A new analysis of tropospheric iodine chemistry suggests that under certain conditions this chemistry could have a significant impact on the rate of destruction of tropospheric ozone. In addition, it suggests that modest shifts could result in the critical radical ratio HO2/OH. This analysis is based on the first ever observations of CH3I in the middle and upper free troposphere as recorded during the NASA Pacific Exploratory Mission in the western Pacific. Improved evaluations of several critical gas kinetic and photochemical rate coefficients have also been used. Three iodine source scenarios were explored in arriving at the above conclusions. These include' (1) the assumption that the release of CH3I from the marine environment was the only iodine source with boundary layer levels reflecting a low-productivity source region, (2) same as scenario 1 but with an additional marine iodine source in the form of higher molecular weight iodocarbons, and (3) source scenario 2 but with the release of all iodocarbons occurring in a region of high biological productivity. Based on one-dimensional model simulations, these three source scenarios resulted in estimated I x (I x =I + IO + HI + HOI + 21202 +INOx) yields for the upper troposphere of 0.5, 1.5, and 7 parts per trillion by volume (pptv), respectively. Of these, only at the 1.5 and 7 pptv level were meaningful enhancements in 03 destruction estimated. Total column 03 destruction for these cases averaged 6 and 30%, respectively. At present we believe the 1.5 pptv I x source scenario to be more typical of the tropical marine environment; however, for specific regions of the Pacific (i.e., marine Upwelling regions)and for specific seasons of the year, much higher levels might be experienced. Even so, significant uncertainties still remain in the proposed iodine chemistry. In particular, much uncertainty remains in the magnitude of the marine iodine source. In addition, several rate coefficients for gas phase processes need further investigating, as does the efficiency for removal of iodine due to aerosol scavenging processes. IntroductionOf the trace gases in the troposphere, ozone, together with the free radicals generated by its photolysis, is most responsible for defining the oxidizing capacity of the troposphere. Within the troposphere, the mixing ratio of this trace gas is influenced by both transport and photochemical processes [e.g., Fabian and Pruchniewicz, 1977; Mahlman et al., 1980; Chameides and Walker, 1973; Fishman and Crutzen, 1977; Liu et al.,1980]. Conventional thinking suggests that it is the reaction of peroxy radicals, (e.g. HO 2 CH302, and RO2, where "R" is any organic grouping) with NO to produce the product species NO 2 that forms the basis of photochemical 03 formation. Photolysis of NO 2 leads to the release of an O atom which, via reaction with 02 , results in the formation of one net 03 molecule. Photochemical destruction occurs when the 03 photolysis product O(•D) reacts with H20 to produce two hydroxyl radicals, OH, or when hydroperoxyl HO 2...