The Distributions And Air-sea Exchange Fluxes Of Methane And Nitrous Oxide In Yellow Sea And The Yangtze River Estuary

N₂O and CH₄ are important atmospheric trace gases, which play significant roles in the global warming and atmospheric chemistry. Global oceans are net natural sources of atmospheric N₂O and CH₄, especially coastal regions which appear to correlate with human activity closely, hence they are responsible for a large part of the oceanic N₂O and CH₄ emissions. Studies on the biogeochemistry of dissolved N₂O and CH₄ in typical coastal sea waters will be helpful to estimate the contribution of oceanic emissions to the atmospheric N₂O and CH₄ on a global scale, and to predict the influence of emissions on the global climate. In the present dissertation, distributions and fluxes of N₂O and CH₄ in the Yellow Sea(YS) ,the North Yellow Sea(NYS) and the Yangtze River Estuaries, are studied.Distributions and fluxes of nitrous oxide and methane were determined during the survey on the YS and North YS in April-2006 and August-2006. The results show that the average nitrous oxide concentrations in the surface and bottom waters of the YS in April are 12.02±1.42 and 12.57±2.49nmol/L, respectively. The average nitrous oxide concentrations in the surface and bottom in the North YS in August are 112.00±3.22 and 13.74±3.19 nmol/L, respectively. The distributions of nitrous oxide in the surface and bottom waters illustrate a decrease of nitrous oxide concentration on shore to offshore, particularly in the waters north of the Yangtze River Estuary and the Yalu River Estuary, both showed high N₂O due to the influence of low salinity river waters.The surface water of some stations in YS is unsaturated in April while the surface water of NYS is supersaturated in August. Liss and Merlivat relationship (LM86) and Wanninkhof relationship (W92) are employed toestimate the sea-to-air N₂O fluxes based on the in situ wind speeds and the measured N₂O concentrations in the surface waters. The results show that nitrous oxide fluxes from the YS in April are 1.20±6.02 and 2.98±9.63μmolm-2d-1 by LM86 and W92, respectively, and nitrous oxide fluxes from the NYS in August are and 16.18±17.85μmolm-2d-1 by LM86 and W92, respectively. Therefore, N₂O fluxes from the YS show obvious seasonal variations, with fluxes in August higher than those in April by a factor of 7.7.Distributions of the methane concentrtions during the two surveys on the NYS in August 2006 and in January 2007 show a same feature that is the decrease of concentration along the freshwater plume from the Yalu River estuary to the open sea. The methane concentration of the Yellow Sea in August is far higher than that in January because the land runoff in January is much less than in August and the biological activity has declined and is not favorable to the production of CH₄ in low temperature. The fluxes of the methane from the NYS in August 2006 and in January 2007 are 0.32±0.95 and 11.64±10.35μmolm-2d-1 by LM86, respectively.Therefore, CH₄ fluxes from the NYS show obvious seasonal variations, with fluxes in August higher than those in January by a factor of 36.Distributions and fluxes of nitrous oxide and methane were determined during the survey on the Yangtze River estuaries and its adjacent areas in June, August and October 2006. The results show a same feature that is the decrease of concentration along the freshwater plume from the river mouth to the open sea. High N₂O concentrations were found in bottom water because of the bottom sediment resuspension in Hangzhou Bay in August and October cruises. The vertical distribution of N₂O and dissolved oxygen in water column show good mirror image at some stations.The surface water of the Yangtze River estuaries is supersaturated with N₂O in the three surveys, which indicates that the Yangtze River estuaries and its adjacent areas are net sources of atmospheric nitrous oxide. The N₂O air-sea exchange fluxes show seasonal variations in Yangtze River estuary and its adjacent waters, with fluxes in August higher than those in June and October by factors of 5.1 and 1.6, respectively.The CH₄ concentrations in the bottom water is slightly higher than those in surface waters in Yangtze River estuary and its adjacent waters which shows the existence of methane source in bottom water. The horizontal distribution of CH₄ show a same feature that is the decrease of concentration along the freshwater plume from the river mouth to the open sea. High CH₄ concentrations were found in bottom waters because of the bottom sediment resuspension in Hangzhou Bay. The estimated average CH₄ fluxes in June and October using long term wind speeds and LM86 relationship are15.9±18.4 and 19.67±27.05μmolm-2d-1, and the average N₂O fluxes in June, August and October are 4.10±1.25,20.86±17.70 and 13.17±5.89μmolm-2d-1 respectively.Therefore, the Yangtze River estuaries and its adjacent areas are strong net sources of atmospheric methane.