Stratospheric chemistry iodine

Bureau H, Keppler H, Metrich N (2000) Volcanic Degassing of Bromine and Iodine Experimental Fluid/Melt Partitioning Data and Applications to Stratospheric Chemistry. Earth Planet Sci Lett 183 51... 

Chlorine nitrate CIONO2 and bromine nitrate B1ONO2 are important reservoir molecules formed by the chain termination reactions, CIO + NO2 and BrO + NO2, in the CIO and BrOx cycles in the stratosphere, respectively. Iodine nitrate IONO2 plays a similar role in the in the iodine chemistry in the troposphere. 

The reactions of halogen atoms and radicals are of fundamental importance in stratospheric chemistry (see Sects. 4.4 and 8.2, 8.3, and 8.4), and the halogen cycle is also of interest in the marine boundary layer in the troposphere (see Sect. 7.5). In this section, among the atmospheric reactions of halogen atoms and radicals, fundamental homogeneous reactions of Cl atoms and CIO radicals are described, and the reactions of bromine and iodine atoms and radicals are discussed in the more phenomenological discussions in Chaps. 7 and 8. 

While gas phase chemistry leads to much higher levels of active bromine compared to chlorine, this is even more the case for iodine. Solomon et al. suggested that iodine might be of importance in ozone depletion. At present there is no information on the amounts of iodine in the stratosphere, but heterogeneous reactions will probably not play a significant role. Conversely, fluorine is almost completely in its deactivated form HF, and also heterogeneous reactions have been found to be immeasurably slow. Hence, fluorine species are not expected to influence stratospheric chemistry. 

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 CIO cycles in much the same way as BrO, e.g.,... 

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. 

Ozone in the lower stratosphere may in principle be affected by iodine chemistry (Solomon et al, 1994). The abundance of total iodine in the troposphere is believed to be of the order of pptv, but the fractional partitioning of iodine free radicals (I and IO) is much higher than in the case of other halogens (chlorine and even of bromine see Section 5.6.3). 

Solomon et al. (1994) implicated iodine in lower stratospheric ozone loss as a result of the rapid vertical transport of precursor gases via convection currents, resulting in photolysis and subsequent chemical transformations at altitudes up to 20 km, and concluded that this route would be 3 orders of magnitude greater than O3 loss resulting from chlorine chemistry. 

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