A long-term programme of measurements of trace and major elements, in air particulate and rainwater at ground level at three rural locations in England, has been maintained over the period 1996-1998. Concentrations of 35 elements in air and 25 elements in rainwater (plus nitrate and sulphate) have been measured on a quarterly basis during this three-year period.

These measurements have also been compared with earlier data from the 1990s along with corresponding historical data, compiled since the early 1970s, to investigate any long-term changes in relation to changes in methods of energy production, energy consumption and emissions control policy.

The main conclusions are:

• Recent annual mean concentrations of heavy metals in air and rainwater at two sites, Chilton and Wraymires, are similar to those measured at other rural locations in the UK. At Styrrup, both air and rainwater concentrations were slightly elevated in comparison. Over the last five years, the average annual mean concentrations of these metals at rural locations in the UK are in the following ranges:

As (0.3 - 0.9); ~3 at Styrrup.

Cd (0.1 - 0.4).

Cr (0.4 - 0.9); ~5 at Styrrup.

Cu (2 - 4); ~8 at Styrrup.

Ni (0.8 - 2); ~4 at Styrrup.

Pb (4 - 20); ~40 at Styrrup.

Zn (16 - 32);~60 at Styrrup.

Rainwater (m g l^-1)

As (0.1 - 0.3); ~0.5 at Styrrup.

Cd (0.1 - 0.2); ~0.3 at Styrrup.

Cr (0.1 - 0.5).

Cu (1 - 2); 6 - 9 at Chilton, Styrrup and Wraymires.

Ni (0.4 - 1); ~2 at Styrrup.

Pb (2 - 5).

Zn (10 - 21);~40 at Styrrup.
 
 

• During the 1990s seasonal differences were evident with increases in air concentrations and in the deposition of some elements from anthropogenic activities occurring in winter. These were accompanied by higher air concentrations and greater deposition of marine-aerosol derived Na and Cl. Little seasonal variation was apparent for the mainly soil-derived elements, Al, Ca, Ce, Fe and Sc. However at Wraymires, the location with a relatively high annual rainfall that has a relatively large seasonal variation, the majority of elements showed a stronger seasonal variation in bulk deposition with elevated depositions occurring in the winter quarters.

• The current annual average air concentrations of several heavy metals at urban sites in the UK exceed those at rural locations. 5-fold increases in concentrations of Cd, Cu, Fe and Ni were seen, while those for Cr and Pb were 4 times greater. Urban airborne levels of Mn, V and Zn were twice those at rural locations.

• The highest annual mean air concentration of Pb (38 ng m^-3) that was measured at rural locations between 1996 and 1998 was only 2% of the current EC Directive limit value of 2 m g m^-3. This concentration is 8% of the limit value for Pb (0.5 m g m^-3) proposed under the new EC Framework Directive and is some 15% of the value recommended by EPAQS as an air quality standard for the UK.

• Recent (1996-1998) annual mean air concentrations of Cd at rural locations in the UK were <1 ng m^-3 and were within the WHO range of guideline values, i.e. <1 - 5 ng m^-3.

• The main source of atmospheric Pb at rural locations is vehicle emissions, as indicated by the Pb/Br ratios and highly significant correlations between their measured concentrations in the airborne particulate material.

• Restrictions, imposed since 1972, on the use of Pb additives in petrol have substantially reduced the concentrations of airborne Pb in the UK rural environment. Comparison of the periods 1972-1979 and 1990-1998 shows that reductions in annual mean air concentrations and depositions of Pb of between 80% - 90 % have been recorded. Further, when the periods 1972-1979 and 1993-1997 were compared the reduction in the average UK total annual estimated emissions of Pb was 80%. This compared well with corresponding reductions in annual mean air concentrations (82%-84%) and depositions (71% - 91%). The effect of the restrictions was particularly noticeable after the most recent regulation in January 1986. The increase in the use of unleaded petrol since 1988 has contributed to further reductions in concentrations of Pb in air and rainwater at rural UK sites.

• Substantial decreases in air concentrations of the heavy metals, As, Cr, Cu, Ni, V and Zn have been measured since the 1970s. The reductions in their average annual mean concentrations between 1972-1979 and 1990-1998 were around 60% - 80%. Similar reductions in the deposition of Cu, Ni, V and Zn have also been recorded.

• The decreases in rural annual mean air concentrations of As, Cr, Ni, Se and V have closely followed corresponding decreases in their estimated emissions to the atmosphere in the UK since the 1970s. The reductions in the average annual mean air concentrations of these metals (particularly Ni, Se and V), between the periods 1972-1979 and 1993-1997, were very similar to the corresponding reduction in estimated emissions. This was also true for the deposition of Ni and V.

• Percentage reductions between 1972-1979 and 1993-1997 for Zn in both air and rainwater were very consistent (between 62% - 68%) at rural locations. However, the corresponding reduction in estimated emissions was only 24%. This reflects the higher uncertainty in the estimates for Zn emissions compared to other metals such as Ni and V. The proportion from fossil fuel combustion is also estimated to be much less for Zn, with larger contributions from other sources such as ferrous and non-ferrous metals production, waste incineration and road transport (tyre wear).

• The major source of airborne As, Cr, Cu, Se and in particular Ni and V, in the UK is the combustion of fossil fuels for public power generation and industrial energy requirements. Therefore, the beneficial impact that changes in methods of energy production and associated abatement technology (most of which was brought about by changes in Government policy), has had on rural air quality with respect to heavy metals in the UK, is illustrated by this long-term measurements programme.

• Considerable reductions in both air concentrations and in the deposition of Al, Fe, Mn and Sc (elements thought to be predominantly soil-derived origin) were apparent since the 1970s. The decreases in their average annual mean concentrations and deposition between 1972-1979 and 1990-1998 were around 60% - 80%, i.e. similar to those for heavy metals of predominantly anthropogenic origin. Consequently, reductions in fly-ash emissions from changes in fossil fuel combustion and abatement technology described above can probably explain the long-term decreases in airborne Al, Fe, Mn and Sc at rural locations in the UK.

• Air concentrations of Ca, K and Mg have also declined by a similar magnitude since the 1970s. Soil dust has long been assumed to be a major source of these elements, together with a contribution from marine-derived aerosols. However as for Al, Fe, Mn and Sc, the large long-term reductions in air concentrations of Ca, K and Mg are likely to be due to decreases in fly-ash emissions. The corresponding decreases in the annual mean deposition of Mg were much less marked and were more similar to those recorded for Na of marine-aerosol origin; (less reliable historical measurements of deposits of Ca and K prevented their investigation). This suggests that the main source of Mg in bulk deposition is marine aerosols. Differences in the sizes of particles (and therefore their source) that are sampled by the air sampler and bulk deposition collector can account for the apparent difference in the long-term changes for Mg in air and in bulk deposition.

• The percentage reduction in annual mean depositions of nitrate at the three rural locations were 35% - 39%, comparing the 1990s to the 1970s. The magnitude of these changes was similar to that for Na and indicate that the long-term deposition of nitrate has been relatively constant (compared to the heavy metals). Estimates of emissions of nitrogen oxides in the UK over the long-term have also remained relatively constant; the decreases in contributions from power generation and industrial sources have been offset by increased emissions from road transport.

• The long-term decreases in the wet deposition of non-marine sulphate that have been recorded at rural locations since the 1970s were comparable to the corresponding reductions in estimated emissions of SO₂. The percentage reductions in the annual mean wet deposition of non-marine sulphate between 1973-1979 and 1993-1997 were in the range 47% - 68% at the three rural sites, compared with a 48% reduction in emissions of SO₂.

5 Recommendations

The following recommendations are:

The Rural Trace Elements network is the longest running network, for the measurement of metals, in the UK. Since the early 1970s this monitoring network has produced a unique time-series dataset for concentrations of 35 elements in air particulate and some 25 elements in rainwater. The continued operation of this small network is recommended to allow further data to be collected. This would allow any further changes in concentrations of airborne trace and major elements at rural UK locations to be quantified. Therefore, any such changes may then be examined and related to changes in methods of energy production, energy consumption and industrial emissions which may be brought about by future changes in relevant UK Government or EC policy and legislation, e.g. the future EC daughter Directives that will set limit values for As, Cd, Hg and Ni concentrations in air.

In view of the future EC daughter Directive on Hg concentrations in air, consideration should be given to more detailed measurements of airborne mercury. Information is only available on particulate-bound mercury from the current network. AEA Technology made measurements of both particulate and total gaseous mercury (TGM) at Chilton over a period of a year in 1995/1996. This work could be re-started, either at Chilton or at another appropriate location(s), so that improved data on airborne Hg levels at UK rural locations can be obtained.

The dataset could be subjected to a more thorough and rigorous statistical examination. This may allow a more detailed investigation of the long-term trends, or a more accurate identification of the source or sources of potentially toxic heavy metals at rural locations in the UK.

An archive of samples, including both air filters and rainwater, has been assembled over the years. Retrospective analysis of those that have produced anomalous results is possible for confirmation. Also many historical measurements, particularly Cd in air particulate and other heavy metals, e.g. As in rainwater, were below the prevailing analytical limits of detection. Appropriate samples could be re-analysed using more appropriate and improved analytical techniques to further improve the quality of the dataset.

Expansion of the current suite of analytes to include the measurement of other metals should be considered. For example, concentrations of platinum, palladium and rhodium could be measured in both current and historical samples, to assess the impact of the introduction of catalytic converters to vehicle exhausts on levels of these metals on the UK rural environment.
 
 
 
 

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