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Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring
STOTEN-21778; No of Pages 8 Science of the Total Environment xxx (2017) xxx–xxx
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Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring Zhen He a,c, Qiu-Lin Liu a, Ying-Jie Zhang a, Gui-Peng Yang a,b,c,⁎ a
Key Laboratory of Marine Chemistry Theory and Technology, Ocean University of China, Ministry of Education/Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao 266100, China Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China c Institute of Marine Chemistry, Ocean University of China, Qingdao 266100, China b
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• VHCs were measured in the BS and NYS. • The environmental factors for influencing the distributions of VHCs were examined. • Both anthropogenic and biological activities affect the distribution of VHCs. • The BS and the NYS was a significant source of the atmospheric VHCs.
a r t i c l e
i n f o
Article history: Received 10 October 2016 Received in revised form 9 January 2017 Accepted 11 January 2017 Available online xxxx Editor: D. Barcelo Keywords: Bohai Sea Distribution North Yellow Sea Sea-to-air flux Volatile halocarbons
a b s t r a c t Concentrations of volatile halocarbons (VHCs), such as CHBr2Cl, CHBr3, C2HCl3, and C2Cl4, in the Bohai Sea (BS) and North Yellow Sea (NYS) were measured during the spring of 2014. The VHC concentrations varied widely and decreased with distance from the coast in the investigated area, with low values observed in the open sea. Depth profiles of the VHCs were characterized by the highest concentration generally found in the upper water column. The distributions of the VHCs in the BS and NYS were clearly influenced by the combined effects of biological production, anthropogenic activities, and riverine input. The sea-to-air fluxes of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 in the study area were estimated to be 47.17, 56.63, 162.56, and 104.37 nmol m−2 d−1, respectively, indicating that the investigated area may be a source of atmospheric CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 in spring. © 2017 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author at: Key Laboratory of Marine Chemistry Theory and Technology, Ocean University of China, Ministry of Education/Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao 266100, China. E-mail address: [email protected] (G.-P. Yang).
Volatile halocarbons (VHCs) are important atmospheric trace gases and carriers of chlorine, bromine, and iodine in the atmosphere. Numerous halocarbons (e.g., CFCs) can act as greenhouse gases that absorb
http://dx.doi.org/10.1016/j.scitotenv.2017.01.065 0048-9697/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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infrared radiation in the troposphere and affect climate (Ramanathan, 1975; World Meteorological Organization (WMO), 2007). VHCs can be photodissociated to produce reactive halogens radicals, such as chlorine and bromine. These radicals participate in a series of catalytic reactions which destroy ozone in the troposphere and reduce it in the stratosphere (WMO, 2007; Wang et al., 2007). Marine macroalgae and microalgae are the main oceanic sources of VHCs (Abrahamsson et al., 1995; Nightingale et al., 1995; Moore et al., 1996; Carpenter and Liss, 2000; Quack and Wallace, 2003; Abrahamsson et al., 2004a; Carpenter et al., 2009). Various hypotheses were proposed to explain the role of halocarbons in the metabolism of marine macroalgae and microalgae. Abrahamsson et al. (1995) verified the involvement of haloperoxidase enzymes in the biosynthesis of the gases by both macro- and microalgae. Anthropogenic or industrial sources for chlorocarbons, such as chloroform and trichloroethene, were identified in the coastal water of the North Sea in addition to natural sources (Huybrechts et al., 2005). Oceans act as a source of atmospheric CHBr3, CHBr2Cl, C2HCl3, and C2Cl4 (Khalil et al., 1983; Abrahamsson and Ekdahl, 1996; Y. Liu et al., 2011). The transfer of oceanic VHCs to the atmosphere is one of the most important removal processes of oceanic VHCs, aside from chemical and biological degradation and adsorption onto particles (Abrahamsson et al., 2004b). Thus, the marine boundary layer is thought to be one of the most important interfaces for the gas exchange between water and the atmosphere (Bravo-Linares et al., 2007). In this study, the concentrations of VHCs (CHBr2Cl, CHBr3, C2HCl3, and C2Cl4) were measured in the Bohai Sea (BS) and North Yellow Sea (NYS) during the spring of 2014. The spatial distributions of these VHCs were investigated. The environmental factors that influence the concentrations of these gases were studied. Their sea-to-air fluxes were estimated, and the possible sources were assessed. 2. Methods 2.1. Study area
of the gases were studied along the section J, which covers different hydrographic regions, including B18 off the Yalu River estuary, B16, B14, and B12 on the shelf. The surface (0–5 m) and depth (0–5 m, 10 m, 30 m, 50 m and bottom depth) samples were collected using 12 l Niskin bottles mounted on a Seabird conductivity, temperature, and depth (CTD) Rosette (General Oceanics Co.). The environmental and hydrographic conditions of the sampling stations, such as surface seawater temperature and salinity, were obtained from the CTD apparatus. 2.3. Analytical methods The concentrations of VHCs were analyzed immediately after sample collection by using a cryogenic purge and a trap system coupled to a gas chromatograph with an electron capture detector. The seawater sample was drawn into a 100 ml glass syringe without ambient air, and then directly injected into a glass bubbling chamber at 40 °C and stripped with ultrapure nitrogen at a flow rate of 100 ml min−1 for 12 min. The extracted gases passed through two glass tubes containing anhydrous Mg(ClO4)2 and ascarite to remove water vapor and carbon dioxide, respectively. The extracted gases were trapped in an inert stainless steel tube immersed in liquid nitrogen. After extraction, the stainless steel tube was heated, and the desorbed gases were introduced into a GC/ECD (Agilent 6890N) for analysis. The target compounds were quantified based on the retention times and peak areas of calibration standards. The analytical precisions of the target compounds were generally 7%–15% in the routine sample analysis. Their detection limits were 0.1–0.5 pmol l−1. A similar analytical setup was described by Lu et al. (2010). Chl-a was measured fluorometrically using a fluorescence spectrophotometer (F-4500, Hitachi Co., Japan) after filtering 300 ml of seawater through a glass fiber filter (Whatman GF/F, 47 mm diameter) and extracting the seawater with 90% acetone for 24 h in the dark at 4 °C according to the method adopted by Parsons et al. (1984). 3. Results and discussion
2
The BS, which has a total surface area of 77,000 km and an average depth of 18 m, is a shallow semi-enclosed sea and the largest inner sea of China. BS can be divided into four parts, including the Liaodong Bay, Bohai Bay, Laizhou Bay, and Central BS. The NYS is separated from the South Yellow Sea (SYS) by a line between Chengshanjiao of the Shandong Peninsula and Changsangot on the Korean Peninsula (Li et al., 2016). NYS is located between mainland China and the Korean Peninsula and connected to the BS by the Bohai Strait. The hydrographic characteristics of the BS and NYS are heavily affected by circulation, such as that the Yellow Sea (YS) Warm Current, the YS, the BS, and the Korea Coastal Currents (Su, 1998; Lee et al., 2000). A protruding hydrological phenomenon in the NYS is the NYS Cold Water Mass (NYSCWM), which usually occupies the deep and bottom layers of the trough of the central YS (Li et al., 2016). The hydrological conditions of the BS are significantly influenced by over forty river discharges, including the Yellow River, Hai River, Liao River, and Luan River (Ning et al., 2010). Especially, the Yellow River carries substantial quantities of fresh water with large amounts of particulates and nutrients that support the high biological productivity of the BS. The anthropogenic pollution in Chinese and Korean coastal regions affects the ecosystems of the YS and BS (S. Liu et al., 2011). Thus, the environment of this region, particularly along the coast, is crucial in the climate change in the entire China Seas and the surrounding mainland. 2.2. Seawater sampling The cruise was conducted aboard the R/V “Dong Fang Hong 2” in BS and NYS from 27 April to 23 May 2014. The sampling stations are shown in Fig. 1. The horizontal distributions of the four VHCs were investigated in the surface seawater of all the stations, and the vertical distributions
The concentrations of Chl-a in the surface water of the BS and NYS varied from 0.38 μg l−1 to 3.7 μg l−1 with a mean value of 1.74 μg l−1. Fig. 2A indicates that Chl-a exhibited elevated patch concentrations at the Yalu River and Yellow River estuary, and the highest value of Chl-a was observed at B18. By contrast, the concentration of Chl-a was very low at the central region of the NYS. The high concentrations of Chl-a in the river estuaries occurred because the river input increases the levels of nutrients, which could enhance phytoplankton biomass. The NYSCWM (at the bottom of the NYS) and thermocline (20–30 m) were formed during the spring (Yu et al., 2006; Yao et al., 2012; Li et al., 2015). Thermocline, which separates the NYSCWM from its upper water, hinders the vertical transportation of nutrients. The nutrients of the bottom water cannot be brought to the upper layer and thus cannot supply the growth of phytoplanktons (Yang et al., 2015), which may reduce the Chl-a concentration in the central NYS waters. 3.1. Surface water concentrations of the VHCs The concentrations of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 in the surface water of the BS and NYS varied from 0.39 to 35.64, 3.01 to 67.41, 10.48 to 97.49, and 15.76 to 45.18 pmol l− 1 with a mean of 12.77, 26.92, 32.83, and 25.18 pmol l−1. The concentrations of VHCs measured in this study were in the same range as those reported in coastal and oceanic waters (Sing et al., 1983; Zoccolillo and Rellori, 1994; Roy et al., 2011). However, the mean value of C2Cl4 was higher by one order of magnitude than the value reported by Sing et al. (1983) in the Eastern Pacific Ocean. The mean of CHBr2Cl, CHBr3, and C2Cl4 were the same order of magnitude as those recorded in the literature (Zoccolillo and Rellori, 1994; Roy et al., 2011). The difference of VHC
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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Fig. 1. Locations of sampling stations.
concentrations was probably associated with varying source types and water masses. Fig. 2B shows that the CHBr2Cl concentrations varied widely in the investigated area in the BS. The highest concentrations of CHBr2Cl appeared in the Bohai strait, which is an important channel for ship navigation. The activities involved in ship navigation include ship wastewater disposal and disinfection of wastewater by chlorination, which leads to the production of a wide range of halogenated organic compounds (Quack and Wallace, 2003). Moreover, Y. Liu et al. (2011) reported that elevated levels of brominated short-lived halocarbons appear in the seawater after a ship passes near a series of nuclear power plants and treated-water outfalls along the coast. Thus, the result may be attributed to anthropogenic contamination as intense shipping was observed during sampling. In the NYS, the CHBr2Cl concentrations in the surface seawater exhibited distinct and decreasing spatial variation from the coastal to the central region. The highest value of CHBr2Cl at B18 corresponded to the relatively high level of Chl-a in this region, which may be caused by the abundant nutrients discharged from Yalu River, leading to the growth of phytoplankton and the emission of CHBr2Cl. Previous research (Nightingale et al., 1995; Carpenter and Liss, 2000) reported that various volatile halogenated compounds were present in the tissue of some species of algae and the release rates of CHBr3, CHBr2Cl, and CH2Br2 were determined in laboratory experiments. CHBr3 concentrations in the surface seawater during spring showed distinct spatial variation that decreased from the coast to the central area (Fig. 2C). Elevated CHBr3 concentrations were observed in the Yalu River estuarine area with relatively higher Chl-a concentrations. Two possible explanations for these high values could be deduced. On one hand, this region was clearly affected by the terrestrial input. The disinfection of seawater could produce a wide range of byproducts, such as trihalomethanes (Quack and Wallace, 2003; Zhou et al., 2005). On the other hand, high CHBr3 concentrations were caused by river discharges with substantial nutrients that stimulate local phytoplankton growth and the emission of CHBr3. Previous studies indicated that phytoplanktons are the primary producers of halocarbons in oceans (Abrahamsson et al., 2004b). However, the results showed that CHBr3
was produced by algae at significantly higher rates than other bromomethanes (Nightingale et al., 1995; Ekdahl et al., 1998; Quack and Wallace, 2003). The concentration of CHBr3 was evidently low in the central region of NYS because NYSCWM forms during spring (Yao et al., 2012; Li et al., 2015). The concentrations of CHBr3 were affected by NYSCWM. Thus, the spatial distributions of CHBr3 in the study area in spring were caused by the comprehensive action of phytoplankton emission of CHBr3, terrestrial inputs, and water mass movement. Table 1 shows that the relationship between CHBr3 and CHBr2Cl and Chl-a was tenuous, in part because of the variations caused by nutrients and light and in part because of other biological sources (e.g., bacteria), besides phytoplankton or even abiotic photochemical sources. These results indicated that Chl-a was a poor indicator of the biological production of bromochloromethane. The C2HCl3 and C2Cl4 concentrations in the BS were higher than those in the NYS (Fig. 2D and E). High concentrations of C2HCl3 and C2Cl4 were observed in coastal regions, such as at B53 and B56. These results were assumed to be related to the anthropogenic inputs from the river runoff, such as Fuzhou River, which is rich in volatile organic compounds. As reported, the load of chlorinated compounds (e.g., chloroform) and other organic compounds in the Yangtze River were estimated to be between 500 kg d−1 and 3500 kg d− 1 (Müller et al., 2008), and pollutant load threatened the health of the marine ecosystem. Moreover, low salinity values observed at these stations indicated the influence of river runoff. The extremely high concentrations of C2HCl3 and C2Cl4 were observed in the central region of NYS (B23). The relatively high concentration of Chl-a was also detected in this region, suggesting that this area is a possible biogenic source of C2HCl3 and C2Cl4 from specific phytoplankton. A previous study showed that C2HCl3 can be released by phytoplanktons (Abrahamsson et al., 1995). B23 was far from the coastal influence of anthropogenic input. Thus, determining the source of these high levels of C2HCl3 and C2Cl4 in the middle shelf is difficult. The chemical decomposition of C2HCl3 can be ruled out because the compound is very stable against hydrolysis in aquatic environments and has a half-life of a hundred years (Mabey and Mill, 1978; Fogelqvist, 1984). Therefore, further investigation is crucial to identify the source in this region. Moreover, lower concentrations of
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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Fig. 2. Horizontal distributions of Chl-a (μg l−1), temperature (°C) and four kinds of VHCs (pmol l−1) in the surface seawater in spring.
C2HCl3 and C2Cl4 were recorded at B18, which was located in the region of the Yalu River runoff. This finding could be attributed to the adsorption of high contents of total suspended matter (e.g., sand grains, clays and particulate organic matter). Investigations in estuaries (Dewulf et al., 1996; Roose et al., 2001; Christof et al., 2002) indicated the adsorption of VHCs is in relation to total suspended matter. Furthermore, Duinker et al. (1982) and Christof et al. (2002) found that increasing
Table 1 Correlation coefficients of linear regression analyses between concentrations of the four VHCs and other oceanographic parameters in spring. Variables
CHBr2Cl
CHBr3
CHBr3 C2HCl3 C2Cl4 Temperature Salinity Chl-a
0.363⁎ 0.118 0.314 0.155 −0.351⁎⁎ 0.123
0.029 −0.155 −0.167 0.100 0.125
C2HCl3
C2Cl4
number of halogen atoms in the water could increase the adsorption to particles. Another reason was that the temperatures of the seawater were low than that of others. These results suggest that the source and outcome of these halocarbons in marine environments are extremely complex. In summary, the surface distribution patterns of the four VHOC were complicated and controlled by riverine runoffs, various current systems, anthropogenic input from terrestrial pollutants (e.g., water chlorination and sewage discharge), and biological processes, such as phytoplankton production. Moreover, oceanographic environmental factors, such as sea surface temperature and salinity, might also play an important role in controlling VHC distributions. 3.2. Vertical distributions of the VHCs
⁎ Correlation is significant at the 0.05 level (2-tailed). ⁎⁎ Correlation is significant at the 0.01 level (2-tailed).
0.438⁎⁎
−0.449⁎⁎ −0.008 0.088
0.010 −0.008 −0.280
This transect extends from the Yalu River estuary to the Shandong Peninsula, with the Yalu River Diluted Water on the northeast side and the NYSCWM dispersing out in the middle shelf. Fig. 3 shows the depth profiles of temperature, salinity, Chl-a, and the four kinds of VHCs at transect J.
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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Fig. 3. Depth profiles of temperature (°C), salinity (‰), Chl-a (μg l−1) and 4 kinds of VHCs (pmol l−1) at transect J.
Fig. 3 indicates that the vertical distributions of the VHCs along transect J were controlled by various factors, including the source strengths of the VHCs and water masses (e.g., Yalu River and NYSCWM). Temperature and salinity, which are influenced by NYSCWM, indicate the occurrence of layer phenomenon. The isotherm of upper portions was concave and bowl-like. The isotherm in the lower part was epirelief and hat-like. The salinity distribution appeared like a saddle. The lower temperature and the higher salinity, which are significantly affected by the NYSCWM, were observed in the bottom water mass (Zhang et al., 2014). B18 of transect J indicated a lower temperature
and salinity in the upper water column than in B16 because of the influence of the cold Yalu River water. The vertical distributions of the concentrations of the four VHCs along transect J were complicated because of the strong influence of the northerly monsoon, complex water masses, and variable point sources of these gases. High concentrations of the four VHCs were observed in the upper layer of the water column, whereas low concentrations were detected in the middle layer (Fig. 3). In the upper mixed layer, the CHBr2Cl concentrations generally decreased with distance from the coast. Relatively high concentrations of
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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CHBr2Cl in the water column were observed in the bottom water at B12. Those findings suggested the existence of local sources, such as the emission from sediments. Moore and Tokarczyk (1993) demonstrated that elevated concentrations of CHBr2Cl near the bottom water could be attributed to a sedimentary source, and Fogelqvist (1984) confirmed this finding and proposed that it resulted from the degradation of naturally occurring brominated organics. The CHBr3 concentrations decreased from the surface to bottom along the east part of transect J. In the water column, the CHBr3 and Chl-a concentrations (Fig. 3C, E) decreased gradually from inshore to offshore with the maxima of CHBr3 and Chl-a appearing in the upper mixed layer and at the coastal stations (e.g., B18). These results suggested the influence of the expansion of the diluted water of the Yalu River. Evidently, high CHBr3 concentration was detected at the bottom water at B12. Therefore, the considerably high concentrations of CHBr3 may also be related to the emission from organically enriched sediments. The production of benthic organisms was supposed to contribute to the elevated CHBr3. The production of VHCs from marine organic aggregates was demonstrated by Hughes et al. (2008). Abrahamsson and Ekdahl (1996) attributed the high concentration of halocarbon in near-bottom water to the production by benthic organisms. Further work is required to determine the exact source and production mechanisms of these VHCs. The distribution of C2HCl3 was similar to the distribution of temperature in the transect J, with the highest concentration in the surface water and the lowest concentration in the middle water at B14, suggesting the effect of the NYSCWM. The distribution of C2Cl4 decreased significantly and gradually from inshore to offshore in the transect J. The highest concentration was observed in the surface water at B18, which is affected by the Yalu River discharge, and the lowest concentration was detected in the middle water at B14, which is influenced by the NYSCWM. 3.3. Relationships between VHCs and oceanographic parameters Table 1 shows the correlation coefficients of the linear regression analyses between concentrations of the four VHCs and other oceanographic parameters in spring. A linear relationship between CHBr2Cl and CHBr3 (R = 0.363, P = 0.023) in this cruise, as previously reported by Chuck et al. (2005), was also confirmed in the study (Table 1). A linear relationship between C2HCl3 and C2Cl4 (R = 0.438, P = 0.006) was observed in this cruise in spring. Those results indicated that C2HCl3 and C2Cl4 share some common sources or removal mechanisms. However, they may also have their own specific producers. The solubilities of the gases generally decrease with increasing water temperature; hence, a negative correlation between temperature and the concentration of C2HCl3 was observed. Correlation analyses showed significant relationships exist between the salinity and CHBr2Cl concentrations in the surface seawater, suggesting the influence of salinity. Linear regression analysis showed no clear correlation between the VHCs and Chl-a concentration in the surface water, which was consistent with previous investigations (Yang et al., 2015; Yuan et al., 2016). These results suggested that Chl-a may be a poor proxy for determining the source of VHCs. The selected halocarbons did not indicate any significant correlation with each other in seawater. 3.4. Sea-to-air flux calculations The fluxes of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 were calculated using the following equation (Liss and Slater, 1974): F ¼ KW ðCW −Ca =HÞ
ð1Þ
where F is the flux (nmol m−2 d−1); Kw (m d−1) is the gas transfer velocity; Cw and Ca are the concentrations of gases in seawater and atmosphere, respectively. H is the temperature dependent Henry's
Law constant, which is derived from the non-dimensional value given by Moore (2000) for CHBr2Cl, CHBr3, C2HCl3, and C2Cl4. Kw was calculated for CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 as described by Liss and Merlivat (1986), respectively. KW ¼ 0:17u10 ðSc=600Þ−2=3 0bu10 ≤3:6
ð2Þ
KW ¼ ð2:85u10 −9:65ÞðSc=600Þ−1=2 3:6bu10 ≤13
ð3Þ
KW ¼ ð5:9u10 –49:3ÞðSc=600Þ−1=2 u10 N13
ð4Þ
where u is the wind speed (m s−1), and Sc is the Schmidt number. The Schmidt numbers for CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 were calculated using the equation of Khalil et al. (1999). The four VHCs did not exhibit in situ measured atmospheric concentrations. The global background atmospheric concentrations of 0.12 pmol l− 1 for CHBr2Cl, 0.30 pmol l−1 for CHBr3, 0.14 pmol l−1 for C2HCl3, and 0.77 pmol l−1 for C2Cl4 (Reifenhäuser and Heumann, 1992; Schall and Heuann, 1993; Carpenter et al., 2003; Rivett et al., 2003; Zhou et al., 2005; Chuck et al., 2005), respectively, to represent their atmospheric concentrations over the study area. The sea-to-air fluxes of the four VHCs across the air–sea interface in the study area were calculated according to the above-described equation and reference atmospheric concentrations of the four VHCs. The mean sea-to-air fluxes (range) of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 were estimated to be 47.17 (−1.16–178.82), 56.63 (−24.07–356.70), 162.56 (2.23–977.83), and 104.37 (1.64–398.39) nmol m−2 d−1, respectively. In Fig. 4, the fluxes of the gases exhibited considerable spatial variability and varied following the wind speeds at most stations. For instance, extremely high fluxes at B36, B56, B64, and BS3 coincided with high wind speeds, whereas the low fluxes of the gases were found at B07, B31, and B50, which had low wind speeds. The fluxes of VHCs at B57 with high wind speeds were insufficient because of the low VHC concentrations and low sea surface temperature. Thus, the spatial variability of the sea-to-air fluxes of VHCs was largely influenced by wind speed, as well as their concentrations and the sea surface temperature. Compared with those reported in the Gran Canaria and Tropical Atlantic coast, the CHBr3 mean flux in the present work was approximately onetenth of those reported by Zhou et al. (2005) in the Gulf of Maine, which was strongly influenced by local macroalgae. The mean fluxes of C2HCl3 and C2Cl4 in this study were higher than the results presented by Ekdahl et al. (1998) in the Gran Canaria. Our study area was significantly influenced by anthropogenic inputs and phytoplankton production. Therefore, the large discrepancies in the fluxes of the VHCs may be caused by the spatial and temporal variations in anthropogenic and biogenic sources of the VHCs, as well as factors that influenced gas exchange (e.g., wind speed and temperature). The BS and NYS represent one of the continental shelves in the world. Thus, our result indicates that the coastal and shelf regions can significantly contribute to the global oceanic VHC emissions and significantly affect the budgets of reactive chlorine and bromine. The major uncertainty in the assessment of sea-to-air fluxes lies in the calculation of gas transfer velocities, which depends on measured wind speed and sea surface temperature. In this work, the instantaneous wind speeds were used to calculate the gas transfer coefficient for the sea-to-air fluxes at each station. Therefore, the preliminary calculation of the fluxes of the four compounds should be regarded as an instantaneous and approximate assessment. Introducing these atmospheric mixing ratios also leads to larger uncertainties in the sea-to-air flux estimates. Further study, which includes simultaneous measurement of concentrations of these VHCs in both aqueous and gaseous phases, is necessary to accurately estimate the sea-to-air fluxes of these VHCs in the study area.
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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Fig. 4. Relationship between sea-to-air fluxes and wind speed of four VHCs in the BS and NYS in spring.
4. Conclusions
Acknowledgements
The spatial distributions and the sea-to-air fluxes of CHBr2 Cl, CHBr3, C2HCl3, and C2Cl4 in the BS and the NYS, as well as their controlling factors, were investigated. The data presented in this study revealed that CHBr2 Cl, CHBr3 , C2 HCl3 , and C2 Cl4 concentrations widely varied in the investigated area of the BS, whereas their concentrations decreased with distance from the coast in the NYS, with the low values observed in the open sea. The vertical distributions of the four VHCs in the water column varied among stations, with the maxima generally occurring in the upper water column and low concentration appearing in the middle. The complex interactions of the various water masses and differences in the source strength were possible reasons causing the strong patchy pattern of the VHCs distributions in the study area. Environmental factors, such as Chl-a concentration, surface seawater salinity, and temperature, possibly played an important role in controlling the distributions of these VHCs. Phytoplankton might substantially control surface CHBr2Cl and CHBr3 concentrations as the elevated concentrations were observed in the coastal area off the Yalu River estuary where the Chl-a levels were quite high. Further study is required to determine the influence of the oceanographic parameters on the distributions of VHCs. During the study period, the mean sea-to-air fluxes of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 were approximately 47.17, 56.63, 162.56, and 104.37 nmol m − 2 d− 1 , respectively. Therefore, we suggested that BS and NYS are sources of the four VHCs to the atmosphere.
We thank the captain and the crew of the R/V “Dong Fang Hong 2” for the help and cooperation during the in situ investigation. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 41506088 and 41320104008), the National Key Research and Development Program of China (Grant No. 2016YFA0601301), AoShan Talents Program of Qingdao National Laboratory for Marine Science and Technology (No. 2015 ASTP), the National Natural Science Foundation for Creative Research Groups (Grant No. 41521064), and the Central University Basic Business Expenses Special Funds for Scientific Research Projects (Grant No. 201513062). References Abrahamsson, K., Ekdahl, A., 1996. Volatile halogenated compounds and chlorophenols in the Skagerrak. J. Sea Res. 35 (1–3), 73–79. Abrahamsson, K., Ekdahl, A., Collen, J., Pedersen, M., 1995. Marine algae—a source of trichloroethylene and perchloroethylene. Limnol. Oceanogr. 40 (7), 1321–1326. Abrahamsson, K., Bertilsson, S., Chierici, M., Fransson, A., Froneman, P.W., Lore'n, A., Pakhomov, E.A., 2004a. Variations of biochemical parameters along a transect in the Southern Ocean, with special emphasis on volatile halogenated organic compounds. Deep-Sea Res. II 51, 2745–2756. Abrahamsson, K., Lorén, A., Wulff, A., Wängberg, S.Å., 2004b. Air-sea exchange of halocarbons: the influence of diurnal and regional variations and distribution of pigments. Deep-Sea Res. II 51, 2789–2805. Bravo-Linares, C.M., Mudge, S.M., Loyola-Sepulveda, R.H., 2007. Occurrence of volatile organic compounds (VOCs) in Liverpool Bay, Irish Sea. Mar. Pollut. Bull. 54 (11), 1742–1753. Carpenter, L.J., Liss, P.S., 2000. On temperate sources of bromoform and other reactive organic bromine gases. J. Geophys. Res. 105 (D16), 20539–20547.
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Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring Zhen He a,c, Qiu-Lin Liu a, Ying-Jie Zhang a, Gui-Peng Yang a,b,c,⁎ a
Key Laboratory of Marine Chemistry Theory and Technology, Ocean University of China, Ministry of Education/Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao 266100, China Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China c Institute of Marine Chemistry, Ocean University of China, Qingdao 266100, China b
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• VHCs were measured in the BS and NYS. • The environmental factors for influencing the distributions of VHCs were examined. • Both anthropogenic and biological activities affect the distribution of VHCs. • The BS and the NYS was a significant source of the atmospheric VHCs.
a r t i c l e
i n f o
Article history: Received 10 October 2016 Received in revised form 9 January 2017 Accepted 11 January 2017 Available online xxxx Editor: D. Barcelo Keywords: Bohai Sea Distribution North Yellow Sea Sea-to-air flux Volatile halocarbons
a b s t r a c t Concentrations of volatile halocarbons (VHCs), such as CHBr2Cl, CHBr3, C2HCl3, and C2Cl4, in the Bohai Sea (BS) and North Yellow Sea (NYS) were measured during the spring of 2014. The VHC concentrations varied widely and decreased with distance from the coast in the investigated area, with low values observed in the open sea. Depth profiles of the VHCs were characterized by the highest concentration generally found in the upper water column. The distributions of the VHCs in the BS and NYS were clearly influenced by the combined effects of biological production, anthropogenic activities, and riverine input. The sea-to-air fluxes of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 in the study area were estimated to be 47.17, 56.63, 162.56, and 104.37 nmol m−2 d−1, respectively, indicating that the investigated area may be a source of atmospheric CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 in spring. © 2017 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author at: Key Laboratory of Marine Chemistry Theory and Technology, Ocean University of China, Ministry of Education/Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao 266100, China. E-mail address: [email protected] (G.-P. Yang).
Volatile halocarbons (VHCs) are important atmospheric trace gases and carriers of chlorine, bromine, and iodine in the atmosphere. Numerous halocarbons (e.g., CFCs) can act as greenhouse gases that absorb
http://dx.doi.org/10.1016/j.scitotenv.2017.01.065 0048-9697/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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infrared radiation in the troposphere and affect climate (Ramanathan, 1975; World Meteorological Organization (WMO), 2007). VHCs can be photodissociated to produce reactive halogens radicals, such as chlorine and bromine. These radicals participate in a series of catalytic reactions which destroy ozone in the troposphere and reduce it in the stratosphere (WMO, 2007; Wang et al., 2007). Marine macroalgae and microalgae are the main oceanic sources of VHCs (Abrahamsson et al., 1995; Nightingale et al., 1995; Moore et al., 1996; Carpenter and Liss, 2000; Quack and Wallace, 2003; Abrahamsson et al., 2004a; Carpenter et al., 2009). Various hypotheses were proposed to explain the role of halocarbons in the metabolism of marine macroalgae and microalgae. Abrahamsson et al. (1995) verified the involvement of haloperoxidase enzymes in the biosynthesis of the gases by both macro- and microalgae. Anthropogenic or industrial sources for chlorocarbons, such as chloroform and trichloroethene, were identified in the coastal water of the North Sea in addition to natural sources (Huybrechts et al., 2005). Oceans act as a source of atmospheric CHBr3, CHBr2Cl, C2HCl3, and C2Cl4 (Khalil et al., 1983; Abrahamsson and Ekdahl, 1996; Y. Liu et al., 2011). The transfer of oceanic VHCs to the atmosphere is one of the most important removal processes of oceanic VHCs, aside from chemical and biological degradation and adsorption onto particles (Abrahamsson et al., 2004b). Thus, the marine boundary layer is thought to be one of the most important interfaces for the gas exchange between water and the atmosphere (Bravo-Linares et al., 2007). In this study, the concentrations of VHCs (CHBr2Cl, CHBr3, C2HCl3, and C2Cl4) were measured in the Bohai Sea (BS) and North Yellow Sea (NYS) during the spring of 2014. The spatial distributions of these VHCs were investigated. The environmental factors that influence the concentrations of these gases were studied. Their sea-to-air fluxes were estimated, and the possible sources were assessed. 2. Methods 2.1. Study area
of the gases were studied along the section J, which covers different hydrographic regions, including B18 off the Yalu River estuary, B16, B14, and B12 on the shelf. The surface (0–5 m) and depth (0–5 m, 10 m, 30 m, 50 m and bottom depth) samples were collected using 12 l Niskin bottles mounted on a Seabird conductivity, temperature, and depth (CTD) Rosette (General Oceanics Co.). The environmental and hydrographic conditions of the sampling stations, such as surface seawater temperature and salinity, were obtained from the CTD apparatus. 2.3. Analytical methods The concentrations of VHCs were analyzed immediately after sample collection by using a cryogenic purge and a trap system coupled to a gas chromatograph with an electron capture detector. The seawater sample was drawn into a 100 ml glass syringe without ambient air, and then directly injected into a glass bubbling chamber at 40 °C and stripped with ultrapure nitrogen at a flow rate of 100 ml min−1 for 12 min. The extracted gases passed through two glass tubes containing anhydrous Mg(ClO4)2 and ascarite to remove water vapor and carbon dioxide, respectively. The extracted gases were trapped in an inert stainless steel tube immersed in liquid nitrogen. After extraction, the stainless steel tube was heated, and the desorbed gases were introduced into a GC/ECD (Agilent 6890N) for analysis. The target compounds were quantified based on the retention times and peak areas of calibration standards. The analytical precisions of the target compounds were generally 7%–15% in the routine sample analysis. Their detection limits were 0.1–0.5 pmol l−1. A similar analytical setup was described by Lu et al. (2010). Chl-a was measured fluorometrically using a fluorescence spectrophotometer (F-4500, Hitachi Co., Japan) after filtering 300 ml of seawater through a glass fiber filter (Whatman GF/F, 47 mm diameter) and extracting the seawater with 90% acetone for 24 h in the dark at 4 °C according to the method adopted by Parsons et al. (1984). 3. Results and discussion
2
The BS, which has a total surface area of 77,000 km and an average depth of 18 m, is a shallow semi-enclosed sea and the largest inner sea of China. BS can be divided into four parts, including the Liaodong Bay, Bohai Bay, Laizhou Bay, and Central BS. The NYS is separated from the South Yellow Sea (SYS) by a line between Chengshanjiao of the Shandong Peninsula and Changsangot on the Korean Peninsula (Li et al., 2016). NYS is located between mainland China and the Korean Peninsula and connected to the BS by the Bohai Strait. The hydrographic characteristics of the BS and NYS are heavily affected by circulation, such as that the Yellow Sea (YS) Warm Current, the YS, the BS, and the Korea Coastal Currents (Su, 1998; Lee et al., 2000). A protruding hydrological phenomenon in the NYS is the NYS Cold Water Mass (NYSCWM), which usually occupies the deep and bottom layers of the trough of the central YS (Li et al., 2016). The hydrological conditions of the BS are significantly influenced by over forty river discharges, including the Yellow River, Hai River, Liao River, and Luan River (Ning et al., 2010). Especially, the Yellow River carries substantial quantities of fresh water with large amounts of particulates and nutrients that support the high biological productivity of the BS. The anthropogenic pollution in Chinese and Korean coastal regions affects the ecosystems of the YS and BS (S. Liu et al., 2011). Thus, the environment of this region, particularly along the coast, is crucial in the climate change in the entire China Seas and the surrounding mainland. 2.2. Seawater sampling The cruise was conducted aboard the R/V “Dong Fang Hong 2” in BS and NYS from 27 April to 23 May 2014. The sampling stations are shown in Fig. 1. The horizontal distributions of the four VHCs were investigated in the surface seawater of all the stations, and the vertical distributions
The concentrations of Chl-a in the surface water of the BS and NYS varied from 0.38 μg l−1 to 3.7 μg l−1 with a mean value of 1.74 μg l−1. Fig. 2A indicates that Chl-a exhibited elevated patch concentrations at the Yalu River and Yellow River estuary, and the highest value of Chl-a was observed at B18. By contrast, the concentration of Chl-a was very low at the central region of the NYS. The high concentrations of Chl-a in the river estuaries occurred because the river input increases the levels of nutrients, which could enhance phytoplankton biomass. The NYSCWM (at the bottom of the NYS) and thermocline (20–30 m) were formed during the spring (Yu et al., 2006; Yao et al., 2012; Li et al., 2015). Thermocline, which separates the NYSCWM from its upper water, hinders the vertical transportation of nutrients. The nutrients of the bottom water cannot be brought to the upper layer and thus cannot supply the growth of phytoplanktons (Yang et al., 2015), which may reduce the Chl-a concentration in the central NYS waters. 3.1. Surface water concentrations of the VHCs The concentrations of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 in the surface water of the BS and NYS varied from 0.39 to 35.64, 3.01 to 67.41, 10.48 to 97.49, and 15.76 to 45.18 pmol l− 1 with a mean of 12.77, 26.92, 32.83, and 25.18 pmol l−1. The concentrations of VHCs measured in this study were in the same range as those reported in coastal and oceanic waters (Sing et al., 1983; Zoccolillo and Rellori, 1994; Roy et al., 2011). However, the mean value of C2Cl4 was higher by one order of magnitude than the value reported by Sing et al. (1983) in the Eastern Pacific Ocean. The mean of CHBr2Cl, CHBr3, and C2Cl4 were the same order of magnitude as those recorded in the literature (Zoccolillo and Rellori, 1994; Roy et al., 2011). The difference of VHC
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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Fig. 1. Locations of sampling stations.
concentrations was probably associated with varying source types and water masses. Fig. 2B shows that the CHBr2Cl concentrations varied widely in the investigated area in the BS. The highest concentrations of CHBr2Cl appeared in the Bohai strait, which is an important channel for ship navigation. The activities involved in ship navigation include ship wastewater disposal and disinfection of wastewater by chlorination, which leads to the production of a wide range of halogenated organic compounds (Quack and Wallace, 2003). Moreover, Y. Liu et al. (2011) reported that elevated levels of brominated short-lived halocarbons appear in the seawater after a ship passes near a series of nuclear power plants and treated-water outfalls along the coast. Thus, the result may be attributed to anthropogenic contamination as intense shipping was observed during sampling. In the NYS, the CHBr2Cl concentrations in the surface seawater exhibited distinct and decreasing spatial variation from the coastal to the central region. The highest value of CHBr2Cl at B18 corresponded to the relatively high level of Chl-a in this region, which may be caused by the abundant nutrients discharged from Yalu River, leading to the growth of phytoplankton and the emission of CHBr2Cl. Previous research (Nightingale et al., 1995; Carpenter and Liss, 2000) reported that various volatile halogenated compounds were present in the tissue of some species of algae and the release rates of CHBr3, CHBr2Cl, and CH2Br2 were determined in laboratory experiments. CHBr3 concentrations in the surface seawater during spring showed distinct spatial variation that decreased from the coast to the central area (Fig. 2C). Elevated CHBr3 concentrations were observed in the Yalu River estuarine area with relatively higher Chl-a concentrations. Two possible explanations for these high values could be deduced. On one hand, this region was clearly affected by the terrestrial input. The disinfection of seawater could produce a wide range of byproducts, such as trihalomethanes (Quack and Wallace, 2003; Zhou et al., 2005). On the other hand, high CHBr3 concentrations were caused by river discharges with substantial nutrients that stimulate local phytoplankton growth and the emission of CHBr3. Previous studies indicated that phytoplanktons are the primary producers of halocarbons in oceans (Abrahamsson et al., 2004b). However, the results showed that CHBr3
was produced by algae at significantly higher rates than other bromomethanes (Nightingale et al., 1995; Ekdahl et al., 1998; Quack and Wallace, 2003). The concentration of CHBr3 was evidently low in the central region of NYS because NYSCWM forms during spring (Yao et al., 2012; Li et al., 2015). The concentrations of CHBr3 were affected by NYSCWM. Thus, the spatial distributions of CHBr3 in the study area in spring were caused by the comprehensive action of phytoplankton emission of CHBr3, terrestrial inputs, and water mass movement. Table 1 shows that the relationship between CHBr3 and CHBr2Cl and Chl-a was tenuous, in part because of the variations caused by nutrients and light and in part because of other biological sources (e.g., bacteria), besides phytoplankton or even abiotic photochemical sources. These results indicated that Chl-a was a poor indicator of the biological production of bromochloromethane. The C2HCl3 and C2Cl4 concentrations in the BS were higher than those in the NYS (Fig. 2D and E). High concentrations of C2HCl3 and C2Cl4 were observed in coastal regions, such as at B53 and B56. These results were assumed to be related to the anthropogenic inputs from the river runoff, such as Fuzhou River, which is rich in volatile organic compounds. As reported, the load of chlorinated compounds (e.g., chloroform) and other organic compounds in the Yangtze River were estimated to be between 500 kg d−1 and 3500 kg d− 1 (Müller et al., 2008), and pollutant load threatened the health of the marine ecosystem. Moreover, low salinity values observed at these stations indicated the influence of river runoff. The extremely high concentrations of C2HCl3 and C2Cl4 were observed in the central region of NYS (B23). The relatively high concentration of Chl-a was also detected in this region, suggesting that this area is a possible biogenic source of C2HCl3 and C2Cl4 from specific phytoplankton. A previous study showed that C2HCl3 can be released by phytoplanktons (Abrahamsson et al., 1995). B23 was far from the coastal influence of anthropogenic input. Thus, determining the source of these high levels of C2HCl3 and C2Cl4 in the middle shelf is difficult. The chemical decomposition of C2HCl3 can be ruled out because the compound is very stable against hydrolysis in aquatic environments and has a half-life of a hundred years (Mabey and Mill, 1978; Fogelqvist, 1984). Therefore, further investigation is crucial to identify the source in this region. Moreover, lower concentrations of
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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Fig. 2. Horizontal distributions of Chl-a (μg l−1), temperature (°C) and four kinds of VHCs (pmol l−1) in the surface seawater in spring.
C2HCl3 and C2Cl4 were recorded at B18, which was located in the region of the Yalu River runoff. This finding could be attributed to the adsorption of high contents of total suspended matter (e.g., sand grains, clays and particulate organic matter). Investigations in estuaries (Dewulf et al., 1996; Roose et al., 2001; Christof et al., 2002) indicated the adsorption of VHCs is in relation to total suspended matter. Furthermore, Duinker et al. (1982) and Christof et al. (2002) found that increasing
Table 1 Correlation coefficients of linear regression analyses between concentrations of the four VHCs and other oceanographic parameters in spring. Variables
CHBr2Cl
CHBr3
CHBr3 C2HCl3 C2Cl4 Temperature Salinity Chl-a
0.363⁎ 0.118 0.314 0.155 −0.351⁎⁎ 0.123
0.029 −0.155 −0.167 0.100 0.125
C2HCl3
C2Cl4
number of halogen atoms in the water could increase the adsorption to particles. Another reason was that the temperatures of the seawater were low than that of others. These results suggest that the source and outcome of these halocarbons in marine environments are extremely complex. In summary, the surface distribution patterns of the four VHOC were complicated and controlled by riverine runoffs, various current systems, anthropogenic input from terrestrial pollutants (e.g., water chlorination and sewage discharge), and biological processes, such as phytoplankton production. Moreover, oceanographic environmental factors, such as sea surface temperature and salinity, might also play an important role in controlling VHC distributions. 3.2. Vertical distributions of the VHCs
⁎ Correlation is significant at the 0.05 level (2-tailed). ⁎⁎ Correlation is significant at the 0.01 level (2-tailed).
0.438⁎⁎
−0.449⁎⁎ −0.008 0.088
0.010 −0.008 −0.280
This transect extends from the Yalu River estuary to the Shandong Peninsula, with the Yalu River Diluted Water on the northeast side and the NYSCWM dispersing out in the middle shelf. Fig. 3 shows the depth profiles of temperature, salinity, Chl-a, and the four kinds of VHCs at transect J.
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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5
Fig. 3. Depth profiles of temperature (°C), salinity (‰), Chl-a (μg l−1) and 4 kinds of VHCs (pmol l−1) at transect J.
Fig. 3 indicates that the vertical distributions of the VHCs along transect J were controlled by various factors, including the source strengths of the VHCs and water masses (e.g., Yalu River and NYSCWM). Temperature and salinity, which are influenced by NYSCWM, indicate the occurrence of layer phenomenon. The isotherm of upper portions was concave and bowl-like. The isotherm in the lower part was epirelief and hat-like. The salinity distribution appeared like a saddle. The lower temperature and the higher salinity, which are significantly affected by the NYSCWM, were observed in the bottom water mass (Zhang et al., 2014). B18 of transect J indicated a lower temperature
and salinity in the upper water column than in B16 because of the influence of the cold Yalu River water. The vertical distributions of the concentrations of the four VHCs along transect J were complicated because of the strong influence of the northerly monsoon, complex water masses, and variable point sources of these gases. High concentrations of the four VHCs were observed in the upper layer of the water column, whereas low concentrations were detected in the middle layer (Fig. 3). In the upper mixed layer, the CHBr2Cl concentrations generally decreased with distance from the coast. Relatively high concentrations of
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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CHBr2Cl in the water column were observed in the bottom water at B12. Those findings suggested the existence of local sources, such as the emission from sediments. Moore and Tokarczyk (1993) demonstrated that elevated concentrations of CHBr2Cl near the bottom water could be attributed to a sedimentary source, and Fogelqvist (1984) confirmed this finding and proposed that it resulted from the degradation of naturally occurring brominated organics. The CHBr3 concentrations decreased from the surface to bottom along the east part of transect J. In the water column, the CHBr3 and Chl-a concentrations (Fig. 3C, E) decreased gradually from inshore to offshore with the maxima of CHBr3 and Chl-a appearing in the upper mixed layer and at the coastal stations (e.g., B18). These results suggested the influence of the expansion of the diluted water of the Yalu River. Evidently, high CHBr3 concentration was detected at the bottom water at B12. Therefore, the considerably high concentrations of CHBr3 may also be related to the emission from organically enriched sediments. The production of benthic organisms was supposed to contribute to the elevated CHBr3. The production of VHCs from marine organic aggregates was demonstrated by Hughes et al. (2008). Abrahamsson and Ekdahl (1996) attributed the high concentration of halocarbon in near-bottom water to the production by benthic organisms. Further work is required to determine the exact source and production mechanisms of these VHCs. The distribution of C2HCl3 was similar to the distribution of temperature in the transect J, with the highest concentration in the surface water and the lowest concentration in the middle water at B14, suggesting the effect of the NYSCWM. The distribution of C2Cl4 decreased significantly and gradually from inshore to offshore in the transect J. The highest concentration was observed in the surface water at B18, which is affected by the Yalu River discharge, and the lowest concentration was detected in the middle water at B14, which is influenced by the NYSCWM. 3.3. Relationships between VHCs and oceanographic parameters Table 1 shows the correlation coefficients of the linear regression analyses between concentrations of the four VHCs and other oceanographic parameters in spring. A linear relationship between CHBr2Cl and CHBr3 (R = 0.363, P = 0.023) in this cruise, as previously reported by Chuck et al. (2005), was also confirmed in the study (Table 1). A linear relationship between C2HCl3 and C2Cl4 (R = 0.438, P = 0.006) was observed in this cruise in spring. Those results indicated that C2HCl3 and C2Cl4 share some common sources or removal mechanisms. However, they may also have their own specific producers. The solubilities of the gases generally decrease with increasing water temperature; hence, a negative correlation between temperature and the concentration of C2HCl3 was observed. Correlation analyses showed significant relationships exist between the salinity and CHBr2Cl concentrations in the surface seawater, suggesting the influence of salinity. Linear regression analysis showed no clear correlation between the VHCs and Chl-a concentration in the surface water, which was consistent with previous investigations (Yang et al., 2015; Yuan et al., 2016). These results suggested that Chl-a may be a poor proxy for determining the source of VHCs. The selected halocarbons did not indicate any significant correlation with each other in seawater. 3.4. Sea-to-air flux calculations The fluxes of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 were calculated using the following equation (Liss and Slater, 1974): F ¼ KW ðCW −Ca =HÞ
ð1Þ
where F is the flux (nmol m−2 d−1); Kw (m d−1) is the gas transfer velocity; Cw and Ca are the concentrations of gases in seawater and atmosphere, respectively. H is the temperature dependent Henry's
Law constant, which is derived from the non-dimensional value given by Moore (2000) for CHBr2Cl, CHBr3, C2HCl3, and C2Cl4. Kw was calculated for CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 as described by Liss and Merlivat (1986), respectively. KW ¼ 0:17u10 ðSc=600Þ−2=3 0bu10 ≤3:6
ð2Þ
KW ¼ ð2:85u10 −9:65ÞðSc=600Þ−1=2 3:6bu10 ≤13
ð3Þ
KW ¼ ð5:9u10 –49:3ÞðSc=600Þ−1=2 u10 N13
ð4Þ
where u is the wind speed (m s−1), and Sc is the Schmidt number. The Schmidt numbers for CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 were calculated using the equation of Khalil et al. (1999). The four VHCs did not exhibit in situ measured atmospheric concentrations. The global background atmospheric concentrations of 0.12 pmol l− 1 for CHBr2Cl, 0.30 pmol l−1 for CHBr3, 0.14 pmol l−1 for C2HCl3, and 0.77 pmol l−1 for C2Cl4 (Reifenhäuser and Heumann, 1992; Schall and Heuann, 1993; Carpenter et al., 2003; Rivett et al., 2003; Zhou et al., 2005; Chuck et al., 2005), respectively, to represent their atmospheric concentrations over the study area. The sea-to-air fluxes of the four VHCs across the air–sea interface in the study area were calculated according to the above-described equation and reference atmospheric concentrations of the four VHCs. The mean sea-to-air fluxes (range) of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 were estimated to be 47.17 (−1.16–178.82), 56.63 (−24.07–356.70), 162.56 (2.23–977.83), and 104.37 (1.64–398.39) nmol m−2 d−1, respectively. In Fig. 4, the fluxes of the gases exhibited considerable spatial variability and varied following the wind speeds at most stations. For instance, extremely high fluxes at B36, B56, B64, and BS3 coincided with high wind speeds, whereas the low fluxes of the gases were found at B07, B31, and B50, which had low wind speeds. The fluxes of VHCs at B57 with high wind speeds were insufficient because of the low VHC concentrations and low sea surface temperature. Thus, the spatial variability of the sea-to-air fluxes of VHCs was largely influenced by wind speed, as well as their concentrations and the sea surface temperature. Compared with those reported in the Gran Canaria and Tropical Atlantic coast, the CHBr3 mean flux in the present work was approximately onetenth of those reported by Zhou et al. (2005) in the Gulf of Maine, which was strongly influenced by local macroalgae. The mean fluxes of C2HCl3 and C2Cl4 in this study were higher than the results presented by Ekdahl et al. (1998) in the Gran Canaria. Our study area was significantly influenced by anthropogenic inputs and phytoplankton production. Therefore, the large discrepancies in the fluxes of the VHCs may be caused by the spatial and temporal variations in anthropogenic and biogenic sources of the VHCs, as well as factors that influenced gas exchange (e.g., wind speed and temperature). The BS and NYS represent one of the continental shelves in the world. Thus, our result indicates that the coastal and shelf regions can significantly contribute to the global oceanic VHC emissions and significantly affect the budgets of reactive chlorine and bromine. The major uncertainty in the assessment of sea-to-air fluxes lies in the calculation of gas transfer velocities, which depends on measured wind speed and sea surface temperature. In this work, the instantaneous wind speeds were used to calculate the gas transfer coefficient for the sea-to-air fluxes at each station. Therefore, the preliminary calculation of the fluxes of the four compounds should be regarded as an instantaneous and approximate assessment. Introducing these atmospheric mixing ratios also leads to larger uncertainties in the sea-to-air flux estimates. Further study, which includes simultaneous measurement of concentrations of these VHCs in both aqueous and gaseous phases, is necessary to accurately estimate the sea-to-air fluxes of these VHCs in the study area.
Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065
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Fig. 4. Relationship between sea-to-air fluxes and wind speed of four VHCs in the BS and NYS in spring.
4. Conclusions
Acknowledgements
The spatial distributions and the sea-to-air fluxes of CHBr2 Cl, CHBr3, C2HCl3, and C2Cl4 in the BS and the NYS, as well as their controlling factors, were investigated. The data presented in this study revealed that CHBr2 Cl, CHBr3 , C2 HCl3 , and C2 Cl4 concentrations widely varied in the investigated area of the BS, whereas their concentrations decreased with distance from the coast in the NYS, with the low values observed in the open sea. The vertical distributions of the four VHCs in the water column varied among stations, with the maxima generally occurring in the upper water column and low concentration appearing in the middle. The complex interactions of the various water masses and differences in the source strength were possible reasons causing the strong patchy pattern of the VHCs distributions in the study area. Environmental factors, such as Chl-a concentration, surface seawater salinity, and temperature, possibly played an important role in controlling the distributions of these VHCs. Phytoplankton might substantially control surface CHBr2Cl and CHBr3 concentrations as the elevated concentrations were observed in the coastal area off the Yalu River estuary where the Chl-a levels were quite high. Further study is required to determine the influence of the oceanographic parameters on the distributions of VHCs. During the study period, the mean sea-to-air fluxes of CHBr2Cl, CHBr3, C2HCl3, and C2Cl4 were approximately 47.17, 56.63, 162.56, and 104.37 nmol m − 2 d− 1 , respectively. Therefore, we suggested that BS and NYS are sources of the four VHCs to the atmosphere.
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Please cite this article as: He, Z., et al., Distribution and sea-to-air fluxes of volatile halocarbons in the Bohai Sea and North Yellow Sea during spring, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.065