Mercury isotopic compositions in fine particles and offshore surface seawater in a coastal area of East China: Implication for Hg sources and atmospheric transformations

Abstract. Isotopic compositions of Hg in atmospheric particles (HgPM) are probably the mixed results of emission sources and atmospheric processes. Here, we present Hg isotopic compositions in daily fine particles (PM2.5) collected from an industrial site (CX) and a nearby mountain site (DMS) in a coastal area of East China, as well as in surface seawater close to the industrial area, to reveal the roles of anthropogenic emission sources and atmospheric transformations in varying Hg isotopes. The PM2.5 samples displayed significant spatial difference in δ202Hg. For the CX, the negative δ202Hg values were similar to those of source materials and HgPM contents were well correlated with other chemical tracers, indicating the dominant contributions of local industrial activities to HgPM2.5. Whereas the observed positive δ202Hg at the DMS was likely associated with regional emissions and extended photo-reduction during transport. Δ199Hg values in PM2.5 from the CX and DMS were comparable positive. The unity slope of Δ199Hg versus Δ201Hg over all data suggests that the odd-MIF of HgPM2.5 was primarily induced by photo-reduction in aerosols. The positive Δ200Hg values with minor spatial difference were probably associated with photo-oxidation of Hg0 which is generally enhanced in the coastal environment. Total Hg in offshore surface seawater was characterized by negative δ202Hg and near-zero Δ199Hg and Δ200Hg, which are indistinguishable from Hg isotopes of source materials. Overall, industrial PM2.5 had comparable δ202Hg values but more positive Δ199Hg and Δ200Hg as compared to surface seawater. The results indicate that atmospheric transformations would induce significant fractionation of Hg isotopes, which obscures Hg isotopes of anthropogenic emissions.