A Comparative Assessment of the Role of Iodine Photochemistry in Tropospheric Ozone Depletion

Over the past two decades, numerous field measurements have established that methyl iodide (CH₃I) is commonly present as a significant trace constituent of the planetary boundary layer, at concentrations in excess of 1 pptv [Lovelock et al., 1973; Rasmussen et al., 1982; Singh et al., 1983; Barnes et al., 1991; Oram and Penkett, 1991]. These field measurements have shown that the marine regions display systematically higher levels of CH₃I than the remote “clean” continental boundary layer, indicating that the oceans provide a source of CH₃I which is emitted directly into the troposphere. CH₃I has also been detected in polar air masses [Rasmussen and Khalil, 1983], in addition to the observation of springtime maxima in the level of particulate iodine in the Arctic [Sturges and Barrie, 1988; Barrie and Barrie, 1990]. The origin of these emissions is believed to be various types of macroalgae and phytoplankton, which may be found in both coastal waters and the open ocean [Lovelock, 1979; Chameides and Davis, 1980; Manley and Dastoor, 1987; Nightingale, 1991]. Although the precise mechanism is uncertain, it seems clear that these organisms produce CH₃I as part of their normal metabolic processes by “methylating” the iodide present in sea-water and, as a result, the water becomes locally super-saturated with CH₃I causing a flux from the aqueous to the gaseous phase [Singh et al., 1982; Nightingale, 1991]. Consequently, in marine locations of high biomass activity, elevated levels of CH₃I (10–40 pptv) have been observed [Rasmussen et al., 1982; Barnes et al., 1991; Oram and Penkett, 1991]. The formation of CH₃I represents, therefore, an important link in the transportation of the life-essential element iodine in the biosphere, providing a means of sea-to-land transport to balance the flux of dissolved I− entering the sea from the land [Lovelock, 1979].