Novel biogenic iodine‐containing trihalomethanes and other short‐lived halocarbons in the coastal east Atlantic

Abstract

Reactive halogen photochemistry and its impact on tropospheric oxidant levels have recently attracted intense research interest following the observation of the iodine oxide radical at midlatitudes. During September 1998, short‐lived organoiodines including CH 3 I, C 2 H 5 I, CH 2 ICl, CH 2 IBr, CH 2 I 2, and the hitherto undetected CHIBr 2, as well as the organobromines CHBr 3, CH 2 Br 2, CHBr 2 Cl, CH 3 Br, and C 2 H 5 Br, were measured in air and seawater at and around Mace Head, on the west coast of Ireland. The release rates of organic bromines and iodines from seaweeds were determined from incubations of 10 species of brown, red, and green macroalgae collected in the intertidal or subtidal zones of the rocky shore. For all the brown algae studied, iodine was released mainly as CH 2 I 2. However, for several seaweeds, the novel iodine‐containing trihalomethanes CHIBr 2 and CHI 2 Cl represented a significant fraction of the released organic iodine. The macroalgae incubation experiments as well as monitoring of the in situ concentrations in a rock pool indicated that natural halocarbon production by seaweeds was stimulated by incident light. The halocarbon fluxes derived from the seaweed incubations, coupled with published detailed biomass surveys, enabled coastal organohalogen seawater concentrations to be estimated. The CHBr 3, CH 2 Br 2, and CHBr 2 Cl concentrations calculated by this method compared well with coastal surface seawater measurements, implying that macroalgae were the major sources of the polybromomethanes. Measured CH 3 Br, CH 3 I, and CH 2 ICl levels were higher than calculated, which may be due to the existence of additional sources. CH 3 Br production by macroalgae accounted for less than 10% of measured levels in coastal waters. Short‐lived iodocarbons such as CH 2 I 2 and CHIBr 2 were depleted in surface seawater compared to calculated levels, implying their photolytic loss within the upper water column.