Biogenic_Cloud_Sources_F10_Lect9

Biogenic_Cloud_Sources_F10_Lect9 - Biogenic Cloud Sources...

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Biogenic Cloud Sources Fall 2010, Lecture 9 1
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Alternative Cloud Formation Method In the previous lecture, we saw various ways in which clouds could form There is another way clouds form, and it may be part of a cybernetic system 2
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Sulfur Cycle Sulfur is carried to the ocean as dissolved sulfate ion Sulfur must be returned from the ocean, if the land is not to be depleted of sulfur Traditionally, it has been stated that hydrogen sulfide gas is emitted from the ocean Hydrogen sulfide is familiar to many people as the smell of a rotten egg 3
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Hydrogen Sulfide Hydrogen sulfide stinks and is quite poisonous If the oceans were emitting large quantities of this gas, we would smell it and travel on the oceans might be hazardous It is quickly oxidized to sulfur dioxide in the presence of oxygen, and is unstable in the air It is quite soluble, and should be washed out of the atmosphere quickly It probably would not have time to be carried over the land 4
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James Lovelock James Lovelock, geophysiologist and creator of the GAIA hypothesis, concludes that hydrogen sulfide cannot be the agent for the return of sulfur to the land Instead, Lovelock believes that dimethyl sulfide ((CH 3 ) 2 S), emitted by many marine organisms, might be the carrier gas 5
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Dimethyl Sulfide Dimethyl sulfide, unlike hydrogen sulfide, has a pleasant odor when dilute, and that the smell of the sea, or of fresh caught marine fish, is partly that of dimethyl sulfide Dimethyl sulfide (DMS) is a naturally produced biogenic gas essential for the Earth's biogeochemical cycles Certain species of phytoplankton, microscopic algae in the upper ocean, synthesize the molecule dimethylsulfoniopropionate (DMSP) which is the precursor to DMS 6
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7 DMSP Decomposition    Note the positive charge on the sulfur, and the negative charge on the proprionic acid part of the DMSP molecule DMS Molecule
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Dimethyl Sulfide Decomposition Dimethyl sulfide decomposes slowly to yield sulfate and methane sulfonate Both compounds can undergo further oxidation to yield sulfuric acid (H 2 SO 4 ) and methane sulfonic acid (CH 3 SO 3 H) Rainfall can remove these compounds and deposit them on the ground, but this cycle takes considerably longer than the breakdown of hydrogen sulfide, thus allowing sulfur to return to the land 8
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Lovelock and DMS Lovelock's own work has contributed one clue about dimethyl sulfide Lovelock traveled on the research vessel Shackleton on a voyage from Wales to Montevideo, Uruguay Using a gas chromatograph he measured dimethylsulfide, carbon disulfide, and halocarbon gases He arranged for colleagues to continue this work after he left the ship 9
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Marine Deserts Later, M. O. Andreae made extensive measurements of dimethyl sulfide over the oceans These measurements confirmed the ability of dimethyl sulfide to be the major carrier of sulfur from ocean to land The vast desert” areas, characterized by low biological productivity, are particularly large generators of dimethyl sulfide, and they cover about 40% of the earth's surface 10
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