Contribution Of Iodinecontaining Organics

In addition to chlorinated and brominated organics, iodine-containing organics are also emitted into the troposphere, primarily by biological processes in the oceans. Methyl iodide is believed to be the major species emitted, but others such as C1CH2I and CH2IBr may also be generated (e.g., see Cicerone, 1981; Klick and Abrahamsson, 1992; Moore and Tokarczyk, 1992; Schall and Heumann, 1993; Gribble, 1994; Happell and Wallace, 1996; and Carpenter et al., 1999), and ethyl iodide has also been measured recently (Yokouchi et al., 1997).

There is a substantial difference in their tropo-spheric chemistry from that of the chlorine and bromine compounds, however (e.g., see Huie and Laszlo, 1995). The carbon-halogen bond is very weak, 57 kcal mol~' in CH3-I compared to 70 kcal molfor CH3-Br, 85 kcal mol"1 for CH3-C1, and 108 kcal mol"1 for CH3-F. In addition, the absorption spectra are red-shifted for the iodine compounds, so that their absorption spectra better overlap with increasing solar intensity. As a result, organoiodine compounds photolyze readily in the troposphere to generate iodine atoms (Calvert and Pitts, 1966). Combined with other fates such as reaction with OH and N03, their tropospheric lifetimes are sufficiently short that they are not expected to reach the stratosphere in sufficient quantities to contribute to ozone destruction.

Because of these rapid removal processes in the troposphere, the contribution of iodine to stratospheric photochemistry has not received much attention. However, Solomon et al. (1994) suggested that rapid transport from the lower troposphere into the upper troposphere and lower stratosphere via convective clouds could provide a mechanism for injecting such compounds into the stratosphere. While the relevant chemistry of iodine is not well known, it would be expected to interact with the C10x cycles in much the same way as BrO, e.g.,

I02C1 (63g)

The overall rate constant for the CIO + IO reaction has been measured to be k(t3 = 5.1 X 10~12e280// cm3 molecule-1 s-1, with a branching ratio of 0.14 + 0.04 for all channels not producing I atoms at 298 K. (Turnipseed et al., 1997). This is in agreement with branching ratios for (63a) of 0.55 ± 0.03, (63c) of 0.20 + 0.02, and (63d) of 0.25 ± 0.02 reported by Bedjanian et al. (1997a).

In addition, BrO-IO cross interactions would be expected; the major channel in this reaction appears to generate Br + OIO, with a branching ratio of ~1 within an uncertainty of ~ 35% (Bedjanian et al., f997b, f998; Laszlo et al., 1997; Gilles et al., 1997). Reaction of IO with H02, O, and NO and photolysis will also occur (DeMore et al., 1997):

Solomon et al. (1994) proposed that below ~20 km, iodine could make a major contribution to 03 destruction if there were f ppt of total iodine in the stratosphere. Episodic transport of iodine compounds to the upper troposphere clearly happens on some occasions, as evidenced by the observation of concentrations of CH3I as high as ~1 ppt at 10-12 km when a typhoon provided strong vertical upward motion (Davis et al., 1996). However, it may be that this is the exception rather than the rule.

For example, Wennberg et al. (1997) used high-resolution spectra taken from the Kitt Peak National Solar Observatory to search for evidence of IO. Combined with simulations using assumed IO chemistry, they conclude that the total stratospheric iodine is ~0.2 ppt, with an upper limit of ~0.3 ppt. Similarly, Pundt et al. (1998) conclude there must be <0.2 ppt iodine at altitudes <20 km, based on solar spectra obrained using balloon platforms. If these small concentrations based on a few measurements are typical, iodine will not be responsible for significant ozone destruction.

In short, it appears likely that sufficient iodine does not reach the stratosphere to make a significant contribution to ozone destruction.

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