Characteristics of the air‒sea exchange of gaseous mercury and deposition flux of atmospheric mercury at an island near the boundary of the Bohai Sea and Yellow Sea

Abstract The variations of dissolved gaseous mercury (DGM, mainly Hg0), air‒sea exchange of Hg0, and deposition of atmospheric mercury are very limited in nearshore regions of North China. Nearly two years measurements of DGM in surface seawater and gaseous elemental mercury (GEM or Hg0) in the atmosphere over the Changdao Island were conducted to elucidate the characteristic of air‒sea exchange of Hg0 based on meteorological parameters. Simultaneously, the wet deposition fluxes of atmospheric Hg were also calculated based on the precipitation amounts and concentrations of total mercury in precipitation (THgp). The results show that both DGM and Hg0 flux exhibit a distinct seasonal pattern with the order of summer (47.7 pg l−1 and 1.46 ng m−2 h−1), spring (24.6 pg l−1 and 0.82 ng m−2 h−1), fall (21.7 pg l−1 and 0.70 ng m−2 h−1), and winter (19.0 pg l−1 and 0.21 ng m−2 h−1). However, a striking feature in winter is that the study site and peripheral area sporadically act as a sink of atmospheric Hg0 as a result of elevated GEM concentrations and low water temperature and solar radiation. The seasonal pattern of saturation was in line with that of DGM. In addition, a distinct diurnal cycle of DGM was observed with maximum concentrations at midday and minimum concentrations in nighttime during the whole study period, especially in summer. These remarkable seasonal and diurnal patterns indicate that the elevated DGM concentration in summer and midday appears mainly a photochemically driven. The mean unfiltered (UTHg) and dissolved (DTHg) seawater THg concentrations were 1.43 ± 0.94 and 0.53 ± 0.19 ng l−1, respectively. On average, the mean DTHg/UTHg ratio was 0.45 ± 0.16, indicating that less than half of THg was concentrated in DTHg. There was no significant relationships between DGM and DTHg. Seasonal concentrations of THgp were 17.3, 7.4, 7.0, and 27.9 ng l−1, whereas seasonal wet deposition fluxes were 1.28, 1.31, 0.82, and 0.62 μg m−2 yr−1 for spring, summer, fall, and winter, respectively. Overall, the THgp concentrations varied from 3.9 to 60.1 ng l−1 with a mean value of 16.4 ng l−1, and the direct wet deposition flux was 4.03 μg m−2 yr−1, representing approximately 57.4% of the annual evasion flux of Hg0 (7.02 μg m−2 yr−1).