target bombarded with X-rays results in absorption and resultant secondary emission of X-rays (fluorescence) characteristic to the irradiated metal. Using appropriate instrumentation, the intensity of the secondary emissions may, by comparison with a suitable standard, be used to give a quantitative measure of the metal. X-ray fluorescence of particulate matter has the advantage over other analytical techniques in that it is rapid, samples need no pre-treatment (i.e. digestion) and it is non-destructive. Typical detection limits for X-ray fluorescence analysis of particulate material collected on filter papers have been reported by Harrison (1986). For arsenic, a value of 100 ng cm^-2 for arsenic has been cited, based upon energy dispersive analysis using radioisotopes utilising a 10 minute count.

Neutron-Activation Analysis (NAA)

NAA is a non-destructive technique used in a number of studies to determine the elemental composition of particulate matter (e.g. Dams et al., 1970). The sample is irradiated with a flux of neutrons which induces the formation of isotopes emitting a characteristic radiation. Specific detectors characterise the individual elements. Although NAA is a relatively sensitive analytical technique, some elements have been shown to be difficult to analyse (e.g. Pb and Cr), and should therefore be determined using other methods.
 
  6.4 Vapour-Phase Arsenic

A recent update to the Health and Safety Executive’s (HSE’s) guideline document Methods for the Determination of Hazardous Substances series MDHS 41 ‘ Arsenic and inorganic compounds of arsenic in air’, includes the determination of vapour phase arsenic (as arsenic trioxide) in the workplace where exposure of individuals to arsenic may occur during the manufacture and use of arsenic compounds.

The procedure relies on the collection of arsenic compounds (particulate and vapour (only arsenic trioxide; arsine is not trapped)) by drawing air through a two-stage sampling unit containing two filters. The first, a cellulose ester membrane filter captures particulate arsenic and inorganic arsenic compounds whilst the second, a sodium carbonate impregnated back-up paper pad, captures arsenic trioxide vapour. This vapour is collected by the reaction with sodium carbonate on the impregnated back-up filter pad through the following stoichiometric relationship:

Analysis of the cellulose ester membrane and sodium carbonate back-up pads is carried out using atomic absorption spectrometry (AAS), This requires the initial digestion of the filters with an acid mixture containing nitric acid, sulphuric acid and hydrogen peroxide. Further details of the AAS method are given above.

6.5 Proposed Sampling Method

The EC Working Group for metals has recommended that the reference method for the determination of ambient metals in air be based upon the Low Volume Sampler (LVS) as described in EN 12341 (CEN, 1998). A low metal background filter material (quartz), and total digestion and analysis with AAS-technology is recommended by the Working Group. Other methods can be used, if their equivalence has been demonstrated.

CEN/TC 264/WG 14 is currently formulating the full Reference Method for the determination of Pb, Cd, As and Ni in ambient air, which will take note of the EC group recommendation. A laboratory and field validation programme is currently being formulated and will contain the following elements:
 

• Sampling of particulates using the LVS (described in EN 12341) for 24-hour sampling periods
• Collection of particulates on quartz fibre and membrane filters (not Teflon)
• Total digestion with and without HF (digestion procedure to be investigated using Certified Reference Material filters).
• Mandatory measurement with AAS, with optional measurements using ICP-MS.

Four long-term rural background sites have been monitoring particulate arsenic levels since 1972; Chilton (Oxfordshire), Styrrup (Nottinghamshire), Windermere (Wraymires, Cumbria) and Trebanos (West Glamorgan). The annual mean concentrations for 1970 to 1998 are shown in Figure 7.1.

Figure 7.1 Annual mean concentrations of arsenic at rural monitoring locations in the UK (1970 - 1998).

Results show a decline in the ambient levels of arsenic in rural areas throughout the UK. This decline is more prominent at the Styrrup site where ambient concentrations have declined from around 30 ng m^-3 in 1972 to around 3 ng m^-3 in 1998. The higher concentrations of arsenic reported at this site, compared to concentrations recorded during the same period (1972 - 1981) for other sites, are likely to be the result of emissions from coal-fired power stations. These occur at a much higher frequency in The Midlands than in other parts of the UK.

Recent analysis of the data obtained from these sites has been presented (Cawes et al, 1994) with the long-term trends having been analysed over the last two decades (covering 1972-1981 and 1982-1991). Results are summarised in Table 7.1.

Table 7.1 : Rural concentrations of arsenic in particulate matter in the UK (1972 - 1991) from Cawes et al, 1994.

The highest arsenic concentration over the period 1972-1981 (15 ng m^-3) was found at the site in Nottinghamshire, whilst the second highest concentration (7 ng m^-3), over the same period, was found at the site in West Glamorgan. In contrast, sites in Oxfordshire and Cumbria showed the lowest concentrations of arsenic, 3-4 ng m^-3, during the same period.

Results presented by Cawes et al (1994) showed lower concentrations of arsenic were observed at all sites covering the period 1982-1991. A similar picture in the ranking of sites with respect to the concentrations was found: sites in Nottinghamshire and West Glamorgan showed the highest concentrations of arsenic with sites in Oxfordshire and Cumbria showing the lower concentrations.

Results from the rural network highlight that ambient concentrations of arsenic in the UK have been steadily declining over the last two decades. This rate of decline has been generally higher in the first decade of monitoring (1972-1981) compared to results of the monitoring carried out for the period 1982-1991. This can be attributed to improvements to abatement technology and the availability of natural gas as a cheaper alternative to traditional coal-burning in the power generation sector.

7.2 Urban Network Data

In addition to the four rural monitoring stations, arsenic in particulate matter has been monitored at a number of urban and residential sites throughout the UK from the mid 1970’s to the late 1980’s. Results for this network have been recently published by Lee et al (1994) and previously reviewed by QUARG (1993), and are shown in Figure 7.2 and are summarised in Table 7.2.

Figure 7.2 Annual mean concentrations of arsenic at urban monitoring locations in the UK (1975 - 1990).

Results of the annual mean urban concentrations show the highest annual mean concentration of arsenic (223 ng m^-3) occurred at an industrial site in Walsall in 1986. This site is in the vicinity of a smelter and has continually recorded the highest concentration of arsenic at any site in the UK since monitoring commenced in the mid 1970s. Annual mean concentrations recorded at other urban monitoring locations show similar values of around 10-15 ng m^-3 in the mid 1970s to values around 5 ng m^-3 in the early 1990s when monitoring ceased.

Analysis of data by Lee et al (1994) has reflect the trends observed in the annual mean data. The highest mean concentrations for the period covering mid 1970s to the late 1980s was recorded at Walsall (93 ng m^-3), whilst the second highest mean concentration over the period of monitoring was recorded at the urban site in Swansea (19.0 ng m^-3). Results reported by Lee at al (1994) indicate that concentrations of arsenic in residential areas are typically in the region of 5 - 7 ng m^-3, approximately 3-4 times higher than those observed at rural monitoring locations.

The seasonal variability of arsenic has been examined by Lee et al (1994) over all urban sites (except Walsall where a single dominant source was known to influence concentrations). This was carried out through determining the ratio of the quarterly mean to the annual mean concentration. In general, higher concentrations of arsenic were found in the first quarter of the year when compared to any other period - a ratio to mean just above 1.4, whilst concentrations of arsenic were very similar in the second and fourth quarters of the year showing a ratio to mean of approximately 0.9. The lowest arsenic concentrations were observed during the third quarter of the year where a quarter mean ratio to annual mean value of approximately 0.7 was reported. Lee et al (1994) offer no explanation for the seasonal variability in this element although it can be hypothesised that the seasonal variability may be explained in terms of its predominant source, coal and fuel-oil combustion. The higher concentrations in the first quarter, when demand for heating and electricity is greater, support this hypothesis. The seasonal variability in concentrations highlights the fact that anomalous results in monitoring programmes will be obtained with respect to the determination of annual mean concentrations where the observation period are based on a short duration, e.g. 3 months in the winter.
 

Table 7.2 : Urban Concentrations of Arsenic in the UK (1972 - 1989) from Lee et al. (1994).

Long-term trend analysis carried out by Lee et al (1994) on arsenic concentrations in the UK indicate a decline in concentration across all urban sites (excluding Walsall) over the last two decades. For example, the mean arsenic concentration for 1975-1978 was 10.3 ng m^-3 which fell by 74% to a concentration of 2.7 ng m^-3 for the period 1986-1989. Lee et al (1994) highlight the decline in domestic emissions of coal covered by the Clean Air Act (1956), as the possible leading factor in the decrease of ambient levels of arsenic in recent years.

7.3 North-Sea Programme

In addition to the rural and urban monitoring stations within the UK, monitoring for arsenic has been carried out at a small number of stations in relation to the North Sea Project of the Natural Environment Research Council.

The Humber Estuary receives significant inputs of arsenic as a result of the relatively dense industrial units located along the River Trent (Millward et al., 1997). Increased arsenic concentrations (of DMA (dimethylarsenic)) in estuarine waters have been shown to occur as far afield as the Wash in Norfolk as a result of atmospheric enrichment from industrial processes located on the Humber estuary (Millward et al., 1997).

In addition to the number of estuarine sites reported by Millward et al. (1997), three land sites have been included in the study at East Ruston (Norfolk), High Muffles (Yorkshire) and Banchory (Kincardinshire). Results for the monitoring period covering 1993 - 1998 are shown in Figure 7.3.

Results show similar concentrations to those observed at rural monitoring locations elsewhere. That is, annual average concentrations of arsenic of around 0.5 to 1.0 ng m^-3.

Figure 7.3 Annual mean concentrations of arsenic at land sites included in the North-Sea monitoring programme (1993 - 1998).

8 Conclusions  
Particulate-phased arsenic concentrations have been monitored in the UK at a number of sites covering rural (Cawes, 1994) and urban locations (Lee et al., 1994) for the past two decades. Results show that concentrations around individual industrial installations can be high (572 ng m^-3 - 3 month mean at Walsall (as cited in Lee et al., 1994)). In general, urban concentrations of particulate-phased arsenic fall within the range 5 - 7 ng m^-3 with rural locations falling within the range 0.8 - 4.5 ng m^-3.

Currently, no air quality standard exists in the UK for arsenic in air. Arsenic is a known carcinogen and the World Health Organisation (WHO, 1987) acknowledges that there is no acceptable limit to be set. However, health risk assessments have been made which report a unit risk value of 1.5 x 10^-3 for each m g m^-3 of arsenic to which an individual is exposed (WHO, 1987).

Emissions estimates for arsenic obtained from both ad-hoc studies and the UK National Air Emissions Inventory (NAEI) indicate that combustion of fossil fuels such as lignite (brown coal), hard coal and heating oil account for the largest proportion of total UK emissions of arsenic. Other relevant emission sources of arsenic to air in the UK include ferrous and non-ferrous production processes. In each case, the emissions of arsenic to air are largely dependent upon the arsenic content of the metals ore and/or fossil fuel and the propensity of the process to produce particulate emissions. Emissions from the chemical, glass and cement industries are generally much smaller than those from other industrial sectors of the UK.

Trend analysis of ambient concentrations of arsenic in the UK indicate that concentrations have been steadily decreasing over the 20 or so years that monitoring has taken place. Such a trend is likely to reflect the advances in clean technology and pollutant abatement schemes (e.g. flue gas desulphurisation (FGD) and electrostatic precipitators (ESP)) integrated as a condition of the authorisation for many of the processes from which arsenic is known to be emitted. In addition, the switch from traditional coal-burning to natural gas as a cheap alternative in the power generation sector are likely to have made the largest significant decrease in emissions of arsenic to air in recent years.

The present review highlights a number of gaps in the current knowledge of arsenic occurrence in the UK. In particular, any detailed monitoring in Scotalnd and Wales. Notably, there has been little work carried out on identifying different species of arsenic in the atmosphere - to date, all ambient monitoring of arsenic carried out has reported total arsenic concentrations in the particulate phase. The review highlights that more consideration is required to determine vapour-phase concentrations (as arsenic trioxide (AsO₃)) of arsenic, but acknowledges that this is likely to be a problem with regards to occupational exposure only.

In addition, the review highlights that a cohesive approach to the monitoring of arsenic is required within the UK in which monitoring methods are standardised with respect to the particulate fraction to be collected. Traditionally, the monitoring of arsenic has been made on the total suspended particulates in air. Recently, an increased understanding of the occurrence of particulates in air highlight the importance of the fine (PM₁₀) and ultra-fine (PM_2.5) fraction (APEG, 1999).

The first Daughter Directive published in 1997 by the European Commission reached the common position in June 1998 for a number of pollutants with the proposals to set limit values for sulphur dioxide, nitrogen dioxide, particulate and lead. The Commission intends that future Daughter Directives will set limit values for a number of other pollutants for which arsenic (among other heavy metals) has been included. Currently, the Department of the Environment, Transport and the Regions is funding consultants, Stanger Science and Environment to undertake 12 months monitoring of lead and heavy metal concentrations in response to these Commission’s proposals. In light of the recent review of the National Air Quality Strategy (HMSO, 1999) the main focus of attention in the 12 month monitoring period is in the vicinity of industrial processes. Results are likely to be made available in December 2000.

References

APEG (1999). Source Apportionment of Airborne Particulate Matter in the United Kingdom. Report of the Airborne Particles Expert Group. January 1999.

Becher, H. & Wahrendorf, J. (1992). Metalle/Arsen. In: H.E. Wichmann, H.W. Schlipkoter, G. Fulgraff: Handbuch Umweltmedizin: Toxikologie, Epidemiologie, Hygiene, Belastungen, Wirungen, Diagnostik, Prophylaxe, Ecomed Verlag, Landsberg, Kap. VI-3.

Butler, O.T. & Howe, A.M. (1999). Development if an international standard for the determination of metals and metalloids in workplace air using ICP-AES: evaluation of sample dissolution procedures through an interlaboratory trial. J. Environ. Monit., 1, 23-32.

Cawes, P.A., Baker, S.J., & Law, D.V. (1994). A survey of atmospheric trace elements in Great Britain, 1972-1991. Report number: AEA/CS/18358008/REMA-039. AEA Technology, National Environmental Technology Centre, Culham Laboratory, Oxfordshire.

CEN (1998). Air Quality - Determination of the PM₁₀ fraction of suspended particulate matter - Reference method and field test procedure to demonstrate reference equivalence of measurement methods. European Committee for Standardization (CEN). Ref. No. prEN 12341: 1998 E.

Dams, R., Robbins, J.A., Rahm, K.A. and Winchester, J.W. (1970). Non-destructive neutron activation analysis of air pollution particulates. Anal. Chem., 42, 861.

Feldmann, J., Grumping, R., & Hirner, A.V. ( 1994). Determination of volatile metal and metalloid compounds in gases from domestic waste disposits with GP/ICP-MS. J. Anal. Chem., 350, 228-234.

Harrison, R.M.(1986). Metal analysis. In: Handbook of Air Pollution Analysis. R.M. Harrison and R. Perry (Eds.). Pub. by Chapman and Hall, Ltd. University Press, Cambridge.

HMSO (1999). Review of the United Kingdom National Air Quality Strategy: A Consultation Document. Published by the Department of the Environment, Transport and the Regions in partnership with the Welsh Office, Scottish Office and Department for the Environment for Northern Ireland, HMSO.

HSE (1995). Arsenic and inorganic compounds of arsenic in air. Part of the HSE’s Methods for the Determination of Hazardous Substances (MDHS 41/2.) Series. Published by the Health and Safety Executive.

Hutton, M and Symon, C. (1986). The quantities of cadmium, lead, mercury and arsenic entering the UK environment from human activities. The Science of the Total Environment, 57, 129-150.

Lahman, E., Munari, S., Amicarelli, V., Abbaticchio, P., & Gabellieri, R. (1986). Environment and Quality of Life. Heavy Metals: Identification of air quality problems and environmental problems in the European Community. Volumes 1 and 2. Commission of the European Communities Report Number EUR 10678, CEC, 1987.

Law, S.L. & Gordon, G.E. (1979). Sources of metals in a municipal incinerators emissions. Environ. Sci. Technology., 13, (4), 432 - 437.

Lee, D.S., Garland, J.A. and Fox, A.A. (1994). Atmospheric concentrations of trace elements in urban areas of the United Kingdom. Atmospheric Environment, 28, 2691-2713.

Ledd, D.S., Espenhahn, S.E., and Baker, S. (1998). Evidence for long-term changes in base cations in the atmospheric aerosol. Journal of Geophysical Research 103, 21955 - 21966.

Millward, G.E., Kitts, H.J., Ebdon, L., Allen, J.I., and Morris, A.W. (1997). Arsenic species in the Humber Plume, UK. Continental Shelf Research, 17 (4), pp. 435-454.

NETCEN (1998). Air Pollution in the UK . A report produced for the Department of the Environment, Transport and the Regions. National Environmental Technology Centre, Culham Laboratory, Oxfordshire.

Pollution Inventory. Environment Agency website address:
http://193.122.103.90/wiyby/html/introduction.htm

QUARG (1993). Urban Air Quality in the United Kingdom. First Report of the Quality of Urban Air Review Group. January 1993. Department of the Environment.

Slooff, W., Haring, B.J.A., Hesse, J.M., .Janu, J.A., and Thomas, R. (1990). Integrated Criteria Document Arsenic. Report No 7104001004.

United Kingdom National Emissions Inventory -
/netcen/airqual/naei/annreport/annrep96/sect6_3.htm

USEPA (1998). Locating and Estimating Air Emissions from Sources of Arsenic and Arsenic Compounds, USEPA, Office of Air Quality Planning and Standards, EPA-454/R-98-013.

WHO (1987). Air Quality Guidelines for Europe. World Health Organisation Regional Office for Europe, Copenhagen. WHO Regional Publications. European Series 23.

WHO (in preparation). Air Quality Guidelines for Europe, 2nd edition. World Health Organisation, Regional Office for Europe, Copenhagen, WHO Regional Publications, European Series, Internet address: http://www.who.dk