Methyl oceanic emission

Methyl iodide is produced by many marine photosynthetic organisms and therefore the ocean is thought to be a major natural source of methyl iodide. Some of this is released to the atmosphere and some reacts with seawater to form methyl chloride. Industrial emissions of methyl iodide may occur in conjunction with its use as a methylating agent and in organic synthesis. Humans are exposed to methyl iodide from the ambient air and from ingesting seafood (United States National Library of Medicine, 1998). 

Biogenic Sulfur Emissions from the Ocean. The ocean is a source of many reduced sulfur compounds to the atmosphere. These include dimethylsulfide (DMS) (2.4.51. carbon disulfide (CS2) (28). hydrogen sulfide (H2S) (291. carbonyl sulfide (OCS) (30.311. and methyl mercaptan (CH3SH) ( ). The oxidation of DMS leads to sulfate formation. CS2 and OCS are relatively unreactive in the troposphere and are transported to the stratosphere where they undergo photochemical oxidation (22). Marine H2S and CH3SH probably contribute to sulfate formation over the remote oceans, yet the sea-air transfer of these compounds is only a few percent that of DMS (2). 

Natural acidity is contributed from emissions of acidic or acidifying compounds from volcanoes (Pyle et al., 1996 Camuffo, 1992), including compounds of S, N, Cl (chloride), and NH3 (ammonia), from the ocean (e.g., methyl sulfonate) (Charlson et al., 1987), and from wetlands (e.g., H2S) (Gorham et al., 1987). 

Some halocarbons are produced by both natural and industrial sources. Unfortunately, the few data concerning the magnitude of the natural sources are conflicting. For example, based on measured concentrations of methyl bromide in the sea and troposphere, Singh et al. (6) estimated that the net annual global flux from ocean to air is about 0.3 teragrams (1 Tg is 1 X 1012 g). This net emission is much larger than that derived from human activities (0.05-0.08 Tg per year) (7, 8). On the other hand, Penkett and co-workers (7) concluded that human activities were the main source of methyl bromide. 

Typical organic precursors for COS, CS2 and the methylated sulfur gases include methionine and cysteine from proteins and isothiocyanates and thiocyanates from plant secondary metabolites. Methanethiol and DMS are also formed in anoxic freshwater sediments from reactions based on H2S and various methyl donors, for example, methoxylated aromatic acids, such as syringic acid from lignins. The rates of DMS emission per unit area are similar for both the oceans and Sphagnum-AommattA wetlands. Only the area of this peat land limited the relative importance of the latter source. 

Methyl chloride, CH3CI 600 ppt Uniform Emissions from ocean, 3 Biomass burning, 0.7 Reaction with OH 1.3 yr... 

Methyl halides such as methyl chloride CH3CI, methyl bromide CHsBr, and methyl iodide CH3I are natural origin species emitted from terrestrial and oceanic sources, but anthropogenic emissions are also important for CH3Br. Among these, CH3I are photolyzed mostly by the actinic flux in the troposphere. 

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