Short‐lived alkyl iodides and bromides at Mace Head, Ireland: Links to biogenic sources and halogen oxide production

Abstract

Abstract. Automated in situ gas chromatograph/mass spectrometer (GC/MS) measurements of a range of predominantly biogenic alkyl halides in air, including CHBr3, CHBr2C1, CH3Br, C2HsBr, CH3I, C2HsI, CH2IC1, CH212, and the hitherto unreported CH2IBr were made at Mace Head during a 3-week period in May 1997. C3H7I and CH3CHICH3 were monitored but not detected. Positive correlations were observed between the polyhalomethane pairs CHBr3/C•r2C1 and CHBr3/CH2IBr and between the monohalomethane pair CH3I/C2HsI, which are interpreted in terms of common or linked marine sources. During periods when air masses were affected by emissions from local seaweed beds, the concentrations of CHBr3, CH2IC1, and CH2IBr not only showed remarkable correlation but also maximized at low water. These are the first field observations to provide evidence for a link between the tidal cycle, polyhalomethanes in air, and potential marine production. The calculated total flux of iodine atoms into the boundary layer at Mace Head from organic gaseous precursors was dominated by photolytic destruction of CH212. Photolysis of CH3I contributed less than 3%. The calculated peak flux of iodine atoms during the campaign coincided with the highest measured levels of iodine oxide radicals, as determined using Differential Optical Absorption Spectrometry (DOAS).

Distribution and sea‐air fluxes of biogenic trace gases in the eastern Atlantic Ocean

“…The Henry's law constant for CH3I has been calculated from the relationship of Elliott and Rowland [1993], and C,, for CH3I was taken to be the mean atmospheric concentration measured during the cruise (0.6 pptv). This value is in good agreement with mixing ratios of CH3I observed at Mace Head during the same period, 0.12-1.47 pptv [Carpenter et al, 1999]. Daily atmospheric NMHC concentrations were used in these flux calculations.…”

Distribution of methyl chloride, methyl bromide, and methyl iodide in the marine boundary air over the western Pacific and southeastern Indian Ocean.

“…4, Table 1), which was consistent with the previous measurements, 0.3-1.8 pptv at Mace Head (Carpenter et al, 1999); <0.004-2.12 pptv at Spitzbergen (Schall and Heumann, 1993); 0.24-2.0 over Asian Seas and 0.05-5.0 pptv over western Pacific (Yokouchi et al, 1997). Its highest concentration was observed in the tropics as CH 3 Cl and CH 3 Br in this study.…”

“…The compound releases iodine atoms mostly in the troposphere through photolytic decay (Lovelock et al, 1973;Zafiriou, 1974), leading to ozone destruction in the boundary layer through rapid production of IO radicals and regeneration of I atoms on photolysis (Chameides and Davis, 1980;McFiggans et al, 2000). Observations up to several pptv IO in the marine boundary layer have been reported (Carpenter et al, 1999). Biological activity in surface waters of the ocean is considered to be a major source of CH 3 I (Singh et al, 1983;Gschwend et al, 1985;Manley and Dastoor, 1988;Reifenhauser and Heumann, 1992), although photochemical production in the open ocean has also been suggested as a dominant pathway (Moore and Zafiriou, 1994;Happell and Wallace, 1996).…”

Marine organohalogens in the atmosphere over the Atlantic and Southern Oceans

“…Methyl iodide is the only known organoiodine to be ubiquitous in the open ocean atmosphere, although additional, more reactive iodine atom precursors such as CH 2 I 2 and CH 2 ClI have been detected in coastal air [Carpenter et al, 1999. At typical concentrations, CH 3 I generates only fractions of a ppt of IO .…”