Elemental mercury in coastal seawater of Yellow Sea, China: Temporal variation and air–sea exchange

Dissolved gaseous mercury (DGM, largely Hg(0)) in coastal seawater and gaseous elemental mercury (GEM or Hg(0)) in the atmosphere were simultaneously measured on the coast of the Yellow Sea, China in four different seasons (2008–09). Mean concentrations (±SD) of DGM and GEM over the study period were 34.0 ± 26.1 pg L−1 and 2.55 ± 0.98 ng m−3, respectively. DGM concentrations and the degree of DGM saturation (Sa) exhibited distinct seasonal variation with the order of summer (DGM: 69.0 ± 23.3 pg L−1, Sa: 11.00 ± 5.92) > fall (27.0 ± 16.4 pg L−1, 3.50 ± 2.60) > spring (23.0 ± 8.7 pg L−1, 2.00 ± 0.98) > winter (16.0 ± 6.0 pg L−1, 0.96 ± 0.39). Under typical meteorological condition with low wind speed and intensive solar radiation in warm seasons, DGM usually exhibited the clear diurnal variation with elevated levels around noon and low levels in morning and afternoon. The diurnal and seasonal variation of DGM indicated the importance of photochemical DGM formation in the seawater. A consistent low DGM levels in high wind speed condition suggested that the biological activity probably influenced the DGM formation. There was no significant correlation between DGM and total mercury (THg), reactive mercury (RHg), dissolved organic carbon (DOC) in the seawater, indicating that THg/RHg and DOC might be not the controlling factors for the DGM formation in our study region. Based on the data of DGM and GEM and a two-layer gas exchange model, Hg(0) fluxes (in the unit of ng m−2 h−1) at air–sea interface were 0.51 ± 1.29 over the entire study period with 0.89 ± 1.84 in fall, 0.88 ± 1.38 in summer, 0.32 ± 0.71 in spring, and −0.06 ± 0.64, a slightly net Hg(0) deposition rate, in winter, respectively.