The quarterly mean concentrations of 35 elements and TSP measured in air at Chilton, Styrrup and Wraymires for each of the years 1996, 1997 and 1998 are tabulated in Appendix 1. These tables also include the annual mean air concentrations for each year.

The quarterly and annual mean concentrations of 25 elements, nitrate and sulphate measured in the soluble fraction of rainwater at these three locations during 1996, 1997 and 1998, are also listed in tables in Appendix 1. The annual mean concentrations are precipitation-weighted. Quarterly and annual deposition data have also been derived, from the rainwater concentrations and the measured rainfall at each site, and are also listed in the tables in Appendix 1.      

The annual mean air concentrations of many of the 35 elements, measured at each of the three sites during the period 1996 to 1998, have been compared to the measurements made over the preceding four years. These measurements (1992-1995), reported in Baker (1997), have been summarised for comparison with the 1996-1998 data in Table 1. Those elements whose annual mean concentrations were frequently below analytical detection limits have been omitted from this summary.

The elements can be broadly classified into the following groups according to average air concentrations that were usually recorded:

1. Concentrations >1000 ng m^-3: Cl.
2. Concentrations between 100 - 1000 ng m^-3: Al, Ca, Fe, K, Mg and Na.
3. Concentrations between 10 - 100 ng m^-3:, Pb and Zn (concentrations of Al and Fe at Wraymires fell within this range).
4. Concentrations between 1 - 10 ng m^-3: Br, Cu, I, Mn, Ni, V (concentrations of Ti were generally <10 ng m^-3 at Chilton and Wraymires; and <20 ng m^-3 at Styrrup).
5. Concentrations of <1 ng m^-3: Ag, Au, Ce, Co, Cd, Cs, Eu, Hg, In, La, Mo, Rb, Sc, Sm and W. Air concentrations of As, Cr, Sb and Se fell into groups (iv) or (v) depending on location. Concentrations of these elements were generally ~1 ng m^-3 at Chilton or >1 ng m^-3 at Styrrup, with concentrations of <1 ng m^-3 recorded at Wraymires (Table 1).

For a given site, the annual mean concentrations measured in 1996, 1997 and 1998 were either similar to the average annual mean for the period 1992-1995, or they tended to fall within the range of values recorded during these five years (Table 1).
 

Figure 2 - Quarterly mean concentrations of heavy metals in air at Chilton, Styrrup and Wraymires (1992-1998)

There were some exceptions, notably Cd, Cr, Cu, Ni, Pb and Zn during 1998, when the annual mean concentrations were below the range of values recorded since 1992 (Table 1). The quarterly mean concentrations of the mainly anthropogenically-derived heavy metals As, Cr, Cu, Hg, Ni, Sb, Se, V and Zn at Chilton, Styrrup and Wraymires between 1992 and 1998 are plotted in Figure 2. Since 1995, Cd has been analysed using ICP-AES (see section 2.3.1) and detectable concentrations have been measured at Chilton and Styrrup; these are shown in Figure 2. At Wraymires, quarterly mean air concentrations of Cd and Ni were mainly below the LODs (typically <0.1 ng m^-3 and <1 ng m^-3, respectively) and are excluded from Figure 2.

The relatively low air concentrations of some of these metals, that were recorded during 1998, are apparent in Figure 2. Cd concentrations have fallen from around 0.5 ng m^-3 to <0.1 ng m^-3 at both Chilton and Styrrup. Quarterly mean air concentrations of Cr, Cu (at all three sites) and Ni and Zn (at Chilton and Styrrup only) were the lowest recorded during the last seven years (Figure 2). Indeed for Cr, Cu, Ni, Pb, V and Zn (at Chilton and Styrrup only) the 1998 annual mean air concentrations at all three sites were the lowest recorded since measurements began in 1972.

Relatively high annual mean concentrations of Zn were found at Styrrup during 1996 and 1997 (Table 1). These were due to anomalously high mean concentrations measured during the last quarter of 1996 (201 ng m^-3) and first quarter of 1997 (437 ng m^-3) (Figure 2). Exclusion of these data reduces the annual means to 66 and 57 ng m^-3 for 1996 and 1997, respectively. These values are typical of those measured at Styrrup between 1992 and 1995 (Table 1).

As in recent years (Baker, 1997) and historically (Cawse et al., 1995), in 1997 and 1998 the lowest annual mean air concentrations of all the elements that were above analytical LODs continued to be recorded at Wraymires. The highest were measured at Styrrup (Table 1). With few exceptions, the annual mean concentrations during 1996, 1997 and 1998 followed the relative order Styrrup > Chilton > Wraymires. The greater industrial influence at Styrrup, with its proximity to the South Yorkshire industrial region, was apparent. The annual mean air concentrations of the mainly industrially-derived heavy metals As, Cr, Pb, Sb, Se, V and Zn were typically ~3 to ~6-fold greater (and up to ~10-fold greater for Cr) at Styrrup compared to Wraymires during the period 1996-1998.

During 1996-1998, the maximum annual mean air concentration of Pb was recorded at Styrrup in 1996 (38 ng m^-3), representing only ~2% of the limit value for airborne Pb concentrations, 2 m g m^-3 as an annual average (EC, 1982). Under the Framework Directive on Ambient Air Quality Assessment and Management (96/62/EC), the European Council (EC) has adopted a proposal for a Directive that sets a new ambient air quality limit value for Pb of 0.5 m g m^-3. Further, the Expert Panel on Air Quality Standards (EPAQS) has recently recommended an air quality standard for Pb in the UK of 0.25 m g m^-3, measured as an annual average (DETR, 1998). Therefore, the 1996 annual mean measured at Styrrup would be only ~15% of this lower value.

The World Health Organisation (WHO) has produced guideline values, with regard to air pollution, for a variety of compounds, including some metals (WHO, 1987). For example, for Cd in air at rural locations, the levels present in 1987 (<1 - 5 ng m^-3 as an annual mean concentration) should not be allowed to increase. At Chilton and Styrrup, the 1996 and 1997 annual mean air concentrations were all below 0.6 ng m^-3 and were <0.2 ng m^-3 during 1998. At Wraymires, Cd air concentrations were <0.2 ng m^-3 during the last three years (Table 1).

As, Cr and Ni are also referred to in the WHO publication. However, no WHO guideline values for these metals are given, due to their carcinogenicity and no safe levels are recommended.

No air quality guideline for Hg has been set, either in the UK or Europe, although the EC are to develop proposals for further daughter Directives which will set limit values for some metal pollutants, including Hg, identified in the Framework Directive (96/62/EC). The other metals are As, Cd and Ni. The potential effects of Hg upon human health were reviewed by the WHO. Although they were unable to recommend a guideline value, a concentration of 1 m g m^-3 as an annual average was considered to be a level which should adequately protect human health (WHO, 1987). Hg is present in the atmosphere in the gaseous phase and attached to particulate material. The above WHO value applies to both forms of Hg.

The sampling method used in this network will collect only the particulate bound Hg (section 2.3.1). During the period 1996-1998, the maximum annual mean air concentration of Hg (0.72 ng m^-3 at Styrrup in 1998, Table 1) represents only 0.07% of the WHO recommended value. However, gaseous Hg is the dominant form present in the atmosphere. Measurements of total gaseous mercury (TGM) were made at the Chilton site between June 1995 and April 1996 (Lee et al., 1996). The average air concentration of TGM over this period was 1.68 ng m^-3, compared with a particulate-bound Hg concentration of ~0.1 ng m^-3, i.e. > 90% of the atmospheric Hg measured at this site was found to be TGM. If this were also true at the Styrrup location in 1998, then the total atmospheric Hg present would still only be 0.7% of the WHO value recommended above.

The quarterly mean air concentrations of Pb and Br, together with Na and Cl, at Chilton, Styrrup and Wraymires for the period 1992-1998 are shown in Figure 3.

The major source of Pb emissions in the UK is the combustion of leaded petrol (Salway et al., 1996 and Salway, 1999). Br is also present (as ethylene dibromide) in leaded petrol, therefore the two elements are associated in vehicle exhaust. The Pb/Br ratio in leaded petrol is 2.59 (Harrison and Sturges, 1983). This can be used, with some caution, as an indicator of vehicle-emitted Pb. Harrison and Sturges (1983) discussed the problems associated with the measurement and interpretation of Br/Pb ratios in airborne particulate material. Another major source of atmospheric Br is marine-derived aerosol (Cawse, 1975; Harrison and Sturges, 1983).

The Pb/Br ratios measured in air particulate material, collected at quarterly sampling intervals between 1992 and 1998 at Chilton, Styrrup and Wraymires, are summarised in Table 2. The corresponding ratios of Cl/Na and Na/Br are also listed.
   

At both Chilton and Styrrup the average quarterly Pb/Br ratio is very close to that for leaded petrol (2.59), however at Wraymires the mean value is much lower. A linear regression analysis of quarterly mean air concentrations of Br against Pb indicated that these elements were highly significantly correlated at Chilton (r = 0.712, p < 0.001) and at Styrrup (r = 0.884, p < 0.001). At Wraymires the two were less significantly correlated (r = 0.434, p < 0.05). This may indicate that sources of Pb and/or Br other than vehicle emissions contributed to air concentrations at Wraymires.

A maritime influence appears to prevail at Wraymires; an average quarterly mean Cl/Na ratio in air of 1.8 was recorded at this site. This is the same as the value derived by Cawse (1974) for bulk sea water. Average Cl/Na ratios in excess of 1.8 were measured at Chilton and Wraymires (Table 2). Combustion of coal was probably responsible for these elevated Cl/Na ratios. At all three locations, the quarterly mean concentrations of Na and Cl were highly significantly correlated (p < 0.001) over the last eight years.

The average Na/Br ratio in air particulate at Wraymires (114) is higher than at Chilton (93) or Styrrup (57) and is closer to the value of 160 derived for bulk sea water (Cawse, 1975). The quarterly mean air concentrations of Na and Br were highly significantly correlated at Wraymires (r = 0.748 p < 0.001), but were less well correlated at Chilton ( r = 0.381, p < 0.05) and at Styrrup (r = 0.423, p < 0.05). This suggests a possible maritime influence on air concentrations of Br at Wraymires. This would account for the lower average Pb/Br ratio in air that was measured at this site.

Figure 3 - Quarterly mean concentrations of Pb, Br, Na and Cl in air at Chilton, Styrrup and Wraymires (1992-1998)

Seasonal variability in the concentrations of elements in air

Historically, air concentrations of many of the elements have shown a seasonal variability (Cawse et al., 1995). This feature was continued during the period 1992-1996, at Chilton, Styrrup and Wraymires. Over this five year period, the average ratio of the quarterly mean to annual mean during each of the four quarters was derived (Baker, 1997). The quarterly mean air concentrations for 1997 and 1998 have now been included in the examination of the data for seasonal differences.

For Pb, Br, Na and Cl the highest ratios tended to occur during the ‘winter’ quarters 1 and 4 at all three sites (Figure 4a).

Generally, this was also the case for many of the heavy metals of anthropogenic origin, with the highest ratios occurring in the first or fourth quarters or both (Figures 4b and 4c). Of these metals, As, Hg, Ni, Sb and V showed the greatest seasonal variations at all three sites (Figure 4b), with higher air concentrations found during the ‘winter’ quarters. The ratios for Co, Cr and Cu at Styrrup and Wraymires were also higher during the first and fourth quarters, whilst at Chilton no clear seasonal differences were apparent (Figure 4c).

These observations, for the data for 1996-1998, are consistent with those for the historical (Cawse et al., 1995) and more recent data (Baker, 1997) from this sampling network. The ‘winter’ increases are attributed to the increased combustion of fossil fuels, together with the persistence of inversion layers in the atmosphere that decrease the dispersion of pollutants. Other meteorological conditions will also contribute to these seasonal variations. For example higher wind speeds during winter, will increase the resuspension of marine aerosol, hence the ‘winter’ maxima in Na air concentrations.

For the elements Al, Ca, Ce, Fe, K, Mn and Sc, which are mainly soil-derived (but some do have anthropogenic sources), the seasonal variation in air concentrations continued to be much less apparent than for the industrially-derived metals (Figure 4d). Moderate seasonal variations are only apparent at Chilton; the highest quarterly mean to annual mean ratios for many of these elements occurred during quarters 2 and 3 during the last seven years (Figure 4d). This could be attributable to increased resuspension of soil dust during relatively dry periods during these ‘summer’ quarters.
   

Figure 4a - Seasonal variability in air concentrations of Pb, Br, Na and Cl at Chilton, Styrrup and Wraymires (1992-1998)
     

Figure 4b - Seasonal variability in air concentrations of As, Hg, Ni, Sb, V and Zn at Chilton, Styrrup and Wraymires (1992-1998)    

Figure 4c - Seasonal variability in air concentrations of Co, Cr, Cs, Cu, Mg and Se at Chilton, Styrrup and Wraymires (1992-1998)    

Figure 4d - Seasonal variability in air concentrations of Al, Ca, Ce, Fe, K, Mn and Sc at Chilton, Styrrup and Wraymires (1992-1998)

Air concentrations of heavy metals at rural locations in the UK

Annual mean air concentrations of As, Cd, Cr, Cu, Ni, Pb and Zn at three other rural locations in the UK, for the period 1994-1997, have recently been reported (Playford and Baker, 1998) and data for 1998 are now available (Playford and Baker, 1999). These sites are located on the east coast of the UK at East Ruston (Norfolk), High Muffles (North Yorkshire) and Banchory (Aberdeenshire). The data from these locations, together with those from Chilton, Styrrup and Wraymires, can be used to derive ‘current’ air concentrations of these metals at rural locations in the UK.

In Table 3 the average, maximum and minimum values for the annual mean air concentrations of As, Cd, Cr, Cu, Ni, Pb and Zn for the period 1994-1998 are listed and are shown in Figure 5. Anomalously high quarterly mean concentrations of Zn measured at Styrrup and High Muffles were excluded in calculating the means that are summarised in Table 3.
 

Notes (i) Measurement period for Cd at Chilton, Styrrup and Wraymires is 1995-1998. Annual mean concentrations of Cd and Ni at Wraymires were below analytical limits of detection.

Air concentrations of all seven metals measured at Wraymires fell within the ranges of values reported for the three east coast sites (Figure 5). Annual mean air concentrations of As, Cd, Cr, Cu and Ni at Chilton were also of the same order, with slightly elevated levels found for Pb and Zn. At Styrrup, average concentrations of all seven metals (particularly As, Cr, Zn and including Pb) exceeded those measured at East Ruston, High Muffles and Banchory over the last five years (Figure 5). The exceedances are attributed to regional influences, e.g. industrial activity and greater consumption of coal for domestic and light commercial industrial combustion, in South Yorkshire and the East Midlands.

Pb concentrations have also been measured in air at two other rural locations in the UK, Cottered and Eskdalemuir. These sampling sites form part of DETR’s Lead in Petrol Monitoring network. For the period 1994-1998, the following annual mean air concentrations of Pb were recorded at these sites (DETR, 1999).

Cottered: Mean 21 ng m^-3,, range 19 -25 ng m^-3.

Eskdalemuir: Mean 8 ng m^-3, range 5 - 10 ng m^-3.

These annual average concentrations of Pb fall within the ranges listed in Table 3.
 
 

Comparison of rural and urban air concentrations of heavy metals in the UK

Urban and rural air concentrations of heavy metals for the period 1992-1996 have been compared previously (Baker, 1997). These data can be compared to help assess what proportion of urban airborne trace metals are due to regional scale transport and how much is locally derived.

Gee (1998) has reported the 1997 annual mean air concentrations of Cd, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn at five urban locations in the UK. These are Motherwell, Glasgow, Leeds, Brent and Central London. The corresponding data for 1998 were obtained (DETR, 1999). The 1997 and 1998 concentrations, averaged for all five urban sites, together with corresponding data from rural locations in the UK, are summarised in Table 4.

The greatest differences between average rural and urban air concentrations during 1997 and 1998 were seen for Cd, Cu, Fe and Ni. Average urban concentrations of these metals were generally ~5-fold greater, with average annual mean urban concentrations of Cd and Ni being ~10 times in excess of corresponding rural levels during 1998. Average Cr and Pb air concentrations at the urban sites were ~4-fold greater, while urban airborne levels of Mn, V and Zn were ~2-fold in excess of those measured at the rural locations during 1997 and 1998.
 

Notes

(2) Urban sites are at Central London, Brent, Leeds, Motherwell and Glasgow (Gee, 1998).
 
 
 
  3.2 Elemental composition of the total suspended particulate
 

At Chilton, Styrrup and Wraymires the annual average percentage (on a dry weight basis) of an element in the TSP has been derived, for each of the years 1996, 1997 and 1998, from the corresponding annual mean air concentrations of that element and the TSP (Table 5). For those elements whose quarterly concentrations were frequently below analytical detection limits, this parameter has not been derived.
 
 
 

Table 5 Mean Elemental Composition of Total Suspended Particulate at Chilton, Styrrup and Wraymires (1996-1998)

These percentages have been summed to indicate the total contribution of these elements to the total air particulate composition (Table 5). It should be noted that the recorded TSP will exclude volatiles, e.g. NH₄NO₃ and NH₄Cl, since the exposed filter papers are oven-dried before they are weighed.

At the three locations, between 20% and 27% of air particulate composition was made up of the 22 trace and major elements listed in Table 5. Cawse (1974) previously established that some 50% by mass of TSP consisted of sulphate, nitrate and ammonium ions. The remainder being silicon, carbon and organic material. The major constituents of the TSP at the three locations were Cl (~8-12%), Na (~4-8%), Ca (~1-3%), K (1-2%), Mg (~1%) and Fe (0.5-1%). Slight variations, in the proportions of some of the anthropogenically-derived heavy metals present in the TSP, according to location were seen. For example, As, Cr, Ni and Zn were higher at Styrrup than at the other two sites. This is due to the relative proximity of this site to sources in industrial areas in South Yorkshire and the East Midlands

>Section 3.3 Long term changes in concentrations of elements in air >