Identification of secondary lead sources in the air of an urban environment

High precision lead isotopic data have been measured from particulates deposited on filters from two suburbs of Sydney (NSW, Australia) and gasoline from two main brands to evaluate the source of lead in air following the phasing out of leaded gasoline in Sydney in 1986 and a 25% reduction of lead in leaded gasoline in 1994. These measures have resulted in a 75% reduction of lead in Sydney air nowadays. Despite these positive figures, our results indicate that gasoline still accounts for more than 90% of the lead in the atmosphere of this city. However, when compared with an earlier investigation carried out in the period 1979-1981, our data show that a background source with a ²⁰⁶Pb/²⁰⁴Pb isotopic ratio higher than petrol has become detectable. We have used a best fitting mathematical procedure to identify the isotopic ratio and percentage contribution of this source to the air. The values that we have obtained are ~ 18.2 and ~ 7%, respectively, for the ²⁰⁶Pb/²⁰⁴Pb ratio and the percentage contribution. Several local background sources are considered, including natural lead from soils, wood burning, coal burning in power stations and aircraft propellants. We also draw attention to the similarity of the background isotopic composition in Sydney air and isotopic ratios measured in snows of Antarctica.