Abstract. The characteristics of the reactive gaseous mercury (RGM) and particulate mercury (HgP) in the marine boundary layer (MBL) is poorly understood due in part to sparse data from sea and ocean. Gaseous elemental Hg (GEM), RGM and size-fractioned HgP in marine atmosphere, and dissolved gaseous Hg (DGM) in surface seawater were determined in the South China Sea (SCS) during an oceanographic expedition (3–28 September 2015). The mean concentrations of GEM, RGM and HgP2.5 were 1.52 ± 0.32 ng m−3, 6.1 ± 5.8 pg m−3 and 3.2 ± 1.8 pg m−3, respectively. Low GEM level indicated that the SCS suffered less influence from human activities, which could be due to the majority of air masses coming from the open oceans as modeled by backward trajectories. Atmospheric reactive Hg (RGM + HgP2.5) represented less than 1 % of total atmospheric Hg, indicating that atmospheric Hg existed mainly as GEM in the MBL. The GEM and RGM concentrations in the northern SCS were significantly higher than those in the western SCS, and the HgP2.5 and HgP10 levels in the Pearl River Estuary were significantly higher than those in the open waters of the SCS, indicating that the Pearl River Estuary was polluted to some extent. The size distribution of HgP in PM10 was observed to be bi-modal with a higher peak (5.8–9.0 μm) and a lower peak (0.7–1.1 μm), but the coarse modal was the dominant size, especially in the open SCS. There was no significant diurnal variation of GEM and HgP2.5, but we found the RGM concentrations were significantly higher in daytime than in nighttime mainly due to the influence of solar radiation. In the northern SCS, the DGM concentrations in nearshore areas were higher than those in the open sea, but this pattern was not significant in the western SCS. The sea–air exchange fluxes of Hg0 in the SCS varied from 0.40 to 12.71 ng m−2 h−1 with a mean value of 4.99 ± 3.32 ng m−2 h−1. The annual emission flux of Hg0 from the SCS to the atmosphere was estimated to be 159.6 tons yr−1, accounting for about 5.54 % of the global Hg0 oceanic evasion though the SCS only represents 1.0 % of the global ocean area. Additionally, the annual dry deposition flux of atmospheric reactive Hg represented more than 18 % of the annual evasion flux of Hg0, and therefore the dry deposition of atmospheric reactive Hg was an important pathway for the input of atmospheric Hg to the SCS.