Studies On Distribution, Sources And Sea-to-Air Flux Of Volatile Halocarbons In The China Shelf Seas

Volatile halocarbons （VHCs） with both natural and anthropogenic sources areimportant atmospheric trace gases, which play significant role in the global warmingand atmospheric chemistry. The ocean plays important and complex roles in theglobal biogeochemical cycles of these VHCs, and the marine boundary layer is one ofthe most important places for gas exchange between water/atmosphere. Therefore,Oceans may act as a sink or source of these compounds. Coastal and shelf regions,with the complex marine environment, may be the sources of the VHCs. Anunderstanding of the biogeochemistry of the VHCs in the shelf seas is important inorder to estimate the contribution of oceanic emission to the atmospheric VHCs on theglobal scale, and to predict the influence of oceanic emission to the global climate.The research presented in this dissertation focused on the spatial and temporalvariations of distributions, sources and sea-to-air fluxes of the VHCs in the Chinashelf seas—the East China Sea, the Yellow Sea and Bohai Sea. The main conclusionsare drawn as follows:1. An analytical method for measurement of marine atmospheric volatile halocarbonshas been developed based on canister sampling and an automated preconcentrationgas chromatography-mass spectrometer system. The method detection limit of thegases of interest was in the range of0.5-4.2pptv. The accuracy of the method was inthe range of4-10%with the precision of1-6%.2. The distributions and sea-to-air fluxes of five kinds of VHCs in the East China Seaand the South Yellow Sea during spring, summer, autumn and winter2011areinvestigated. The results showed that the mean （range） concentrations of CHCl₃,C₂HCl₃, C₂Cl₄, CHBr₂Cl and CHBr₃were62.45（10.33-250.50）,29.67（5.22-72.04）,14.55（1.71-38.90）,44.29（1.44-242.81） and134.62（57.36-512.37） pmol L^-1in spring,respectively. Summer concentrations are51.96（15.85-129.04）,10.85（1.70-78.30）,9.47（3.15-29.45）,27.11（6.94-90.20） and57.63（28.46-90.41） pmol L^-1, respectively.Autumn concentrations are63.91（24.63-361.23）,28.46（1.82-85.77）,21.04（9.85-89.3）,20.92（7.98-59.89） and75.91（0.04-537.04） pmol L^-1, respectively, and winter concentration are33.50（6.77-275.58）,35.06（5.30-83.15）,9.86（0.96-40.06）,16.10（10.18-65.22） and19.84（2.99-80.61） pmol L^-1, respectively. In general, withthe exceptions of C₂HCl₃, the mean concentrations of other four kinds of VHCs in thesurface water of the East China Sea and the South Yellow Sea showed a notableseasonal variation with high values in spring and autumn and lowest ones in winter.The spatial distributions of VHCs in the East China Sea and the South Yellow Sea areobvious influenced by the Yangtze River effluent and the oligotrophic Kuroshiowaters as well as the biogenic release. When the distribution patterns are comparedwith each other in different seasons, they are nearly synoptic and generally exhibited adecreasing trend with distant from the coast, with being strongly biased by temporalchange. The vertical distributions of five kinds of VHCs during the spring and antumnare studied. The results showed that the vertical profiles of the five VHCs differamong stations, and the vertical distributions of VHCs were different from each otherduring different seasons, with maxima generally appearing in the mixed layer. Inaddition, the VHCs concentrations exhibited obvious diurnal variation in the surfaceseawater. The highest concentrations of VHCs appearing around noon time may beassociated with biogenic production and photosynthesis of the algae, while therelatively high concentration in the night may be attributed to algal respiration andtidal frequency. Liss and Merlivat relationship （LM86） and Wanninkhof relationship（W92） are employed to calculate the sea-to-air fluxes of VHCs based on the in-situwind speeds and the measured VHCs concentrations in the surface waters. Thesea-to-air fluxes of VHCs showed obvious seasonal variations duo to the influence ofwind speeds and concentrations of the VHCs. For example, the higher VHCsconcentrations in spring and large wind speeds in winter contribute to the large fluxesof VHCs. Sea-to-air fluxes indicated that the East China Sea and the South YellowSea was a source for the fiver VHCs in the atmosphere during the study periods.3. The concentrations of four kinds of VHCs including CHCl₃, C₂HCl₃, C₂Cl₄andCHBr₃in seawater were determined in the East China Sea during23December2009-5January2010, and the sources of the VHCs were studied. The result showedthat the means （ranges） of the concentrations of CHCl₃, C₂HCl₃, C₂Cl₄and CHBr₃inthe surface waters were23.04（6.04-107.81）,18.81（10.67-32.35）,3.72（0.39-9.77）and24.33（13.44-33.01） pmol L^-1, respectively. The concentrations of VHCs nearshore were higher than those in the open sea. In the PN section the verticaldistribution of VHCs had a common feature with the maxima in the upper mixed layer. The distributions of four kinds of VHCs were clearly influenced by the Yangtze Rivereffluent, the Kuroshio water and biological activity. A marked correlation between theC₂HCl₃and C₂Cl₄concentrations was observed in the surface waters, suggesting thatthey might have some common sources. Besides, a positive correlation was foundbetween chlorophyll a and CHBr₃concentrations in the surface seawater, indicatingthat phytoplankton biomass might play an important role in determining thedistribution of CHBr₃in the study area. Our data indicated that the entire East ChinaSea shelf acted as a source for atmospheric CHCl₃, C₂HCl₃and CHBr₃during thestudy period.4. The concentrations of six VHCs in the East China Sea were measured in Novemberand December2010. Mean （range） concentrations of CHCl₃, C₂HCl₃, C₂Cl₄, CH₃CCl₃,CCl₄and CHBr₃in the surface water were16.90（0.40-62.92）,16.27（2.78-83.33）,2.40（0.39-9.33）,32.29（19.72-57.68）,1.70（0.39-8.73） and17.11（4.33-34.46） pmolL^-1, respectively. With the exception of C₂HCl₃, the concentrations of other five kindsof VHCs generally exhibited a decreasing trend with distance from the coast, with thelow values found in the open sea. The anthropogenic sources contributed to theelevated levels of CCl₄and CH₃CCl₃, whereas a combination of the anthropogenicand biogenic sources might be responsible for the elevated levels of CHCl₃, C₂HCl₃,C₂Cl₄and CHBr₃. In the depth profiles, vertical distributions of the six VHCs in thewater column were complicated, with the maxima occurring at0-100m depths. Themean sea-to-air fluxes of CHCl₃, C₂HCl₃, C₂Cl₄and CHBr₃were estimated to be21.08,29.94,2.05and35.50nmol m-2d-1, respectively, indicating that the East ChinaSea was a source for the four VHCs in the atmosphere.5. The concentrations of8VHCs, including methyl chloride （CH₃Cl）, methyl bromide（CH₃Br）, trichloroethene （C₂HCl₃）, tetrachloroethene （C₂Cl₄）, methyl chloroform（CH₃CCl₃） and three chlorofluorocarbibons （CCl₃F （CFC-11）, CCl2FCClF2（CFC-113）and CCl2FCF3（CFC-114））, were measured using canister sampling technique and anautomated preconcentration gas chromatography-mass spectrometer system in themarine atmosphere of the South Yellow Sea in May2012. The mean （range） marineatmospheric mixing ratios for CH₃Cl, CH₃Br, C₂HCl₃, C₂Cl₄, CH₃CCl₃, CFC-11,CFC-113and CFC-114were606.1（412.0-870.6） pptv,16.0（9.9-22.1） pptv,27.3（9.5-52.6） pptv,29.5（10.7-49.0） pptv,9.1（5.8-14.3） pptv,225.1（213.5-233.7） pptv,77.1（69.0-87.9） pptv and18.8（16.7-24.2） pptv, respectively. The diffusion andtransportation of terrestrial pollutants from the coastal area play an important role in the source of selected halocarbons over the South Yellow Sea. Our data indicated thatCH₃Cl, CH₃Br and CFCs had strong local sources from the Shandong Peninsula. Thebackward trajectory analysis suggested that enrichment of CH₃CCl₃in the south of thestudy area might be caused by the long-range transportation of air-masses from theChina mainland. Our results implied the signs of successful phase-out of CFCs andCH₃CCl₃, but substantial releases of C₂HCl₃and C₂Cl₄. The sea-to-air fluxes ofCH₃Cl, CH₃Br and C₂HCl₃were estimated based upon the simultaneous measurementof atmospheric and seawater concentrations of these three gases and the resultsconfirmed that coastal and shelf waters constitute important sources of atmosphericCH₃Cl, CH₃Br and C₂HCl₃.6. The distributions and sea-to-air fluxes of the five kinds VHCs are investigated inthe North Yellow Sea and Bohai Sea in May and Nov.2012. The mean （range）concentrations in the surface water of CHCl₃, C₂HCl₃, C₂Cl₄, CHBr₂Cl and CHBr₃inspring are402.62（63.06-926.72）,18.99（3.65-34.21）,12.61（0.84-28.29）,4.00（0.96-11.28） and20.62（3.29-79.87） pmol L^-1, respectively, whereas those in theautumn are189.13（11.31-310.99）,55.69（17.46-136.54）,41.11（2.67-78.00）,20.35（5.12-53.65） and46.18（15.86-94.54） pmol L^-1, respectively. The results show the fiveVHCs concentrations show obvious seasonal variations. The concentrations of CHCl₃in spring are higher than that in autumn, while the concentrations of C₂HCl₃, C₂Cl₄,CHBr₂Cl and CHBr₃in autumn are higher than that in spring. The anthropogenicsource as well as biogenic source may contribute to the seasonal variations of the fiveVHCs. The high concentrations of CHCl₃in the spring may be related toanthropogenic input, whereas the high concentrations of C₂HCl₃, C₂Cl₄, CHBr₂Cl andCHBr₃may be associated with a combination impact of biogenic release andanthropogenic input. Overall, the distributions of the five VHCs in the surface waterin the North Yellow Sea and Bohai Sea during spring and autumn show significantspatial variations, with high concentrations appearing in the Yellow River estuary,southwest of the Liaodong Peninsula as well as the northwest and east of theShandong Peninsula. Elevated concentrations of the VHCs might be associated withterrestrial input such as Yellow River runoff, biological production as well as thecomplex water masses. Correlation analyses have been used to investigate possiblecontrols on the concentrations of these gases. The CHBr₂Cl concentrations appearedto be significantly correlated with chlorophyll a levels in the study area duringautumn, while other VHCs show no relationship with chlorophyll a during both seasons, suggesting that biological production are not the dominant source of theVHCs. For the physical parameters, sea surface temperature was positively correlatedwith CHBr₂Cl during spring, indicating that the distributions of CHBr₂Cl may beinfluenced by sea surface temperature. A significant correlation was observedbetween C₂HCl₃and C₂Cl₄as well as between CHBr₂Cl and CHBr₃, implied that thesource and sink patterns of C₂HCl₃and C₂Cl₄as well as CHBr₂Cl and CHBr₃in thesurface water were similar. The spatial variability in the sea-to-air fluxes of the fiveVHCs was largely controlled not only by the wind speed, but also by their surfaceconcentrations. Sea-to-air fluxes indicated that the North Yellow Sea and Bohai Seawas a source for the fiver VHCs in the atmosphere during the study periods.7. The temporal and spatial distributions of these compounds vary significantly in theEast China Sea, Yellow Sea and Bohai Sea. The mean annual concentrations of CHCl₃,C₂HCl₃and C₂Cl₄in the East China Sea and South Yellow Sea are higher than thosein the North Yellow Sea and Bohai Sea, while the mean annual concentrations ofCHBr₂Cl and CHBr₃in the East China Sea and South Yellow Sea are lower than thosein the North Yellow Sea and Bohai Sea. These variations are attributable to thedifference of these areas in latitude and longitude and environment, especially in theaspects of primary productivity and the variations of water masses and the influenceof anthropogenic activity. Based on the average fluxes of VHCs and the area of theEast China Sea and the South Yellow Sea, the annual oceanic emissions of CHCl₃,C₂HCl₃, C₂Cl₄, CHBr₂Cl and CHBr₃are estimated to be0.45Gg Cl yr^-1,0.17Gg Clyr^-1,0.12Gg Cl yr^-1,0.23Gg Br yr^-1and0.71Gg Br yr^-1by LM86, respectively.Although the East China Sea and the South Yellow Sea occupies only0.27%of thetotal world ocean area, the contribution of the East China Sea and the South YellowSea to the global oceanic emissions of CHCl₃, C₂HCl₃, C₂Cl₄, CHBr₂Cl and CHBr₃isestimated to be1.4%,8.5%,6%,5%and0.8%, respectively, which means that thecoastal shelf regions may contribute significant amount to the global oceanicemissions of these gases.