Lead concentrations in fine particulate matter after the phasing out of leaded gasoline in Xi'an, China

Daily concentrations of lead (Pb) were determined for PM_2.5 samples collected from an urban location in Xi'an, China from 2007 to 2009 to assess the effects of the phasing out of leaded gasoline in 2000. The Pb concentrations (annual average: 0.306 μg m^-3, range: below detection limit to 2.631 μg m^-3) have declined after the phasing out of leaded gasoline, but the concentrations were still higher than those reported in many other cities. Seasonal variations of Pb were significant, with high concentrations in winter, presumably due to the burning of coal, and low concentrations in summer, due to a deep mixed layer and scavenging of aerosols by precipitation. Correlation analyses and enrichment factor calculations both indicated that anthropogenic sources had a large influence on atmospheric Pb. The lead isotope ratios were low in winter (the average ²⁰⁷Pb/²⁰⁶Pb ratio was 0.843 ± 0.032; ²⁰⁸Pb/²⁰⁶Pb was 1.908 ± 0.058) and high in summer (²⁰⁷Pb/²⁰⁶Pb was 0.860 ± 0.032; ²⁰⁸Pb/²⁰⁶Pb was 2.039 ± 0.057), suggesting that coal combustion was the major Pb source in winter and vehicular emission was the major Pb source in summer. Positive Matrix Factorization receptor model indicated that there were five major sources for Pb in PM_2.5. Coal combustion was the major contributor, accounting for 39.0% PM_2.5 mass, followed by vehicular emissions (30.4%). Other contributors included 17.8% from industrial emissions, 11.6% from biomass burning, and 1.2% from fugitive dust.