Reactive and organic halogen species in three different European coastal environments

[摘要] We present results of three field campaigns using active longpathDOAS (Differential Optical Absorption Spectroscopy) for the studyof reactive halogen species (RHS) BrO, IO, OIO and I₂. Tworecent field campaigns took place in Spring 2002 in Dagebüll atthe German North Sea Coast and in Spring 2003 in Lilia at theFrench Atlantic Coast of Brittany. In addition, data from acampaign in Mace Head, Ireland in 1998 was partly re-evaluated.During the recent field campaigns volatile halogenated organiccompounds (VHOCs) were determined by a capillary gas chromatographcoupled with an electron capture detector and an inductivelycoupled plasma mass spectrometer (GC/ECD-ICPMS) in air and water.Due to the inhomogeneous distribution of macroalgae at the German North SeaCoast we found a clear connection between elevated levels of VHOCsand the appearance of macroalgae. Extraordinarily highconcentrations of several VHOCs, especially CH₃I and CH₃Brof up to 1830 pptv and 875 pptv, respectively, were observed atthe coast of Brittany, demonstrating the outstanding level ofbioactivity there. We found CH₂I₂ at levels of up to20 pptv, and a clear anti-correlation with the appearance of IO.The IO mixing ratio reached up to 7.7±0.5 ppt(pmol/mol)during the day, in reasonable agreement with model studiesdesigned to represent the meteorological and chemical conditionsin Brittany. For the two recent campaigns the DOAS spectra wereevaluated for BrO, OIO and I₂, but none of these species couldbe clearly identified (average detection limits around 2 ppt,3 ppt, 20 ppt, resp., significantly higher in individual cases).Only in the Mace Head spectra evidence was found for the presenceof OIO. Since macroalgae under oxidative stress are suggested tobe a further source for I₂ in the marine boundary layer, were-analyzed spectra in the 500–600 nm range taken during the1998 PARFORCE campaign in Mace Head, Ireland, which had notpreviously been analyzed for I₂. We identified molecular iodineabove the detection limit (~20 ppt), with peak mixingratios of 61±12 ppt. Since I₂ was undetectable duringthe Brittany campaign, we suggest that iodine may not be releasedinto the atmosphere by macroalgae in general, but only by aspecial type of the laminaria species under oxidative stress. Onlyduring periods of extraordinarily low water (spring-tide), theplant is exposed to ambient air and may release gaseous iodine insome way to the atmosphere. The results of our re-analysis ofspectra from the PARFORCE campaign in 1998 support this theory.Hence, we feel that we can provide an explanation for thedifferent I₂ levels in Brittany and Mace Head.

[学科分类] 大气科学