The Distributions And Sea-to-air Fluxes Of Methyl Iodide And Production By Marine Phytoplankton

Volatile halogenated organic compounds (VHOC) are important atmospheric trace gases which play significant role in the global warming and atmospheric chemistry.Methyl iodide is a major carrier of gas phase iodine from the ocean into the atmosphere. In the atmosphere, methyl iodide is photo dissociated by near-UV light, and participates in ozone destruction and aerosol formation in the troposphere.In the ocean, the production sources and sinks, including marine phytoplankton and air-sea flux, of methyl iodide must be investigated to improve the understanding of the methyl iodide global cycle.The research presented in this dissertation focused on the spatial and temporal variations of distributions, sources and sea-to-air fluxes of themethyl iodidein the East China Sea and the Yellow Sea. Effects of temperature, salinity, illumination, acidification, nutrients and iron concentrations on methyl iodide production by phytoplankton culture were also studied. The main conclusions are drawn as follows:1. An analytical method for measurement of methyl iodide in the ocean has been developed based on purge and trap gas chromatography. The method detection limit of the gases of interest was in the range of 0.01-0.05 pmol L-1. The accuracy of the method was in the range of 2-9% with the precision of 3-11%.2. Methyl iodide concentrations have been measured in surface waters of the Yellow Sea in May 2012 and they ranged from 0.65 to 13.56 pmol L-1 with an average of 4.84pmol L-1.The distribution of methyl iodide was influenced by the Yangtze River, the Yalu River and the Yellow Sea Warm Current, with a decreasing trend from inshore to offshore.Apositive correlation was found between the chlorophyll a and methyl iodide concentrations in the surface seawater. Methyl iodide waspositively correlated with methyl chloride and dimethylsulfide (DMS), suggestedthat these compounds had a common source. The sea-to-air fluxes of methyl iodide in the study area ranged from 0.020 to 170.5 nmol m-2 d-1, with a mean of 27.82 nmol m-2 d-1, indicating that the Yellow Sea was a source of atmospheric methyl iodide during the investigation. The surface methyl iodide concentration and wind speed were two main factors controlling the flux of methyl iodide.3. The distributions and sea-to-air fluxes of methyl iodide in the East China Sea during December 2011, June 2012, October 2012, August 2013 were investigated. The results showed that the mean (range) concentrations of methyl iodide were 1.74 pmol L-1 (0.14-8.63 pmol L-1) in December,3.31 pmol L-1 (1.03-10.15 pmol L-1) in June, 3.88 pmol L-1 (1.66-10.55 pmol L-1) in October,4.06 pmol L-1 (0.48-11.35 pmol L-1) in August. In general, the concentrations of methyl iodide were highest in summer (June) and lowest concentrations in winter (December). The concentrations of methyl iodide in fall were higher than spring. The mean (range) sea-to-air fluxes were 32.84 nmol m-2 d-1 (0.62-260.24 nmol m-2 d-1) in December,80.43 nmol m-2 d-1 (3.46-286 nmol m-2 d-1) in June,35.29 nmol m-2 (0.21-152.91 nmol m-2 d-1) in October,124.58 nmol m-2 d-1 (3.25-388.25 nmol m-2 d-1) in August. The methyl iodide sea-to-air fluxes in summer were higher than fall, winter, and influenced by wind and temperature. Aclear diurnal variation of methyl iodide was observed during August in 2013. The concentration of methyl iodide increased from 08:00 to 11:00, then decreased from 11:00 to 22:00, showing a similar trend with CFCs, indicating that the concentration of methyl iodide were influenced by light, phytoplankton and physical process.4. The production of methyl iodide was studied in different growth stages of Phacodactylum tricornutum, Skeletonema costatum, Navicula salinrum, Prorocentrum donghaiense, Phaeocystis globosa. Moreover we have studied the effects of varying of salinity, temperature, illumination intensity and nutrients on cell abundances, methyl iodide concentrations and the rate of production. On the whole, Phaeocystis globosa produced the largest amount of methyl iodide. The effectof temperature on the growth and methyl iodide production was more prominent than that of salinity or light intensity. The range (average) production rates of methyl iodide at 15℃,20℃,25℃ were 0.89～2.73×10-8 pmol cell-1 d-1(1.75×10-8 pmol cell-1 d-1),0.79～3.98×10-8 pmol cell-1 d-1(2.46×10-8 pmol cell-1 d-1),1.19～4.69×10-8 pmol cell-1 d-1(3.07×10-8 pmol cell-1 d-1).Low or high concentrations of nitrate and Fe3+ had an inhibition function on methyl production of Phaeocystis globosa. The promotion function of methyl iodide production by Phaeocystis globosa was caused by high temperature, high light intensity, high salinity and suitable nitrate.Ocean acidification had an inhibition function on methyl production of Phaeocystis globosa.