The changing nature of the 206Pb/207Pb isotopic ratio of lead in rainwater, atmospheric particulates, pine needles and leaded petrol in Scotland, 1982-1998

The inductively coupled plasma-mass spectrometry (ICP-MS)-determined Pb-206/Pb-207 ratio of 145 samples of rainwater collected at 25 locations around Scotland during December 1997 and January 1998 and at three long-term monitoring stations in the northeast, central belt and southeast of the country from November 1997 to December 1998 averaged 1.144 +/- 0.017 (1s). This represents a significant increase from the mean value of 1.120 +/- 0.016 recorded for the long-term sites in 1989-1991, only partly attributable to a concomitant increase in the Pb-206/Pb-207 ratio of leaded petrol from 1.075 +/- 0.013 to 1.088 +/- 0.007. The rainwater Pb-206/Pb-207 data for the late 1990s also contrast markedly with the lower Pb-206/Pb-207 ratios found for pine needle and atmospheric particulate samples from Scotland (e.g. Glasgow: 1.085 +/- 0.012 in 1985-1986, 1.099 +/- 0.007 in 1991-1992), England and Western Europe in this study for the period 1982-1992 when emissions of lead to the atmosphere from petrol-engined vehicles in the UK were similar to 2-9 times higher. The observed change in the lead isotopic signature of rainwater predominantly reflects the impact of measures, such as the introduction and growing uptake of unleaded petrol, to reduce car exhaust emissions of lead to the atmosphere in the UK. Based on the rainwater data, 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%.