Executive Summary
 
 

Information on the long-term trends of trace and major elements is required to assess the impact of changes in emissions regulation policies on air quality in the United Kingdom. AEA Technology’s National Environmental Technology Centre has continued to operate a long-term sampling and measurements programme at rural locations in England. These measurements commenced in 1972. Atmospheric particulate material and rainwater (bulk deposition) have been sampled continuously at three locations, i.e. Chilton (Oxfordshire), Styrrup (Nottinghamshire) and Wraymires (Cumbria).

The concentrations of 35 elements have been measured in atmospheric particulate material. In rainwater, the concentrations of 25 elements, nitrate and sulphate have been measured (in the soluble fraction only).

This report provides a summary of data obtained during the period 1996-1998. These data have been compared with historical data to examine long-term changes in concentrations of trace elements in air particulate and in atmospheric deposition.

Concentrations of potentially toxic metals, e.g. As, Cd, Cr, Cu, Ni, Pb and Zn, in air and rainwater are comparable with measurements at other rural locations in the UK. From these measurements, current ‘rural UK background’ concentration ranges of these metals in air and rainwater have been established. These have been compared with measurements of airborne metals at urban locations in the UK. Up to 5-fold increases were found at the urban sites.

Seasonal differences were evident. Increases in concentrations, of many anthropogenically-derived elements, in both air particulate and bulk deposition were found in winter. The differences were less apparent for the predominantly soil-derived elements, except in bulk deposition at Wraymires. At this location, the large seasonal variations at this relatively high rainfall location have influenced the deposition of elements, with increased deposition of the majority of the 25 elements occurring during the winter quarters.

Both atmospheric concentrations and depositions of Pb at rural sites were well correlated with estimated UK emissions to the atmosphere between 1972 and 1997. Restrictions on the use of Pb additives in petrol have substantially reduced the concentrations of airborne Pb in the rural environment. Reductions in annual mean air concentrations and annual mean depositions of 80% - 90% have occurred since the 1970s. In more recent years (since 1988), the increased usage of unleaded petrol has also contributed to these reductions.

Road transport is still the largest source of Pb emissions in the UK and is thought to be the major source of atmospheric Pb at rural locations. During 1996-1998, the maximum annual mean air concentration was 38 ng m^-3, which represents only ~2% of the existing EC Directive limit value of 2 m g m^-3, or 8% of the proposed limit value under the new EC Framework Daughter Directive. This concentration is ~15% of the annual mean value recommended by EPAQS as an air quality standard for the UK.

Decreases in atmospheric concentrations of As, Cr, Ni, V and Zn have been measured between the periods 1972-1979 and 1993-1997. The major source of their emissions is the combustion of fossil fuels. Percentage reductions in annual mean air concentrations of these metals of ~60% to ~80% were recorded at Chilton, Styrrup and Wraymires and were comparable to corresponding reductions in estimated atmospheric emissions in the UK, particularly for Ni and V, however Zn was an exception. The changes in annual mean air concentrations were well correlated (although less so for Zn) with the changes in estimated reductions in annual total UK emissions to the atmosphere between 1972 and 1997. It is likely that estimates for Zn emissions are more uncertain than for other metals such as Ni and V. For Zn the proportion from fossil fuel combustion is thought to be much less, with larger contributions from other sources such as ferrous and non-ferrous metals production processes, road transport (tyre wear) and waste incineration.

Considerable reductions in both annual mean air concentrations and depositions of the predominantly soil-derived or ‘crustal’ elements (Al, Fe, Mn and Sc) have been measured at all three rural sites since the 1970s. These decreases were of a similar magnitude to those recorded for the anthropogenically-derived heavy metals. Reductions in fly-ash emissions, brought about by long-term changes in fossil fuel combustion for power generation, industrial requirements and domestic and industrial space heating, together with improved abatement technology in power generation and industry, are the likely explanation. Average annual mean air concentrations of Ca, K and Mg have declined similarly, by ~60% - 80%. Although these elements are also thought to be mainly soil-derived (and have a marine-aerosol source also), such large decreases are also attributed to decreases in emissions of fly ash.

The measurement of sulphate wet deposition at Chilton, Styrrup and Wraymires provides a useful additional dataset to the Acid Deposition monitoring networks. At all three sampling locations the annual total wet deposition of non-marine sulphate decreased between 1970s and the 1990s. The percentage reductions in the annual mean deposition of non-marine sulphate recorded at these sites between 1973-1979 and 1993-1997, i.e. 47% - 68% were in agreement with the corresponding estimated reduction in annual emissions of SO₂ (48%). Operational protocols are not as appropriate for sulphate measurement as those used in the Acid Deposition monitoring network; the current network was primarily established for the measurement of trace and major elements. Nevertheless, these data are further evidence that wet S deposition in the UK has declined along with UK emissions over the period 1973 to 1997, at least in England.