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).

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Episodes of elevated bromine oxide (BrO) concentration are known to occur at high latitudes in the Arctic boundary layer and to lead to catalytic destruction of ozone at those latitudes; these events have not been observed at lower latitudes. With the use of differential optical absorption spectroscopy (DOAS), locally high BrO concentrations were observed at mid-latitudes at the Dead Sea, Israel, during spring 1997. Mixing ratios peaked daily at around 80 parts per trillion around noon and were correlated with low boundary-layer ozone mixing ratios.

Abstract. We present results of three field campaigns using active longpath DOAS (Differential Optical Absorption Spectroscopy) for the study of reactive halogen species (RHS) BrO, IO, OIO and I 2 . Two recent field campaigns took place in Spring 2002 in Dagebüll at the German North Sea Coast and in Spring 2003 in Lilia at the French Atlantic Coast of Brittany. In addition, data from a campaign in Mace Head, Ireland in 1998 was partly re-evaluated. During the recent field campaigns volatile halogenated organic compounds (VHOCs) were determined by a capillary gas chromatograph coupled with an electron capture detector and an inductively coupled plasma mass spectrometer (GC/ECD-ICPMS) in air and water. Due to the inhomogeneous distribution of macroalgae at the German North Sea Coast we found a clear connection between elevated levels of VHOCs and the appearance of macroalgae. Extraordinarily high concentrations of several VHOCs, especially CH 3 I and CH 3 Br of up to 1830 pptv and 875 pptv, respectively, were observed at the coast of Brittany, demonstrating the outstanding level of bioactivity there. We found CH 2 I 2 at levels of up to 20 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 studies designed to represent the meteorological and chemical conditions in Brittany. For the two recent campaigns the DOAS spectra were evaluated for BrO, OIO and I 2 , but none of these species could be 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 presence of OIO. Since macroalgae under oxidative stress are suggested to be a further source for I 2 in the marine boundary layer, we re-analyzed spectra in the 500-600 nm range taken during the 1998 PARFORCE campaign in Mace Head, Ireland, Correspondence to: C. Peters (christina.peters@iup.uni-heidelberg.de) which had not previously been analyzed for I 2 . We identified molecular iodine above the detection limit (∼20 ppt), with peak mixing ratios of 61±12 ppt. Since I 2 was undetectable during the Brittany campaign, we suggest that iodine may not be released into the atmosphere by macroalgae in general, but only by a special type of the laminaria species under oxidative stress. Only during periods of extraordinarily low water (spring-tide), the plant is exposed to ambient air and may release gaseous iodine in some way to the atmosphere. The results of our re-analysis of spectra from the PARFORCE campaign in 1998 support this theory. Hence, we feel that we can provide an explanation for the different I 2 levels in Brittany and Mace Head.

Abstract. Atmospheric measurements were performed during a 1 month period in early summer of 1997 at the Dead Sea in Israel in an attempt to identify bromine monoxide BrO, and evaluate its effect on ozone chemistry. The differential optical absorption spectroscopy (DOAS) technique was utilized to identify and measure BrO present in the air masses. Concurrent to the DOAS measurements, continuous monitoring of SO2, NO/NOx, O3, and CO was performed. Filter samples for aerosol analysis and whole air canister samples for bromocarbon analysis were also collected. The present paper reports the complete comprehensive data set of the measurements at the Dead Sea site and is a continuation to our preliminary communication [Heberstreit et al., 1999]. The more complete data now available enable a more detailed examination of the sources and mechanisms of the reactive halogen species and the presentation of new conclusions. The results showed a diurnal repeating cycle of 03 and BrO variations, correlated with solar radiation and wind direction. During the elevated BrO events, where bromine oxide rose to daily maximum values oRen exceeding 100 ppt, a clear negative correlation with 03 was observed. During these episodes, the 03 regularly decreased from noontime levels of 50-80 ppb or higher down to 10-30 ppb and occasionally to levels below the detection limit of 2 ppb. The enhanced BrO levels were associated with southerly winds that are typical for the location during midday hours. This suggests that a possible source for the reactive bromine species is the interaction of atmospheric oxidants with bromide at the surface of the large salt pans located at the southern end of the Dead Sea. Research flights flown over the area showed that ozone destruction to levels well below the background values were observed over large areas of the Dead Sea Valley.