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

Abstract

Automated in situ gas chromatograph/mass spectrometer (GC/MS) measurements of a range of predominantly biogenic alkyl halides in air, including CHBr 3, CHBr 2 Cl, CH 3 Br, C 2 H 5 Br, CH 3 I, C 2 H 5 I, CH 2 ICl, CH 2 I 2, and the hitherto unreported CH 2 IBr were made at Mace Head during a 3‐week period in May 1997. C 3 H 7 I and CH 3 CHICH 3 were monitored but not detected. Positive correlations were observed between the polyhalomethane pairs CHBr 3/CHBr 2 Cl and CHBr 3/CH 2 IBr and between the monohalomethane pair CH 3 I/C 2 H 5 I, 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 CHBr 3, CH 2 ICl, and CH 2 IBr 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 CH 2 I 2. Photolysis of CH 3 I 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).