Studies On The Kinetics Of Photochemistry Of Dimethylsulfide And Influencing Factors In Seawater

DMS (dimethylsulfide) is the primary volatile biogenic sulfur compoundemanating from the ocean, and its oxidation products in the atmosphere playimportant roles in the global climate change and the formation of acid rain.Photochemical degradation is one of the most important removal processes of DMS,which influence the concentration and sea-to-air flux of DMS. However, thephotochemical degradation of DMS can be affected by all kinds of complex factorsin seawater. Studies on the photochemical process of DMS as well as influencingfactors would be very helpful to further understand the biogeochemical cycle ofDMS and its climate effect.In the present dissertation, we selected a xenon lamp as the sunlight source inthe laboratory photochemical studies, using a laboratory simulation approach, tostudy the photochemical degradation behaviors of DMS and influencing factorsunder different wavebands of UVB (280,295and305nm), UVA (320,345and395nm) and visible light (435and495nm) bands. Shipboard experiments wereconducted in the East China Sea to ascertain the relative importance of photolysis,biological consumption, and sea-air flux in the removal of DMS from seawater. Themain results are showed as follows:(1) The photo degradation reaction of DMS showed first-order reaction kineticbehavior (ln (Ct/C0)=-kt). The degradation rates of DMS varied a lot underdifferent light wavelengths, with the higher under UVB rather than UVAand visiblelight. The maximum rate of DMS was found under the light wavelength of280nm.Colored dissolved organic matter (CDOM) plays a very crucial role in thephotochemical degradation of DMS, because the interaction between the solarenergy radiation and CDOM is the main source of active molecules. As the photodegradation of DMS going on, all the spectral absorption coefficients of CDOMincreased under different wavelengths, with the maximum at280nm. Moreover, good relationships were found between CDOM absorption coefficients and DMSconcentrations under different light wavelengths, suggesting the important role ofCDOM in the photo degradation of DMS.The transfer ratio of DMS photo degradation into DMSO under the8differentwavelengths were31.3%,28.2%,25.5%,22.5%,27.4%,29.6%,20.4%and24.1%after the irradiation time of six hours. The results showed that the transfer ratios ofDMS were larger under the irradiation of UVB rather than UVA and visible light,with the maximum one under280nm.(2) Shipboard experiments were conducted in the East China Sea during thecruise in June-July2013to ascertain the relative importance of photolysis,biological consumption, and sea-air flux in the removal of DMS from seawater.Throughout the study area, the concentrations of DMS ranged from1.16to11.61nmol·L-1, with an average of4.78nmol·L-1.The horizontal distributions of DMS insummer displayed a general decrease in concentration from inshore to offshore sites,which were obviously affected by anthropogenic activities and terrestrial runoff. Asa result, a significant correlation appeared between DMS and chlorophyll aconcentrations in the surface layer, which suggested that phytoplankton biomassmight play an important role in determining the distribution of DMS in the studyarea.The results of shipboard experiments were agreed with the laboratoryphotochemical studies. As a result, the DMS photolysis rate of UVB-filteredcondition was faster than the photolysis rate under UVA-filtered conditions andvisible light. The DMS photolysis rate in alongshore station were fastest. Since thephotolysis rates of DMS were also directly dependent on CDOM concentration andthe alongshore stations were affected seriously by human behavior.In the subsurface water of the East China Sea, the DMS turnover time forDMS photolysis was consistent with the biological consumption turnover time thatare both about1d. The sea-to-air flux turnover times were0.70-11.90d. For thereason that, the photochemical oxidation of DMS is as important as the microbialdegradation in the migration and transformation process of DMS. These results suggest that DMS photolysis is a significant removal pathway when compared tothe atmospheric removal and bacterial consumption, respectively.The laboratory photochemical studies of the subsurface water of the EastChina Sea were compared with the laboratory photochemical studies of thesubsurface water of the Yellow Sea. As a result, the DMS photolysis rate in the EastChina Sea was slower than the DMS photolysis rate in the Yellow Sea. Although,the concentrations of DMS and the illumination wavelength are both same in thelaboratory photochemical studies, the structure and compositionof matter inseawater are very different in different areas. Therefore, the process ofphoto-oxidation of DMS in surface water is a complex and dynamic process, thereaction rate is affected by the illumination wavelength, the concentrations ofCDOM and the environment of the study area.