AEA Technology, on behalf of the Air and Environment Quality Division of the Department of the Environment, Transport and the Regions (DETR) (formerly the Department of Environment); the Scottish Executive; the National Assembly for Wales; and the Department of the Environment in Northern Ireland, has operated three long-term sampling stations for trace elements in the atmosphere in rural environments in England since the early 1970s. The overall aims of this programme of measurements are to quantify and follow changes at rural locations in air particulate and atmospheric deposition with respect to inorganic species, and to examine any variation in relation to changes in methods of energy production, energy consumption and emissions control policy.

The objectives of the present study were as follows:

    (i) to maintain the existing measurements programme;

     (ii) to establish the seasonal and long-term changes in elemental concentrations in air and rainwater at rural locations and deposition to soil;

     (iii) to provide data for comparison with urban air quality.

This report provides a summary of data obtained during the period 1996 to 1998. The data obtained between 1992 and 1996 has been reviewed and reported previously (Baker, 1997). These data were compared with the historical data to investigate long-term changes in concentrations of trace elements in air particulate and atmospheric deposition. A review of the data obtained during the period 1972-1991 was undertaken by Cawse et al. (1995).    

2 Methods of Sampling and Analysis
 
 

Chilton, Oxfordshire (National Grid Reference: SU 468861, altitude 130 m). This site is in central southern England, about 90 km from the English Channel. The sampling equipment is situated on site at AEA Technology’s Harwell Laboratory. The surroundings are predominantly rural, with large areas devoted to cereal cultivation.

Styrrup, Yorkshire. In 1982, the sampling station was re-located to Sandbeck Park (National Grid Reference SK 561906), altitude 15 m). This site is 4-5 km to the west of the original site at Styrrup (National Grid Reference SK 606898, altitude 15 m). For reasons of continuity, the original name of the station is retained. The station is in a rural area with some arable farming in the immediate surrounding area. There are some industrial influences, e.g. Sheffield, with its steel industry lies some 25 km to the west.

Wraymires, Cumbria (National Grid Reference SD 362974, altitude 84 m). This is a rural site adjacent to Esthwaite Water, near Lake Windermere. It is approximately 25 km from the Irish Sea and is ~30 km to the north-east of Barrow-in-Furness, where there is some shipbuilding industry. Approximately 40 km to the south of Wraymires is the Heysham oil refinery. The station is rural with a moderate maritime influence and relatively high annual rainfall. There is little arable farming in the area which might give rise to resuspension of local soil dust.

Atmospheric particulate material and rainwater is sampled continuously. The sample collectors (described below) are changed each month by local site operators. The samples are sealed prior to their despatch to the National Environmental Technology Centre (NETCEN). All samples are collected at 1.5 m above ground level to reduce contamination by soil splashing during heavy rainfall. The sampling equipment and methods used were chosen to obtain continuous, trouble-free operation that required a minimum of attention. Each sampling station is subject to an annual maintenance visit.

Airborne particulate material is sampled by drawing air through a cellulose filter (Whatman 40 grade paper, 6 cm diameter) held in a polypropylene duct. The inlet nozzle faces downwards to avoid the intake of rain. A plastic backing disc (1.5 mm mesh) prevents the collapse of the filter paper. Air is drawn through the filter paper by an electrically driven pump. The volume of air sampled (~250 m³ per month) is measured by a calibrated gasmeter. The sampling characteristics, e.g. collection efficiency has been established experimentally (Pattenden and Wiffen, 1976) and has been briefly described in Cawse et al. (1995). Although they do not conform to a PM₁₀ specification, these air samplers tend not to collect large particles efficiently and are therefore reasonable surrogates for respirable particle samplers. The filter papers are oven-dried and pre-weighed to allow determination of the total suspended particulate (TSP) in air. It should be noted that the recorded TSP will exclude volatiles, especially NH₄NO₃ and NH₄Cl, since the exposed filter papers are oven-dried before they are weighed.

Rainwater is collected each month using a polythene funnel and bottle. The collecting funnel contains a polythene debris filter (0.5 mm mesh) to exclude insects. The bottle is enclosed in black polythene to prevent the growth of algae in the collected rainwater sample. The collector retains the bulk deposition, i.e. the wet deposition, plus the dry deposit that falls upon the funnel and is washed into the bottle by subsequent rain. In contrast to the restrictions on particle size collected by the air sampler, the bulk precipitation collector may retain larger particles in the atmosphere. The monthly rainfall at each site is recorded by a standard Meteorological Office rain gauge at 0.3 m above ground level.

Air filters

Representative portions of each monthly air filter are bulked to obtain quarterly (i.e. three-monthly) samples for analysis. To avoid contamination by trace and major elements, sample processing is carried out in a laminar air-flow cabinet. Air filter holders are only opened to remove the filter paper under these conditions. Polythene gloves and polythene apparatus are used for sample handling to avoid contamination, e.g. by metals. All apparatus is cleaned with 5% v/v acetic acid AR grade, then rinsed with demineralised water before use. A portion of each monthly sample is retained for retrospective analysis, if required.

The following 35 elements are analysed in atmospheric particulate material. Ag, Al, As, Au, Br, Ca, Cd, Ce, Cl, Co, Cr, Cs, Cu, Eu, Fe, Hg, I, In, K, La, Mg, Mn, Mo, Na, Ni, Pb, Rb, Sb, Sc, Se, Sm, Ti, V, W and Zn. Historically, instrumental neutron activation analysis (INAA) was the preferred technique for the majority of these analytes and is still currently used. The exceptions were Cd, Cu, Ni and Pb which, since 1995, have been determined by Inductively-coupled plasma atomic emission spectroscopy (ICP-AES). Previously, Cu, Ni and Pb were analysed by X-ray Fluorescence (XRF). It should be noted that the sampling method only collects non-volatile particulate material and will not truly represent elemental mercury and volatile mercury compounds. Appropriate ‘blank’ samples are also analysed.

Errors in analysis by INAA are based on 1s counting statistics. Detection limits are based on the rejection of errors >30%. A Standard Reference Material, i.e. National Bureau of Standards (NBS) citrus leaves, is also analysed for Quality Control (QC) purposes.

Uncertainty in the measurements made by ICP-AES is estimated to be + 10% for results that were an order of magnitude greater than the LOD. For each element, the LOD was determined at the time of analysis. Higher uncertainties are associated with measurements that fall within an order of magnitude of the LOD. An independent QC standard is measured during the analysis of samples using this technique.
 
 

Rainwater/Bulk Deposition

Monthly rainwater samples are bulked, according to rainfall amount each month, to obtain a quarterly sample for analysis. All monthly samples are stored at -5 ° C upon receipt until they are bulked into quarterly samples. A portion of each monthly sample is stored frozen, should retrospective analysis be required. The quarterly rainwater samples are filtered (using Whatman 42 grade cellulose filter paper) to separate the soluble and insoluble fractions. Filtered samples are then concentrated prior to their analysis.

ICP-AES and Inductively-coupled plasma mass spectrometry (ICP-MS) techniques are used to determine the following 25 analytes in the soluble fraction of the rainwater samples; Ag, Al, As, Au, Ca, Cd, Co, Cr, Cu, Fe, Hg, In, K, Mg, Mn, Mo, Na, Ni, Pb, Sb, Se, Ti, V, W and Zn. The soluble fraction is known to contain a higher percentage of potentially toxic elements, whereas the insoluble component contains a high proportion of elements derived from the soil, e.g. Al, Ce, Fe and Sc (Cawse et al, 1995). Uncertainties in the measurements made by ICP-AES/ICP-MS are described above (section 2.3.1).

Nitrate and sulphate were also analysed in the filtered rainwater samples using Ion Chromatography. The pH of both the monthly samples as received, and the bulked quarterly samples, is measured by a standard pH electrode and meter.