The Distribution And Air-Sea Fluxes Of N₂O In The Coastal Waters Of China

N₂O is an important atmospheric trace gas,which plays significant roles in the global warming and atmospheric chemistry.Global oceans are net natural sources of atmospheric N₂O.Coastal regions such as continental shelves,rivers and bays appear to be responsible for a large part of the oceanic N₂O emission.Therefore studies on the biogeochemistry of dissolved N₂O in coastal waters will be helpful to evaluate the contribution of oceanic emission to the atmospheric N₂O.In the present dissertation,distribution,sources,sinks and fluxes of N₂O in the coastal waters of China,i.e.the East China Sea(ECS),the main rivers and estuaries in Hainai and the Sanggou Bay,are studied systematically.The main research work is as follows:1.Distribution and fluxes of N₂O are determined during surveys on the ECS in November-December 2006 and February- March 2007.The results show that the average N₂O concentrations in the surface waters in the ECS in autumn and winter are 8.76±1.30 and 16.17±6.82nmol/L,respectively.In general,N₂O concentrations in the waters of the ECS show obvious seasonal variations,with those in winter higher than in autumn by a factor of 2.N₂O enrichment in the near-bottom waters is found at all stations of the ECS in both autumn and winter, which underlines that there exists important sources of N₂O in deep waters or bottom sediments.The waters near shore may be influenced by the N₂O-rich Yangtze River effluent.Hence the horizontal distributions of N₂O in the surface waters in autumn illustrate a decrease of N₂O concentration onshore to offshore. The horizontal distributions of N₂O in the surface waters in winter illustrate a decrease of N₂O concentration from the northeast to the southwest,with a high N₂O concentration found in the center of the ECS shelf influenced by Kuroshio. The surface water of the ECS is supersaturated in autumn and winter,which indicates that the ECS is a net source of atmospheric N₂O.Liss and Merlivat relationship(LM86) and Wanninkhof relationship(W92) are employed to estimate 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 N₂O fluxes from the ECS in autumn are 0.14-13.2 and 0.65-25.81μmol/m²/d by LM86 and W92, respectively,and N₂O fluxes from the ECS in winter are -11.73-148.32 and 0.23-251.92μmol/m²/d by LM86 and W92,respectively.Therefore,N₂O fluxes from the ECS show obvious seasonal variations,with fluxes in winter higher than those in autumn by a factor of 6.Based on the average N₂O fluxes from the ECS and the area of ECS,the annual N₂O emission from ECS is estimated to be 15.6×10²-27.2×10^-2 Tg N₂O/yr.Although the ECS occupies only 0.2%of the total ocean in area,the contribution of the ECS to the net global sea to air N₂O flux is estimated to be～3%,which means that shelf waters contribute significant amount to the total oceanic N₂O flux compared to open waters.2.Distributions and fluxes of nitrous oxide are determined during two surveys on the Wanquan river,Wechang and Wenjiao River and their estuaries in the east part of Hainan and the adjacent marine areas in December 2006 and August 2007.The results show that the average N₂O concentrations in the surface waters in the east part of Hainan in winter are 17.38±12.0nmol/L.The average N₂O concentrations in the surface waters in rivers of Hainan and its adjacent areas in summer are 22.26±20.6nmol/L.The N₂O concentrations in rivers and estuaries are much higher than those in the adjacent marine areas.The surface water in all stations of the studied areas is supersaturated with N₂O in summer,which indicates that it is a net source of atmospheric N₂O.The average N₂O fluxes from the studied areas in summer are 5.95±4.3 and 10.34±7.7μmol/m²/d by LM86 and W92, respectively.3.Nitrous oxide concentrations in the surface waters of the Sanggou Bay are determined during four surveys between 2006 and 2007,and the results show obvious seasonal variation with the highest concentration occurring in winter and the highest saturation occurring in summer.The surface water of the Sanggou Bay is supersaturated with N₂O during all the year,which indicates that the Sanggou Bay is a net source of atmospheric N₂O.The horizontal distributions of N₂O in the surface waters have the similar features except in spring as following:The N₂O concentrations shows decreases from the western,southern and northern coast to the central part.