GEOPHYSICAL RESEARCH LETTERS, VOL. 11, NO. 4, PAGES 353-356, APRIL 1984 A COMPARISON OF PHOTOLYSIS AND SUBSTITUTION DECOMPOSITION RATES OF METHYL IODIDE IN THE OCEAN Rod G. Zika and Louis T. Gidel University of Miami, Rosenstiel School of Marine and Atmospheric Science 4600 Rickenbacker Causeway, Miami, Florida 33149 Douglas D. Davis Georgia Institute of Technology, School of Geophysical Science Atlanta, Georgia 30332 Abstract. Measurements of the quant•n yield of the west coasts of North and South America between 29N methyl 'iodide (CH•I)in seawater and other solvents and 29S in December, 1981. They found a lack of cot- were made in order-to determine the potential of direct relation between the concentrations of CH_I and CH_C1, photolysis to compete in global surface waters with the particularly at depth, and concluded that their•ata decomposition of CH•I via S.2 (substitution, nucleo- provided no direct support for the hypothesis that philic, btmolecular)•reaction•with C1-ion. Asa de- chloride ion reactions with methyl iodide may be the composition pathway it is potentially significant be- dominant oceanic source of methyl chloride. Methyl cause photolysis does not yield methyl halides as pro- chloride and methyl bromide were found to be pos- ducts and may thus modulate the degree of supersatura- itively correlated. The mean supersaturations for tion of surface water with respect to methyl chloride methyl iodide, methyl chloride and methyl bromide were and methyl bromide. However the reduction in quantum 275%, 340% and 250% respectively. yield (fgctor of 10) and the large hypsochromic shift Thephotolysis of methyliodide in seawater may be (116 cm -•) for methyl iodide in going from nonpolar a missingfactor in the sea-air chemistry of iodine solvents to water reduces the importance of the direct species. Numerous studies on the photochemistry of photolysis in surface waters over most of the globe. CH_I in the gas phase (Meyer, 1968) and in non-polar Because of the temperature dependence of the S• teac- so{vents (Hacobian and Iredale, 1950) demonstrate its tion of C1-, the halflife of CHAI increases with depth facile decomposition by light. and latitude from 5-6 days in tropical surface water to The chemical complexity of seawater and the major 150 days at high latitudes. This increase means that differences in its solvating properties compared to correlations in concentrations of CH3I andits decore- nonpolar solvents donot allowfor an extrapolation of position products, CH_Cl and O/_Br may be destroyed by quantum yields or photolysis products determined in either faster transport or primary production processes solvents other than seawater. This is especially true for CH3I. in the case of iodinespecies which have a very com- plicated ground state chemistry in seawater (Wong, Introduction 1980). Thus a study of the photolysis of CH_I in sea- water was considered necessary for a better understand- The origin, chemistry and fate of methyl iodide ing of the geochemistry of iodine. (CH3I) hasreceived considerable attention recently ' since it was first discovered in seawater and the mar- Experimental ine atmosphere by Lovelock et al. (1973). The lowest concentrations on the globe are found over remote re- Sunlight irradiation of methyl iodide solutions gions while the highest levels are found near indus- were performed in • 24/40 single neck round bottom trialized cities and over the openocean (Chameides and quartz flasks. The flasks were stopperedwith Teflon 2 Davis, 1980; Rasmussen et al., 1982; Singh et al., mmbore stopcocks mounted on • 24/40 Teflon clad 1983). Miyake and Tsunogai (1963)_•ve estimated that joints. The flasks were filled so thatnoair space an oceanic sourceof 0.5 Tg(I) yr would balance the waspresent. Theywere immersed in a water bath on the iodine cycle onEarth. It has been estimated that t_h• roof so that the flask was just below the surface of surface ocean produces CH3I ata rate of 1-2 Tg a• 1 the water. An Eppley Integrating Radiometer (Model (Zafiriou, 1975; Singh et al., 1983; Rasmussen et ., 411) was located directly adjacent to the bath; during 1982; Chameidesand Davis, 1980). The latter authors believe that the marine production rate has the poten- tial to affect the atmospheric chemical processes be- cause of therapid gas phase photolysis of CH3I by sun- light (halflife in the tropical atmosphere, 3.5 days) the experiments it provided a continuous record of the solar flux for wavelengths less than 385 nm. Dark con- trol samples were maintained in the same bath. Several replicate 1-2 ml samples were withdrawn at each sam- pling period from the light exposed flask, the dark to iodine atoms which canin turn react with 03, H 0 control,and from a freshly prepared standard solution. and NO x compounds. Methyl iodide also reacts w•t• 1,1,2-Trichlorotrifluoroethane was added as an internal chlorideion via an S• reactionin seawater to pro- standard in cases where the experiments wereof long duce methyl chloride 'gas which further adds to the duration to all of these solutions. Samples to be halocarbon content of the atmosphere (Zafiriou, 1975). analyzed were removed from the reaction vessels with an Singh et al. (1983) made simultaneousmeasurements of all glass-Teflon syringe and syringe needle. To avoid methyl iodide, methyl chloride and methyl bromidealong introducing a headspace in the reaction vessel a volume of fresh methyl iodide/internal or methyl iodide stan- Copyright 1984 by the American Geophysical Union. dard solution was added to replace the volume removed at each sampling. This was accomplished by sampling Paper number 4L0412. and replacing the sample directly through the orifice 0094-8276/84/004L-0412503.00 of the attached stopcock. Since only small volumes 353