Initial Source of Halogens

Although the release of Cl by this reaction is limited to the areas under direct influence of polluted air, such as coastal zone of large cities and harbors where ship emissions prevail, the contribution of this reaction is thought to be large even globally since the condition applies to many coastlines worldwide. Recently, CINO2 has been observed not only in the coastal zone (Osthoff et al. 2008), but 

On the other hand, the reaction of HNO3 and sea salt (Sect. 6.2.5) 

As mentioned in Sect. 7.5.4, CI2 has been observed sporadically in the marine boundary layer. As for the process of the release of CI2, the surface reaction of OH on deliquescent sea salt has been confirmed experimentally (Oum et al. 1998a Knipping et al. 2000). While the reaction of O3 with sea salt (see Sect. 6.2.1) is known to be the most important process of the release of Br2 as will be described later, it has been reported that the reaction of O3 with Fe -doped NaCl releases CI2 (Sadanaga et al. 2001), and the possibility of CI2 release after the depletion of Bt2 has been suggested. Since rather ubiquitous distribution of CI2 in the marine boundary layer has been known recently, further studies are necessary for the processes of the CI2 release into the atmosphere. Since the absorption spectrum of CI2 extends to visible region (Fig. 4.36) and it is easily photolyzed to release Q atoms (see Sect. 4.4.1), it is an important active species in the troposphere. 

On the other hand, since the absorption cross section of tribromomethane (bromoform, CHBt3) released from seaweed is small in the spectrum region of actinic flux (GiUotay et al. 1989), it is not a very important source of photoactive bromine species in the troposphere. 

The absorption spectrum of iodomethane is shown in Fig. 4.32 in Chap. 4. Organoiodine compounds including iodomethane have strong absorption in the tropospheric actinic flux region so that they are easily photolyzed to release I atoms (see Sect. 4.3.8 for CH3I). 

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