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.

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