Seasonal and Annual Variations in Atmospheric Hg and Pb Isotopes in Xi'an, China

We present a 3-year time series of lead (Pb) and mercury (Hg) concentrations and isotope signatures in total suspended particulate (TSP) matter and as total gaseous Hg (TGM) in Xi'an, Northwestern China. Mean concentrations of TSP (299 ± 120 μg m^-3), Pb_TSP (0.33 ± 0.15 μg m^-3) and Hg_TSP (0.64 ± 0.54 ng m^-3), and TGM (5.7 ± 2.7 ng m^-3) were elevated. We find that atmospheric Pb levels in winter in Xi'an have decreased by 4.6% per year since 2003, yet remain elevated relative to air quality guidelines and therefore a major health concern. δ²⁰²Hg_TSP and Δ¹⁹⁹Hg_TSP averaged -0.80 ± 0.30‰ (1σ) and -0.02 ± 0.10‰ (1σ) and δ²⁰²Hg_TGM and Δ¹⁹⁹Hg_TGM averaged -0.08 ± 0.41‰ (1σ) and 0.00 ± 0.04‰ (1σ). Relative to raw coal from Shaanxi and surrounding provinces, δ²⁰²Hg_TSP is enriched in the light Hg isotopes, whereas δ²⁰²Hg_TGM is enriched in the heavy isotopes. TSP and TGM Δ¹⁹⁹Hg signatures are indistinguishable from raw coal, indicating little photochemical mass independent fractionation of atmospheric Hg in the near-field urban-industrial environment. δ²⁰²Hg_TGM correlates significantly with TGM levels (r² = 0.3, p < 0.01) and likely reflects binary mixing of local industrial TGM emissions with global background TGM.