B. Alicke

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

Automated in situ gas chromatograph/mass spectrometer (GC/MS) measurements of a range of predominantly biogenic alkyl halides in air, including CHBr3, CHBr2Cl, CH3Br, C2H5Br, CH3I, C2H5I, CH2ICl, CH2I2, 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/CHBr2Cl and CHBr3/CH2IBr and between the monohalomethane pair CH3I/C2H5I, 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, CH2ICl, 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 CH2I2. 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).

Iodine oxide in the marine boundary layer

A striking example of the influence of halogen chemistry on tropospheric ozone levels is the episodic destruction of boundary-layer ozone during the Arctic sunrise by reactive halogen species, . We detected iodine oxide in the boundary layer at Mace Head, Ireland (53°20′ N, 9°54′ W) during May 1997, which indicates that iodine chemistry is occurring in the troposphere.

Chemistry and oxidation capacity of the nitrate radical in the continental boundary layer near Berlin

The nitrate radical is in many situations the most important nighttime oxidizing species, removing, for example, hydrocarbons, which would otherwise be available to daytime ozone formation. In spite of its importance in the night and probably also under certain conditions during the day, our understanding of the NO3 chemistry and its impact on the oxidation capacity of the atmosphere is still incomplete. Here we present measurements of NO3 by differential optical absorption spectroscopy (DOAS) and a number of other atmospheric trace gases performed during the Berliner Ozonexperiment (BERLIOZ) campaign at Pabstthum near Berlin, Germany, to quantify the contribution of NO3 to the atmospheric oxidation rate of volatile organic compounds (VOCs) and NOx removal. The measurements show that only two NO3 sinks were of importance: (1) About 50–30% (depending on the distance (0.1–3 km) to a near forest) of the NO3 was lost due to reaction with biogenic hydrocarbons. (2) The major part of the remaining loss probably can be attributed to the indirect loss via the reaction of N2O5 on aerosol surfaces. Assuming that heterogeneous hydrolysis of N2O5 is occurring, the nonphotolytical conversion of NOx to HNO3 via N2O5 was found to be comparable with daytime conversion by the reaction of OH with NO2. In combination with measurements of the OH concentration, it was possible for the first time to derive a relative contribution of 28% (24-hour average) for the NO3-initiated oxidation to the total VOC degradation.

CHEMISTRY OF HALOGEN OXIDES IN THE TROPOSPHERE : COMPARISON OF MODEL CALCULATIONS WITH RECENT FIELD DATA

Reactive halogen species (RHS = X, XO, HOX, OXO; X = Cl, Br, I) are known to have an important influence on the chemistry in the polar boundary layer (BL), where they are responsible for ozone depletion events in spring. Recent field campaigns at Mace Head, Ireland, and the Dead Sea, Israel, identified for the first time iodine oxide (IO) at mixing ratios of up to 6.6 ppt and 90 ppt bromine oxide (BrO), respectively, by DOAS also at lower latitudes. These results intensified the discussion about the role of the RHS in the mid-latitude BL.Photochemical box model calculations show that the observed IO mixing ratios can destroy ~0.45 ppb ozone per hour. This is comparable to the rates of the known O3-loss processes in the boundary layer. The model studies also reveal that IO, at these levels, has a strong influence on the BL photochemistry, increasing the OH/HO2- and the NO2/NO - ratios. In combination these changes lead to a reduction of the photochemical ozone formation, which - in addition - reduces ozone mixing ratios by up to 0.15 ppb/h.The studies for the Dead Sea case give no information on the heterogeneous process responsible for the bromine release, but they show that a total of 2 – 4 ppb of total bromine have to be released to explain the observed complete depletion of 60 ppb ozone in 2 – 3 hours.

Comparison of tropospheric NO3 radical measurements by differential optical absorption spectroscopy and matrix isolation electron spin resonance

Despite the importance of NO 3 in the nighttime atmosphere only two techniques, Differential Optical Absorption Spectroscopy (DOAS) and Matrix Isolation Electron Spin Resonance (MIESR) have been applied to its detection in ambient air to date. Here we report the results of the first intercomparisons of these techniques in the atmosphere carried out at rural sites in Germany, at Deuselbach in 1983 and near Berlin in 1998. The simultaneously measured NO 3 mixing ratios, which were in the range from 9 to 20 ppt and at one measurement near 100 ppt, were in good agreement within the error limits. A regression analysis yields a linear relationship between the DOAS and MIESR data with a correlation coefficient of R = 0.99 and a slope of 0.83 ± 0.03 (1σ error) at a negligible intercept of 0.33 ± 0.73 ppt (1σ error). The deviation from unity is within the total systematic error of both measurement techniques. This result shows the reliability of the two techniques over the past 15 years.