Interactive comment on “ Distribution and air-sea exchange of mercury ( Hg ) in the Yellow Sea ”

The Yellow Sea, surrounded by East China and the Korea Peninsula, is a potentially important receptor for anthropogenic mercury (Hg) emissions from East Asia. However, there is little documentation about the distribution and cycle of Hg in this marine system. During the cruise covering the Yellow Sea in July 2010, gaseous elemental mercury (GEM or Hg(0)) in the atmosphere, total Hg (THg), reactive Hg (RHg) and dissolved gaseous mercury (DGM, largely Hg(0)) in the waters were measured aboard the R/V Kexue III. The mean (+/- SD) concentration of GEM over the entire cruise was 2.61 +/- 0.50 ng m(-3) (range: 1.68 to 4.34 ng m(-3)), which were generally higher than other open oceans. The spatial distribution of GEM generally reflected a clear gradient with high levels near the coast of East China and low levels in open waters, suggesting the significant atmospheric Hg outflow from East China. The mean concentration of THg in the surface waters was 1.69 +/- 0.35 ng l(-1) and the RHg accounted for a considerable fraction of THg (RHg: 1.08 +/- 0.28 ng l(-1), %RHg/THg = 63.9%). The mean concentration of DGM in the surface waters was 63.9 +/- 13.7 pg l(-1) and always suggested the supersaturation of Hg(0) in the surface waters with respect to Hg(0) in the atmosphere (the degree of saturation: 7.8 +/- 2.3 with a range of 3.6-14.0). The mean Hg(0) flux at the air-sea interface was estimated to be 18.3 +/- 11.8 ng m(-2) h(-1) based on a two-layer exchange model. The high wind speed and DGM levels induced the extremely high Hg(0) emission rates. Measurements at three stations showed no clear vertical patterns of DGM, RHg and THg in the water column. Overall, the elevated Hg levels in the Yellow Sea compared with other open oceans suggested that the human activity has influenced the oceanic Hg cycle downwind of East Asia.


Introduction
Mercury (Hg) is a persistent pollutant of global concern known to be transported long distances in the atmosphere into remote ecosystems (Schroeder and Munthe, 1998).Hg subsequently transfers to methylmercury (MMHg) and accumulates into the food chain (Morel et al., 1998;USEPA, 1997;Fitzgerald et al., 2007).Consumptions of fish with high MMHg levels can lead to adverse health effects in humans and wildlife (USEPA, 1997).Therefore, there is an increasing interest to understand the global biogeochemistry of mercury.
Numerous studies suggested that the Hg cycle in the ocean is one of the key processes in its global biogeochemical cycle (Mason and Sheu, 2002;Seigneur et al., 2001;Strode et al., 2007Strode et al., , 2010;;Selin et al., 2007;Hedgecock et al., 2006;Soerensen et al., 2010b).It has been well-established that the atmospheric Hg deposition is the principal source of Hg to open ocean (Mason and Sheu, 2002;Lindberg et al., 2007).In earlier studies, the wet deposition was considered the only primary pathway (Mason et al., 1994).Recently, due to the significant instrumental improvement for atmospheric Hg speciation measurements (Landis et al., 2002), increasing field studies have shown that gaseous elemental mercury (GEM, or Hg(0)) in the marine boundary layer (MBL) can be rapidly oxidized to form reactive gaseous mercury (RGM) in situ (e.g., Hedgecock et al., 2003;Laurier et al., 2003;Laurier and Mason, 2007;Chand et al., 2008;Sprovieri et al., 2003Sprovieri et al., , 2010a;;Soerensen et al., 2010a).Because of the high water solubility and surface reactivity, the dry deposition of RGM (direct or uptake by sea-salt aerosol) represents an important fraction of Hg deposition flux to the ocean (Mason and Sheu, 2002;Holmes et al., 2009).
Divalent Hg (Hg(II)) is the dominant Hg species of the atmospheric deposition to the ocean (Lindberg et al., 2007).Once deposited, Hg(II) in the water can convert to DGM (largely Hg(0)) mediated by the photochemistry (e.g., Amyot et al., 1997;Lanzillotta et al., 2002) and the microbe activity (e.g., Rolfhus and Fitzgerald, 2004;Fantozzi et al., 2009), leading to the surface water supersaturation of Hg(0) with respect to the atmospheric Hg(0) (Kim and Fitzgerald, 1986;Amyot et al., 1997;Andersson et al., 2007).Supersaturation of Hg(0) in the surface water implies a net emission of Hg(0) from the sea surface to the atmosphere.The oceanic transformation reaction and subsequent emission to the atmosphere of Hg(0) is a critical fraction of the global Hg cycle (Strode et al., 2010).Mason and Sheu (2002) estimated that the Hg(0) re-emission from the sea surface was approximately equivalent to the global anthropogenic emission source.Using an updated global atmospheric Hg model coupled with a mixed layer slab ocean, Soerensen et al. (2010b) indicated that the Hg(0) re-emission constitutes 80% of the previously atmospheric Hg deposition flux.
The Yellow Sea is located in the downwind region of East Asia, which is the strongest anthropogenic Hg emission source in the world (Pacyna et al., 2010).High anthropogenic Hg emissions with a considerable fraction of RGM and PHg (Schroeder and Munthe, 1998) can induce the elevated atmospheric Hg deposition, probably resulting in a corresponding increase in the oceanic Hg pool.Other pollutions and nutrients input to the Yellow Sea via the river/sewage discharge and atmospheric deposition have influenced the nutrient structure and biogeochemistry of this marine ecosystem (Liu et al., 2003).This change may potentially affect the behavior and fate of Hg in the Yellow Sea.However, there is little documentation of Hg distribution and cycle in this marine environment due to the challenge of ultra-trace Hg analysis.Here, based on the measurements of various Hg species in the MBL and the waters of the Yellow Sea during the cruise in July 2010, we investigated the distribution of Hg (GEM in the atmosphere, total Hg, reactive Hg and DGM in the water) in this marine environment and further estimated the Hg(0) flux at the air-sea interface.

Site region
The Yellow Sea is a semi-closed marginal sea and a representative shallow continental shelf sea with an average depth of about 40 m.It is the link for material and energy exchange between the Bohai Sea (inner sea) and the East China Sea.The hydrological characteristics of this region are impacted by the Bohai Sea waters, the China and Korea costal waters and the East China Sea open waters.Several rivers heavily impacted by human activities (including Huaihe, Yalu, Han, Taedong and Geum Rivers) carrying large land-sourced materials enter the Yellow Sea.This marine region is an important fishing area in East Asia because the special current field induces the high nutrient concentrations and subsequently results in high primary production.
This open cruise was organized by the Institute of Oceanography Chinese Academy of Sciences (IOCAS) for the duration of 10 days (9-18 July 2010) aboard the R/V Kexue III, which circumnavigated the western and central Yellow Sea, originating from Qingdao, through Dalian and Shanghai, and then returned to Qingdao (see Fig. 1).

Atmospheric GEM measurements
Atmospheric GEM was continually measured using RA-915 + Hg analyzer (Lumex, St. Petersburg, Russia), which based on differential Zeeman Atomic Absorption Spectrometry with High-Frequency Modulation of light polarization (ZAAS-HFM) and a multi-path analytical cell (for more detail on this instrument, see Ci et al., 2011b;Sholupov et al., 2004).This analyzer has been successfully used in various types of GEM measurements (e.g., Southworth et al., 2004;Kim et al., 2006;Wang et al., 2007, Ci et al., 2011a) and showed good agreement with the traditional gold trap/CVAAS system (e.g., WA-4 model, Nippon, Japan, Kim et al., 2006).In this cruise, the analyzer was installed inside the ship laboratory and the ambient air was delivered via a Teflon tube.The air inlet was installed on the upper deck about 8 m a.s.l. to avoid the contamination from ship emissions.Based on three times standard deviation of the system blank, this analyzer has a detection limit of 0.30 ng m −3 with sampling time of 1 min at a flow rate of 20 l min −1 (Ci et al., 2011b).GEM data with 1-min interval were modified to hourly mean for data analysis.Unfortunately, some air samplings contaminated by the exhaust plume of the R/V were found at three occupied stations (GEM concentrations: ∼20-70 ng m −3 ).Total duration time of the pollution episode was about 2 h and the GEM data in these periods was removed.

Back-trajectory model
For investigating the influence of air mass movements on the GEM levels, we calculated 72-h back-trajectories of atmospheric transport using the NOAA-HYSPLIT model (Draxler and Rolph, 2003).National Centers for Environmental Prediction's (NCEP) Global Data Assimilation System (GDAS) meteorological dataset was used as the model input (http://www.emc.ncep.noaa.gov/gmb/gdas/).The start height of back-trajectory was 500 m a.s.l., generally representing the typical height of the planetary boundary layer.

Water sampling
This cruise included a total of 47 stations and surface water samples (∼0.5 m depth) for Hg analysis were collected at 40 surface stations, with additional vertical water samples (50-80 m depth) were obtained at three selected stations of these 40 stations (Fig. 1).Ultra-trace Hg clean techniques (USEPA, 2002) were applied during the entire cruise.All containers and Teflon tubes were cleaned in the laboratory by soaking in HNO 3 (20%, v/v) and trace-metal grade HCl (2%, v/v) and rinsed three times with Milli-Q water (>18.2M cm −1 ).Powder-free gloves were worn throughout the procedure.The surface water samples were manually collected using a 1 l rigorously acid-washed borosilicate glass bottle.The sampling bottles were rinsed with the water three times prior to collection.There was no headspace within the sampling bottle to avoid loss of DGM by evasion to the headspace.Special attention was paid to the orientation of the R/V and wind direction to avoid contamination.

Seoul
Three stations were selected in the deep region of the Yellow Sea (water depth of 50-80 m) for studying the vertical distribution of Hg.A water sampling system consisting of 12-5 l Teflon lined Go-Flo bottles was used to collect the vertical samples.Samples for Hg analysis were immediately decanted from the Go-Flo bottle into the acid-washed glass bottle.

DGM collection and analysis
A detailed DGM collection and analysis procedure is given by Ci et al. (2011a), following Mason et al. (2001) and Lamborg et al. (2008).Briefly, the water samples were immediately taken to the laboratory aboard the R/V, slowly decanted into a 1.5 l borosilicate glass bubbler and purged with the Hg-free ambient air for 30 min at a flow rate of 0.5 l min −1 .The DGM released from water was captured onto the gold trap and sealed with Teflon endcaps and then shipped to the laboratory for analysis.To prevent photolytic reactions, the bubbler covered by the aluminum foil.
The Hg collected in gold trap was quantified using a Cold Vapor Atomic Fluorescence Spectrophotometer (CVAFS, Model III, Brooks Rand, USA) based on the two-stage gold amalgamation method (Gill and Fitzgerald, 1987).The field blanks (n = 8) were obtained by replacing the gold trap after the initial purge and the sample was purged for an additional 30 min.The concentrations of DGM in field blanks were 5.0 ± 2.0 pg l −1 , accounting for 5-10% of the raw DGM in water samples.The method detection limit (three times standard deviations of system blanks) for 1.0 l water sample was 6.0 pg l −1 .All reported DGM concentrations were blank corrected.
After the DGM collections, the subsamples were transferred to the acid-clean bottles, acidified using the tracemetal grade HCl, placed into the double bags and shipped to the laboratory on shore for analysis of total Hg (THg) and reactive Hg (RHg).

THg and RHg determinations
THg and RHg in unfiltered acidified water samples were determined using USEPA Method 1631 for ultra-trace Hg analysis in natural water (USEPA, 2002), following Gill and Fitzgerald (1987) and Balcom et al. (2008).For THg determination, 100 ml samples were pre-oxidized with BrCl, NH 2 OH•HCl pre-reduction, SnCl 2 reduction, Hg-free N 2 purging in a glass bubbler, gold trap pre-concentration, thermal desorption and CVAFS quantification.For RHg determination, 100 ml samples were purged onto the gold trap using Hg-free N 2 after direct reduction with SnCl 2 without BrCl pre-oxidation.However, at present the RHg determination in water has not been standardized, as to compare the RHg concentrations obtained by different studies, it is important to take account of the storage and preservation methods adopted by researchers (Parker and Bloom, 2005).Based on the three times standard deviation of the system blank, the detection limits for the THg and RHg were ca.0.10 ng l −1 .

Model for estimating air-sea Hg(0) flux
The calculation of air-sea Hg(0) flux and the discussion of its uncertainties are given in Ci et al. (2011a) in detail.Briefly, Hg(0) fluxes were estimated using a two-layer gas exchange model developed by Liss and Slater (1974).
where F is the Hg(0) flux (ng m −2 h −1 ) and K w is the water mass transfer coefficient (m h −1 ).K w is used as the overall mass transfer coefficient because the resistance to Hg(0) exchange at the air-water interface mainly lies in the water film (> 99%, Poissant et al., 2000).C w and C a are Hg(0) concentrations in the surface water (DGM, pg l −1 ) and in the atmosphere (GEM, ng m −3 ), respectively.H is the dimensionless Henry's Law constant corrected for the given water temperature.
where u 10 is the wind speed (m s −1 ) 10 m above the sea surface, Sc Hg is the Schmidt number and defined as, where v is the kinematic viscosity (cm 2 s −1 ) of sea water and D is the aqueous diffusion coefficient (cm 2 s −1 ) of Hg(0).The kinematic viscosity of sea water at the given temperature is calculated according to the method described by Wanninkhof (1992).The diffusion coefficient of Hg(0) is calculated using the method recently developed by Kuss et al. (2009).
The dimensionless Henry's Law constant is corrected for the desired temperature according to the study of Sanemasa (1975), where M w is the molar weight of water, 18.01 × 10 −3 (kg mol −1 ), R is the gas constant, 8.2058 × 10 −2 (atm L K −1 mol −1 ), ρ w is the density of water, and T is the water temperature (K).The degree of saturation (S a ) of Hg(0) in the water (DGM) with respect to the atmospheric Hg(0) (GEM) was calculated with Eq. ( 5).
where C w , H , and C a are as identical as described above.S a value greater than 1 indicates the supersaturation of DGM, otherwise under saturation is indicated.

GEM distribution in the MBL
GEM concentrations during the cruise ranged from 1.68 to 4.34 ng m −3 with a mean value of 2.61 ± 0.50 ng m −3 (median: 2.61 ng m −3 ).During the past three decades, a large number of GEM/TGM 1 measurements over the ocean have been carried out (Sprovieri et al., 2010b and references therein;Soerensen et al., 2010a).The GEM levels in the MBL over the Yellow Sea was higher than those of oceans in the South Hemisphere, such as the South Atlantic ocean (1.00-1.50ng m −3 , Slemr et al., 2003;Temme et al., 2003;Soerensen et al., 2010a), the Antarctic Ocean (1.30-1.50ng m −3 , Soerensen et al., 2010a), the India Ocean (1.00-1.50ng m −3 , Witt et al., 2010;Soerensen et al., 2010a).Compared with these oceans in the North Hemisphere, the GEM concentrations in the Yellow Sea also were higher than those of many other oceans, such as the Mediterranean Sea (1.5-2.0 ng m −3 , Sprovieri et al., 2003Sprovieri et al., , 2010a) ) and the Adriatic Sea (1.6 ± 0.4 ng m −3 , Sprovieri and Pirrone, 2008), but were comparable to the Atlantic Ocean (∼1.5-2.5 ng m −3 , Temme et al., 2003;Laurier and Mason, 2007;Soerensen et al., 2010a) and the North Pacific Ocean (2.5 ± 0.5 ng m −3 , Laurier et al., 2003).Elevated Hg(0) evasion from the sea surface and long-range transport of anthropogenic emissions from industrial regions might explain those elevated atmospheric Hg levels (Soerensen et al., 2010a).
During the past decade, atmospheric Hg cycle in the downwind region of East Asia has received increasing attentions.Many atmospheric Hg measurements based on the various platforms (including ground, shipborne and airborne) have been conducted.Figure 2 shows the atmospheric Hg measurements at the costal, rural or open ocean regions downwind of China.The GEM/TGM levels reported by all these measurements generally reflected the elevated values compared to the North Hemisphere background regions (e.g., 1.5-1.7 ng m −3 , Sprovieri et al., 2010b).The GEM concentrations in this cruise were lower than at sites near the Korea Peninsula (e.g., An-Myun and Jeju, Nguyen et al., 2007Nguyen et al., , 2010) ) and were close to those in CST (Ci et al., 2011b), ACE-Asia campaign over the Yellow Sea (Friedli, et al., 2004) and the cruise covering the South China Sea (Fu et al., 2010;Xia et al., 2010), and were slightly higher than those in HSO (Jaffe et al., 2005).The measurements of atmospheric Hg speciation at a high-elevation background station in Taiwan Island (Lulin station) reflected the background levels of atmospheric Hg in the North Hemisphere (Slemr et al., 2009) because this site is generally under the influence of free troposphere (Sheu et al., 2010).
As shown in Fig. 3, the GEM concentrations were generally elevated at the coast of China compared to the open ocean.This supports other studies that showed atmospheric Hg emission from anthropogenic sources in East Asia enhances the atmospheric Hg levels in the downwind region (e.g., Fu et al., 2010;Nguyen et al., 2010).This trend at the inlet of the sample line and/or the long and unheated sampling line was applied.In this paper if there was no otherwise indicated, we did not consider the difference between TGM and GEM, which all refer to the original literatures.also has been successfully reproduced by atmospheric modeling (Lin et al., 2010).
The air mass movements also influenced the GEM levels in the study period (see Fig. 3).
During 10-11 July 2010 (from Station 3600-1 to 3600-6), although the R/V was near the Shandong Peninsula, the GEM levels (2.09 ng m −3 ) in this period were lower than the campaign average (2.61 ng m −3 ) and slightly higher than the background levels of the Northern Hemisphere (1.5-1.7 ng m −3 , Sprovieri et al., 2010b).The back-trajectories of 500 m showed that the air masses originated from the East China Sea and crossed the Yellow Sea to the R/V.The GEM levels in this period were close to those originated from open ocean surface reported by Fu et al. (2010) in the South China Sea cruise.During 12-13 July 2010 (from Station B-8 to 3675-2), the R/V navigated along the west coast of the Yellow Sea (near Shandong Peninsula and Liaoning Peninsula).In this period, the GEM concentrations generally fell in 2.50-3.50ng m −3 .The back-trajectory analysis suggested that the air masses in this period largely originated from anthropogenic source regions (such as Shandong Peninsula, Fig. 3).The measurements during 14-17 July (from Station 124-5 to 3500-1) more clearly reflected the influence of air mass movements on the GEM levels.From midday on 14 July to afternoon on 15 July, the air masses originated from the East China Sea before reaching the R/V (Fig. 3).The GEM concentrations in this period were very close to those of Stations 3600-1 to 3600-6, reflecting the regional background GEM levels of East Asia.As the R/V progressed, the source region of the air masses moved from the East China Sea to the southeastern coast of China.At 00:00 h on 16 July the air masses reaching the R/V had passed directly over large industrial and port areas around Southeast China (Fig. 3).The GEM concentrations started to sharply increase from ca. 2.00 ng m −3 to 4.00 ng m −3 and remained ca.3.00-3.50ng m −3 in last period of the cruise (from Station 3300-1 to 3500-1).

THg and RHg in the surface water
The mean concentration of THg in the surface waters was 1.69 ± 0.35 ng l −1 with a range of 0.89-2.26ng l −1 , which were higher than those in open ocean, such as the Pacific Ocean (0.23 ng l −1 , Laurier et al., 2004), the Mediterranean Sea (0.26-0.30ng l −1 , Horvat et al., 2003;Kotnik et al., 2007), the Atlantic Ocean (∼0.5-0.6 ng l −1 , Mason et al., 1998;Mason and Sullivan, 1999), the Black Sea (0.32-2.08 ng l −1 , Lamborg et al., 2008) and were comparable to values reported in some near-shore environments, such as the Long Island Sound (0.46-3.98 ng l −1 , Rolfhus and Fitzgerald, 2001) and the Connecticut River estuary, ∼2.0 ng l −1 , Rolfhus et al., 2003), and lower than the measurements in the coastal site of the Yellow Sea (2.68 ng l −1 , Ci et al., 2011a).In the estimate of the global anthropogenic Hg emissions, Pacyna et al. (2010) suggested that the anthropogenic atmospheric Hg emissions from Asia contribute about half www.atmos-chem-phys.net/11/2881/2011/ of the global emission sources.For the anthropogenic Hg emission, RGM and particulate Hg (PHg) generally present a large fraction and they deposit quickly due to the high water solubility and surface reactivity (Schroeder and Munthe, 1998).Because the Yellow Sea is located on the Eastern Asian continental margin and surrounded by the industrial zone of East China and Korea, the atmospheric wet/dry deposition can contribute to the Hg pool of this marine system (Lin et al., 2010).Recently, Fu et al. (2010) reported the THg levels of 1.2 ng l −1 in the South China Sea, which were slightly lower than our study and also were higher than those of open oceans.
The spatial distribution of THg in the surface waters is illustrated in Fig. 5.The mean THg concentration of water samples taken from stations within 100 km from land (n = 22) was significantly higher than those of other stations (ttest, p < 0.001), suggesting the terrestrial source and resuspended sediment might slightly enhance the THg concentration in the near-shore water column.Stations CJ-2 to CJ-7 are near the Yangtze River mouth and under the influence of Yangtze Diluted Water (low salinity in surface water of Station CJ-7, see Fig. 6); however, the mean concentration of THg for these six stations was no significantly higher than those of other stations (t-test, p = 0.662).The studies of Hg accumulation in the estuary of the Yangtze River suggested that the estuary is an important sink for aquatic Hg (Shi et al., 2005;Fang and Chen, 2010).Based on the estimate of Fang and Chen (2010), about 50% of the Hg from river discharge was accumulated in the sediment of the shore.Then the low THg levels of Stations CJ-2 to CJ-7 might be explained by that the six stations were far from the river mouth and the Hg input via river discharge had been removed by the sedimentation process in the coastal region because of the high coefficient between aquatic Hg and suspend particulate matter (Fitzgerald et al., 2007).However, more detailed integrated researches on the Hg cycle in river-delta-estuarysea ecosystems of the Yangtze River are urgently needed.
The mean RHg concentration in the cruise was 1.08 ± 0.28 ng l −1 with a range of 0.54-1.66ng l −1 , which accounted for the dominant fraction of THg (%RHg/THg = 63.9%).
The RHg concentrations in the Yellow Sea were generally higher than many other open oceans (e.g., Atlantic Ocean: 0.34 ng l −1 , Mason and Sullivan, 1999; Mediterranean Sea: 0.09 ng l −1 , Horvat et al., 2003) and were higher or comparable to those of near-shore environments (e.g., Long Island Sound: 0.26-0.90ng l −1 , Rolfhus and Fitzgerald, 2001; Lower St. Lawrence Estuary: <0.04-0.22ng l −1 , Cossa and Gobeil, 2000).However, the RHg/THg ratios of 63.9% in the Yellow Sea were higher than many other near-shore environments (e.g., 20% in Long Island Sound and Lower St. Lawrence Estuary, Rolfhus and Fitzgerald, 2001;Cossa and Gobeil, 2000) and were similar to those in open oceans (e.g., 60% in the Atlantic Ocean, Mason and Sullivan, 1999;57% in the Mediterranean Sea, Horvat et al., 2003)     RHg in aquatic ecosystem is generally considered as the fraction of Hg for the biogeochemical transformation (e.g., reduction to Hg(0) and methylation to MMHg, Morel et al., 1998 and references therein;Amyot et al., 1997;Whalin et al., 2007), the high RHg levels and RHg/THg ratios suggest that the turnover time of Hg in the Yellow Sea may be shorter than other marine systems and potentially indicate the more dynamic cycling of Hg in this marine system.

DGM concentrations
The DGM concentrations in the surface waters ranged from 33.8 pg l −1 to 93.8 pg l −1 with the mean concentration of 63.9 ± 13.7 pg l −1 , which accounted for 3.6% of the Hg pool of water (%DGM/(THg + DGM)).The ratios were generally comparable to those in near-shore environments (e.g., Rolfhus et al., 2001;Ci et al., 2011a) and were lower than those in open oceans (e.g., Mason et al., 1998;Kotnik et al., 2007).The DGM concentrations in the Yellow Sea were generally higher than those observed from most of open oceans, such as the Equatorial Pacific Ocean (6-45 pg l −1 , Kim and Fitzgerald, 1986), the Mediterranean Sea (30-46 pg l −1 , Horvat et al., 2003;Kotnik et al., 2007) Fu et al. (2010) reported that the mean DGM concentration of 36.5 pg l −1 in the South China Sea, which also were lower than the Yellow Sea.Recently, for understanding the seasonal and diurnal variations of DGM in the coastal region of the Yellow Sea, we conducted the DGM measurements covered four seasons at CST (site location refers to Fig. 2) during 2008-09 (Ci et al., 2011a).This measurement showed a seasonal variation of DGM with elevated levels in warm seasons and low levels in cold seasons.Interestingly, the measurement in the summer campaign (August 2009) at CST suggested the highest seasonal average with 69.0 pg l −1 (Ci et al., 2011a), which was consistent with this cruise.Figure 4 illustrates the spatial distribution of DGM and generally showed the significantly high levels in near coast stations (within 100 km) and low levels in open waters (t-test, p < 0.001).This trend has been extensively reported (e.g., Fu et al., 2010).
As mentioned in the Introduction, many field measurements and laboratory incubation experiments indicated that the DGM formation in the water column is mediated by the abiotic and biotic mechanisms.Among all factors, the solar radiation (Amyot et al., 1997;Lanzillotta et al., 2002;Rolfhus and Fitzgerald, 2004) and microorganism activities (Mason et al., 1995;Lanzillotta et al., 2004;Poulain et al., 2004;Fantozzi, et al., 2009) are considered as the principal forces for the RHg reduction to DGM.The high DGM levels in this cruise and elevated levels near coast might be explained as follows.First, this cruise was conducted in July, corresponding to the high solar radiation, which would promote the photochemical reduction of RHg.Second, due to the special current pattern, the river input and the atmospheric deposition of nutrients (e.g., N and P ), the primary production in the Yellow Sea is high (Liu et al., 2003), then the RHg reduction mediated by biochemical processes may be promoted and subsequently contributes to the elevated DGM levels.Particularly, in recent years the massive green algae (Enteromorpha prolifera) blooming often occurred in the coastal region of the Yellow Sea in the period of post spring to early summer (Sun et al., 2008).In this cruise, we also observed the green algae bloom, which might directly or indirectly stimulate the DGM formation via organism activity in the water column, especially the coastal region.Interestingly, we did not find strong correlation between DGM and THg/RHg in this study, which was consistent with our previous study at the coastal site (CST) of the Yellow Sea (Ci et al., 2011a).This result might further indicate the importance of biochemical processes on the DGM formation in the Yellow Sea.However, at present, there is no focused study is performed to investigate the influence of green algae blooming on the Hg cycle in this marine environment.

Degree of DGM saturation
The mean degree of DGM saturation (S a ) was 7.8 ± 2.3 with a range of 3.6-14.0,indicating that there was always supersaturation of Hg(0) in the surface waters with respect to Hg(0) in the atmosphere (Table 1).This S a values were higher than some marine environments (e.g., 3.51 ± 2.67 in the Long Island Sound, Rolfhus and Fitzgerald, 2001) and were comparable to those measurements in coastal site (CST) of the Yellow Sea in summer 2009 (Ci et al., 2011a) and the South China Sea in July 2007 (Fu et al., 2010).Comparable values also were reported in some studies performed in warm seasons, such as 4-40 in the Pacific Ocean (Mason and Fitzgerald, 1993), ∼7-8 in the Mediterranean Sea (Andersson et al., 2008b;Gårdfeldt et al., 2003) and 10 in the Tokyo Bay (Narukawa et al., 2006), although special attention should be paid to that some studies mentioned above (Fu et al., 2010;Andersson et al., 2008b) used a different Henry's law constant developed by Andersson et al. (2008a).
However, as one evaluates the different works on estimating the air-water Hg(0) flux, it is important to keep in mind that the estimated Hg(0) flux using the two-layer gas exchange model is influenced by the choice of gas transfer parameterizations and diffusion coefficient of Hg(0) (Rolfhus and Fitzgerald, 2001;Andersson et al., 2007;Ci et al., 2011a).According to Eqs. ( 2) and ( 3), the relationships between gas transfer velocity and wind speed at 10 m height and the method for estimating D jointly decide the method for calculating K.For a given method for estimating D, the difference between algorithms of K is principally due to adopting the parameter to treat the wind speed.As shown in Fig. 9 in the study of Andersson et al. (2007), five algorithms that widely used for calculating the wind-induced K were compared and suggested that at high wind speed (e.g., >10 m s −1 ), there was great difference between these algorithms.The parameterization of Wanninkhof (1992) employed in this study represents a moderate strength for calculating K among these five algorithms.The choice of D also influences the calculation of air-sea Hg(0) flux.For example, Kuss et al. (2009) suggested a lower oceanic Hg(0) emission flux using the D developed by Kuss et al. (2009) than Poissont et al. (2000) in a global Hg model (GEOS-Chem, Strode et al., 2007).
As descried in Eq. ( 1), the Hg(0) flux is jointly determined by the Hg(0) gradient at the air-sea interface and windinduced K.Because the model is very sensitive to the wind speed, the wind field can temporally influence the Hg(0) exchange (Wanninkhof, 1992;Ci et al., 2011a).According to Eq. ( 2), the K of wind speed of 15 m s −1 (∼1.0 m h −1 ) and 10 m s −1 (∼ 0.4 m h −1 ) is about 10-fold and 4-fold of wind speed of 5 m s −1 (0.1 m h −1 ), respectively.Due to the influence of the Asian Summer Monsoon in this cruise, extremely high wind speed was recorded (see Table 1).This high wind speed principally contributed the elevated Hg(0) emission rates in this cruise.

Vertical distribution of DGM, RHg and THg in the water column
The vertical samplings at the three selected stations were performed to determine the penetration of these surface signals into the thermocline waters of the Yellow Sea.
The data in vertical profiles of THg, RHg, DGM, water temperature and salinity at the three stations (Stations CJ-7, 124-7 and 3500-9) are shown in Fig. 6.On the whole, the distribution of Hg in the three stations showed no clear vertical variations and suggested the comparable levels with the surface stations, indicating the well vertically mixed in the Yellow Sea.The mean THg b Wind speed at 10 m concentration was 1.26 ± 0.57 ng l −1 (0.88-2.39 ng l −1 ), 1.10 ± 0.21 ng l −1 (0.87-1.49ng l −1 ) and 1.30 ± 0.79 ng l −1 (0.74-2.89 ng l −1 ) for Stations CJ-7, 124-7 and 3500-9, respectively.The mean %RHg/THg ratio in three stations ranged from 46.0% to 57.1%, indicating the similar value with the surface stations.The elevated THg levels were found at the water-sediment interface (52 m in Station CJ-7 and 69 m in Station 3500-9), probably resulting from the elevated suspend particulate matter due to the resuspension of bottom sediment (Guo et al., 2010).
During the cruise aboard the R/V Kexue III from 9-18 July 2010, GEM in the atmosphere, THg, RHg and DGM in the open waters of the Yellow Sea were measured for the first time.The spatial distribution of GEM in the atmosphere, THg and RHg in the water suggested the importance of Hg outflow from East China, which has affected the Hg cycling in the downwind region.The elevated RHg levels and RHg/THg ratios in the waters might indicate the Hg cycle in the Yellow Sea is more active.Using a two-layer gas exchange model, the estimated Hg(0) flux at the interface between atmosphere and the Yellow Sea showed a considerably high levels due to the elevated DGM concentrations and high wind speed.
Atmospheric GEM distribution over the Yellow Sea and the typical backctories of air masses during the cruise in July 2010.See text for more information e calculation of the back-trajectory.0° E).

Fig. 3 .
Fig. 3. Atmospheric GEM distribution over the Yellow Sea and the typical back-trajectories of air masses during the cruise in July 2010.See text for more information on the calculation of the backtrajectory.

Figure 4 .
Figure 4. Spatial distribution of DGM (unit in pg l -1 ) in the surface waters of th Yellow Sea during the cruise in July 2010.

Fig. 4 .Figure 5 .
Fig. 4. Spatial distribution of DGM (unit in pg l −1 ) in the surface waters of the Yellow Sea during the cruise in July 2010.

Fig. 5 .
Fig. 5. Spatial distribution of the total Hg (THg, unit in ng l −1 ) and reactive Hg (RHg) and RHg/THg ratios in the surface waters of the Yellow Sea during the cruise in July 2010.

Table 1 .
Summary of station information, atmospheric GEM, surface water DGM and relevant data for air-sea Hg(0) flux calculation in the Yellow Sea during the cruise in July 2010.