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An isotope binary mixing model was applied for high precision measurement of mercury isotope ratios in samples with low mercury concentrations by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Standard addition was used to evaluate the precision and accuracy of the isotope composition calculations resulting from the isotope binary mixing model. A high, steady 202 Hg signal of approximately 2.13 V was achieved, with the mercury concentration reaching 3 ng/mL. The isotopic composition of three standards (NIST SRM 1646a; NIST SRM 1575a; BCR 482) and natural samples were precisely determined. The standards and natural samples were diluted to low mercury concentrations (low to 0.90 ng/mL) and mixed with standard solutions (NIST SRM 3133) with high mercury concentrations (50 ng/mL); the isotopic compositions of low mercury concentration samples were calculated using an isotope binary mixing model after the isotopic compositions of the mixing solutions were measured. The results showed that the uncertainty of the calculated mercury isotopic compositions was in an acceptable range and the calculation isotope data were in good agreement with direct measurements. Our method allows the precise determination of mercury isotope composition in mercury solutions of concentrations (0.90 ng/mL) below the detection limit of the current system (3.00 ng/mL).

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Abstract. Mass independent fractionation (MIF) of even mercury (Hg) isotopes has long been observed in atmospheric related samples and is confirmed to be generated in the atmosphere, but its exact mechanism is covered up by the Hg sources and atmospheric transformations and stays unclear. Here, we present the first Hg isotope compositions of particulate bound mercury (PBM) in the Northwest Pacific and observe highly positive Δ200Hg values (up to 0.42 ‰). The MIF signatures are mainly controlled by photoreduction, gaseous elemental mercury (GEM) oxidation, and even-MIF dominated oxidation processes. Mercury in a small part of samples influenced by anthropogenic emissions is recognized by Hg concentrations and Δ199Hg signatures. The correlation between Δ200Hg and light conditions confirms that even-MIF is linked to photochemical reactions. The correlation between Δ200Hg and altitudes suggests that a max even-MIF signatures existed in the troposphere. We use Δ199Hg/Δ200Hg ratios and ternary isotopic mixing model to estimate the contributions of photoreduction, GEM oxidation and even-MIF dominated oxidation. Our results demonstrate that atmospheric transformations are far more important than Hg sources in shifting Hg isotope compositions of PBM samples, especially in the marine boundary layer of the open ocean, which is characterized by less anthropogenic influences and has implications for our understanding of the mechanism of even-MIF and subsequently Hg behaviors in the atmosphere.