THE PHASING OUT OF LEADED PETROL AND CHANGED IN THE 206Pb/207Pb ISOTOPIC RATIO OF ATMOSPHERIC LEAD IN SCOTLAND

THE PHASING OUT OF LEADED PETROL AND CHANGES IN THE ²⁰⁶Pb/²⁰⁷Pb ISOTOPIC RATIO OF ATMOSPHERIC LEAD IN SCOTLAND

John G. Farmer (J.G. Farmer@ed.ac.uk), Lorna J. Eades, Margaret C. Graham (Department of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, Scotland, UK) and Jeffrey R. Bacon (Soil Science Group, Macaulay Land Use Research Institute, Aberdeen AB15 8QH, Scotland, UK)

ABSTRACT

The ²⁰⁶Pb/²⁰⁷Pb ratio of 145 samples of rainwater collected at various locations around Scotland during 1998 averaged 1.144±0.017 (±1 s.d.). This represents a significant increase from the mean value of 1.120±0.016 recorded in 1989-1991, only partly attributable to a concomitant increase in the ²⁰⁶Pb/²⁰⁷Pb ratio of leaded petrol from 1.075±0.013 to 1.088±0.007 during a period when lead emissions from petrol-engined vehicles in the UK were estimated to have declined by about 65%. Source apportionment calculations suggest a general decline in the contribution of leaded petrol to atmospheric lead in Scotland from 53-61% in 1989-1991 to 32-45% in 1997-1998, with a corresponding decline in the urban environment from 84-86% to 48-58%.

INTRODUCTION

Lead ore deposits from Australia (²⁰⁶Pb/²⁰⁷Pb=1.04) have been used, along with Canadian lead (²⁰⁶Pb/²⁰⁷Pb=1.16), to produce alkyllead anti-knock additives (²⁰⁶Pb/²⁰⁷Pb~1.07) for petrol consumed in the UK (Delves, 1988). The significant difference between the ²⁰⁶Pb/²⁰⁷Pb ratio of the lead emitted by car exhausts and those of other sources of anthropogenic lead, e.g. coal burning (²⁰⁶Pb/²⁰⁷Pb ~1.18), has afforded an opportunity, at least in principle, for the quantitative assessment of the relative contributions from specific sources to the environmental lead burden (Sugden et al, 1993; Farmer et al, 1994, 2000). In the UK, measures introduced in 1986 to reduce the maximum permitted concentration of lead in petrol from 0.40 g l^-1 to 0.15 g l^-1 and to promote the use of unleaded petrol resulted in a reduction of emissions of lead from petrol-engined vehicles to the atmosphere from an estimated 6.5 x 10³ tonnes in 1985 to 0.8 x 10³ tonnes in 1997 (DETR, 1998).

This paper presents ²⁰⁶Pb/²⁰⁷Pb data for rainwater (n=47) collected at 25 locations around Scotland during December 1997 and January 1998 and at three long-term monitoring stations (n=98) in the northeast (Glensaugh), central belt (Hartwood) and southeast (Sourhope) of the country from November 1997 to December 1998. These data are interpreted in the context of long-term ²⁰⁶Pb/²⁰⁷Pb data for leaded petrol (n=33) from 1989-1998, for rainwater (n=31) from 1989-1991, and for atmospheric particulates collected at urban (n=25, Glasgow, Edinburgh) and rural (n=10) locations in Scotland during 1985-1992 (Farmer et al, 2000).

METHODS

Petrol samples were collected directly at pumps in Edinburgh between February 1989 and November 1998 and the lead extracted using the iodine monochloride/solvent extraction method prior to final solution preparation in 2% v/v (0.32 M) nitric acid for analysis. Rainwater samples were collected predominantly by open gauge (O) and filter gauge (F) methods and were concentrated by evaporation and acidified prior to analysis. Atmsopheric particulates collected on membrane filters of pore size 0.45 mm attached to an electric pump or by hi-volume air filtration were leached with 1.6 M nitric acid and the resultant extracts diluted in 2% v/v nitric acid for analysis (Sugden et al, 1993; Farmer et al, 2000).

Samples were analysed by ICP-MS using VG PQ 1, 2 or 3 instruments. The National Institute of Standards and Technology (NIST) common lead isotopic reference standard SRM 981 (²⁰⁶Pb/²⁰⁷Pb = 1.0933) was used for calibration and mass bias correction. Mean internal analytical precision (±1 s.d.) on the measured ²⁰⁶Pb/²⁰⁷Pb ratios was ±0.24%, ±0.34% and ±0.42%, for the petrol, rainwater and atmospheric particulate samples, respectively. Mean external analytical precision on ²⁰⁶Pb/²⁰⁷Pb, as determined by repeated analysis (n=55) of the IAEA SL-1 Lake Sediment reference material over several years, was ±0.36% (Sugden et al, 1993; Farmer et al, 2000).

RESULTS AND DISCUSSION

The ²⁰⁶Pb/²⁰⁷Pb results for rainwater are summarised in Table 1 and Fig. 1. Almost 79% of the results (n=47) for the 25 locations in Scotland in December 1997 and January 1998 were between 1.120 and 1.159, with an overall mean (±1 s.d.) of 1.144±0.021. For the long-term sites during November 1997-December 1998, ²⁰⁶Pb/²⁰⁷Pb ratios fell in the range 1.130-1.159 for 90%, 57% and 92% of samples from Glensaugh (N.E.), Hartwood (central belt) and Sourhope (S.E.), respectively. For the total of 98 samples collected at these long-term sites, almost 88% of the ²⁰⁶Pb/²⁰⁷Pb ratios were between 1.120 and 1.159, with a mean value of 1.144±0.014. The complete set of rainwater samples (n=145) collected at all sites in Scotland from November 1997 to December 1998 had a mean ²⁰⁶Pb/²⁰⁷Pb ratio of 1.144±0.017 (Table 1, Fig. 2a), with 85% of values lying between 1.120 and 1.159. For the three long-term sites, the corresponding mean ²⁰⁶Pb/²⁰⁷Pb ratio for 1989-1991 was 1.120±0.016 (Bacon and Bain, 1995) (Table 1, Fig. 2a).

Table 1: Comparison of ²⁰⁶Pb/²⁰⁷Pb ratios in rainwater from Scotland 1989-1991 to 1997-1998.

Location

Date of

Collection

²⁰⁶Pb/²⁰⁷Pb

(±1 s.d.)

Range Mean

Glensaugh

1989-1991

1997-1998 (O)

1997-1998 (F)

1.101-1.152

1.113-1.163

1.135-1.161

1.119±0.013

1.140±0.010

1.144±0.008

Hartwood

1989-1991

1997-1998 (O)

1997-1998 (F)

1.104-1.153

1.123-1.207

1.118-1.149

1.129±0.014

1.164±0.024

1.141±0.010

Sourhope

1989-1991

1997-1998 (O)

1997-1998 (F)

1.081-1.132

1.112-1.153

1.129-1.155

1.110±0.018

1.140±0.008

1.147±0.008

Scotland (25 locns.)

Scotland (all)

Dec.1997-Jan.1998

1997-1998

145

1.112-1.218

1.144±0.021

1.144±0.017

Figure 1: Histogram of ²⁰⁶Pb/²⁰⁷Pb ratios for rainwater samples collected at the 25 sampling sites in Scotland during December 1997 and January 1998 and at the three long-term rainwater collection sites, Glensaugh, Hartwood and Sourhope, from November 1997 to December 1998.

For atmospheric particulates, a mean ²⁰⁶Pb/²⁰⁷Pb ratio of 1.118±0.006 was found for the predominantly rural locations in 1985 (Fig. 2a). In contrast, urban atmospheric particulates collected in Glasgow in 1985 had a lower mean ²⁰⁶Pb/²⁰⁷Pb ratio of 1.085±0.012 (n=4), a value similar to the 1.099±0.007 (n=7) found for the same city in 1991-1992 and the 1.092±0.011 (n=14) for urban Edinburgh in 1990-1991, yielding a mean value of 1.094±0.010 for 1990-1992 (Fig. 2a).

The individual ²⁰⁶Pb/²⁰⁷Pb results (n=33) for petrol collected in the Edinburgh area from February 1989 to November 1998 ranged from 1.056±0.001 (February 1989) to 1.098±0.0014 (November 1997) with an overall mean of 1.076±0.011. The mean ²⁰⁶Pb/²⁰⁷Pb values (n=9) for individual periods of collection from 1989 to 1998 displayed an average increase of 0.2% (~0.002) in ²⁰⁶Pb/²⁰⁷Pb per year over the decade (Fig. 2b).

Using the averages of 1.120±0.016 and 1.144±0.017 for ²⁰⁶Pb/²⁰⁷Pb in rainwater in 1989-1991 and 1997-1998, respectively, there is a statistically significant (p<0.01, t-test) increase in the atmospheric ²⁰⁶Pb/²⁰⁷Pb ratio of 0.024±0.023. Over the same time period, although the ²⁰⁶Pb/²⁰⁷Pb ratio for petrol increased (p<0.05, t-test) by 0.013±0.015 from 1.075±0.013 (1989-1991) to 1.088±0.007 (late 1997-1998), there was a major reduction in the emission of lead to the atmosphere from car exhausts from 2.6 x 10³ tonnes in 1989 to 0.8 x 10³ tonnes in 1997 (Fig. 2c). This suggests that the growing use of unleaded petrol, amounting to 71.9% of sales by 1997 (DETR, 1998) has contributed to the observed increase in the ²⁰⁶Pb/²⁰⁷Pb ratio for rainwater in Scotland.

Source apportionment calculations using an equation (Farmer et al, 1994) such as

R_A = R_P.X_PA + R_O.X_OA

where R_A, R_P and R_O are the ²⁰⁶Pb/²⁰⁷Pb ratios for the atmosphere, leaded petrol and other sources, respectively, and X_PA and X_OA (=1-X_PA) are the fractional contributions of leaded petrol and other sources of lead, respectively, to atmospheric lead, can be used to estimate the influence of car-exhaust emissions of lead upon the atmospheric lead burden. This assumes that leaded petrol is the predominant source of lead emissions with the characteristic isotopic signature influenced by ²⁰⁶Pb-depleted Australian lead and also relies upon estimates of the ²⁰⁶Pb/²⁰⁷Pb ratios for other sources. Thus, using 1.088 for R_P, 1.144 for R_A, and R_O values of 1.17 (19^th Century “industrial”), 1.18 (coal) and 1.19 (“geochemical”) (Farmer et al, 1999, 2000), values of X_PA are calculated to be 0.32, 0.39 and 0.45, respectively, for 1997-1998. This contrasts with corresponding values of X_PA of 0.53, 0.57 and 0.61 when the 1989-1991 values of 1.075 for R_P and 1.120 for R_A are used. When the urban particulate ²⁰⁶Pb/²⁰⁷Pb ratio of 1.094 is used for R_A in 1990-1992, along with the mean ²⁰⁶Pb/²⁰⁷Pb ratio of 1.079 for petrol in 1990-1991 as R_P, the corresponding values are 0.84, 0.85 and 0.86. If the mean rainwater ²⁰⁶Pb/²⁰⁷Pb ratio of 1.131 for seven locations in the central belt of Scotland is used for R_A, along with 1.088 for R_P in December 1997/January 1998, the calculated values for X_PA are 0.48, 0.53 and 0.58. Thus, on this basis, the contribution of car-exhaust emissions of lead from leaded petrol to atmospheric lead in Scotland from the late 1980s/early 1990s until 1998 has declined from 53-61% to 32-45% (using rainwater data from the long-term sites and across Scotland), and, with respect to the urban environment, from 84-86% (using urban atmospheric particulate data) to perhaps 48-58% (using rainwater data from the central belt).

In view of the wide ranging use made of anthropogenic lead isotopic ratios as tracers and source indicators in environmental and earth sciences, it will be important to continue monitoring atmospheric ²⁰⁶Pb/²⁰⁷Pb ratios at this time of change and to identify contributions of lead from sources such as waste incineration and the recycling and the refining of battery lead (Nriagu, 1998).

REFERENCES

Bacon JR, Bain DC (1995), Environ. Geochem. Health 17: 39-49.

Delves HT (1998), Chem. Brit. 24: 1009-1012.

DETR (1998), Digest of Environmental Statistics No. 20. London, The Stationery Office.

Farmer JG, Sugden CL, MacKenzie AB, Moody GH, Fulton M (1994), Environ. Technol. 15: 593-599.

Farmer JG, Eades LJ, Graham MC (1999), Environ. Geochem. Health 21: 257-272.

Farmer JG, Eades LJ, Graham MC, Bacon JR (2000), J. Environ. Monit. 2: 49-57.

Nriagu JO (1998), Science 281: 1622-1623.

Sugden CL, Farmer JG, MacKenzie AB (1993), Environ. Geochem. Health 15: 59-65.

Figure 2: (a) Mean ²⁰⁶Pb/²⁰⁷Pb (±1 s.d.) data for rural atmospheric particulates (o), urban atmospheric particulates (ÿ) and rainwater (à) in Scotland; (b) mean ²⁰⁶Pb/²⁰⁷Pb (±1 s.d.) data for petrol in Scotland; (c) emissions of lead to the atmosphere from petrol-engined vehicles in the UK (DETR, 1998).