Studies On The Distributions And The Sea-Air Fluxes Of Methyl Chloride And Methyl Bromide In The East China Sea And The Yellow Sea

CH3Cl and CH3Br are important atmospheric trace gases and the dominate carriers of atmospheric Cl and Br, which play significant roles in the global warming and atmospheric chemistry. Global oceans are net natural sources of atmospheric CH3Cl and CH3Br. Although coastal regions only occupy a small part of the world ocean, they appear to be responsible for a large part of the oceanic CH3Cl and CH3Br emission. Therefore, studies on the biogeochemistry of CH3Cl and CH3Br in coastal waters will be helpful to estimate the contribution of oceanic emissions to the atmospheric CH3Cl and CH3Br on global scale, and to predict the influence of oceanic emissions to the global climate change.In the present dissertation, distributions, sources, sinks, sea-air fluxes of CH3Cl and CH3Br in the coastal waters of China, i.e. the Yellow Sea (YS) and the East China Sea (ECS), and factors influencing them are studies detailedly and systematically for the first time. The main research work is as follows:1. The purge-and-trap technique coupled with gas chromatography and electron capture detector (GC-ECD) has been developed in our laboratory for the determination of CH3Cl and CH3Br in seawater. The optimum conditions of purge-and-trap system were as follows: 100mL of seawater is purged with high purity nitrogen at 50 mL·min-1 for 12min at 40℃, immerging the adsorption tube (stainless steel, long 30cm, ID 0.8mm ) in liquid nitrogen (-150℃) when purging, heating the tube for 2min in boiling water (100℃) and then enter the GC for separation. The injector and the detector are at 110℃and 275℃, respectively. The flow-rate of the carrier (N2) and the makeup gas for the detector are at 2.1 mL·min-1 and 15 mL·min-1, respectively, and the split ratio is 10:1. The initial temperature of the column is set at 45℃and held for 10min, then increased to 200℃at the rate of 15℃·min-1, held for 5min. The detection limits are 1.8 and 0.2 pmol·L-1, and the precisions are <2% and <8%, and the recovery are 89%-97% and 75%-85% for CH3Cl and CH3Br, respectively. Sample storage experiments have been done to test reliability of the storage procedure used in this study, and the results show that there is no significant change within 24h. Usually analysis was ended in 6h in situ.2. Distributions of CH3Cl and CH3Br are determined during surveys in the YS and the ECS during October-November 2007. The results show that in fall, the average concentrations of CH3Cl and CH3Br in the surface seawater in the Northern Yellow Sea (NYS) are 84.4±18.8 and 6.22±3.17 pmol·L-1, respectively, while those in the Southern Yellow Sea (SYS) are 105±23.2 and 3.93±1.30 pmol·L-1, in the ECS are 83.5±11.8 and 2.74±1.49 pmol·L-1, respectively. The concentrations of CH3Cl and CH3Br are higher in the YS than in the ECS. The distributions of CH3Cl and CH3Br in the YS and the ECS are influenced by the Yangtze River effluent and the anthropogenic activity. Hence the horizontal distributions of CH3Cl and CH3Br concentrations in the surface waters in fall illustrate a decreasing trend from inshore to offshore and a ligule distribution comes into being off the Yangtze River estuary. Analytical results show that there is significant linear correlation between the concentrations of CH3Cl and CH3Br in the surface seawater in the YS and the ECS. There exist significant diurnal variations of CH3Cl and CH3Br concentrations in the surface seawater. The highest concentrations of CH3Cl and CH3Br are present at 12:00-16:00 and 07:00-12:00, respectively, but their variations have no correlation with Chl-a. Results of the vertical profiles of CH3Cl and CH3Br at different sections and stations show that the distribution patterns are influenced by YS Cool Water, Yangtze River diluted water and Kuroshio, so the vertical distributions of CH3Cl and CH3Br along different sections and stations are highly variable.3. The concentrations of CH3Cl and CH3Br in the surface seawater of the YS and the ECS are supersaturated in fall. This means that the YS and the ECS are a net source of atmospheric CH3Cl and CH3Br. The sea-to-air fluxes of CH3Cl and CH3Br in the Northern Yellow Sea (NYS) are estimated to be 190±339 and 50.9±80.8 nmol·m-2·d-1, respectively, while those in the Southern Yellow Sea (SYS) are 289±234 and 22.5±26.7nmol·m-2·d-1, in the ECS are 431±367 and 19.9±34.8 nmol·m-2·d-1, respectively. Because there are common sources and sinks for CH3Cl and CH3Br in seawater, a significant linear correlation is found between the supersaturation concentrations, saturation anomaly (SA) and their fluxes. There is also a significant negative linear correlation between SA of CH3Br concentrations in surface seawater and the surface seawater temperature (SST).