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20 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.1.2 Studies of Reactive Halogen Species (RHS) in the Marine and mid- Latitudinal Boundary Layer by Active Longpath Differential Optical Absorption Spectroscopy (DOAS) Participating scientist Christina Peters Abstract Appearance and distribution of a variety of reactive halogen species like BrO, IO, OIO and I2 in coastal areas was investigated within the framework of this PhD thesis. Figure 2.2: I2 time series recorded during the Mace Head campaign in 1998. The dotted line indicates the mean detection limit of 20 ppt. The shaded areas indicate I2 clearly identified above the detection limit (with respect to the individual error Background The importance of reactive halogen species (RHS) in the troposphere is on the one hand due to their strong effect on tropospheric ozone levels, which was discovered during ozone depletion events in the polar boundary layer during polar sunrise. On the other hand recent field and laboratory studies are indicating a great relevance of reactive iodine in new particle formation processes. Since particles in the marine atmosphere affect the microphysical properties of clouds, they have a potential impact on climate. Funding PhD Thesis in the framework of the AFO2000 program, funded by the BMBF. Methods and results Within the framework of this thesis three field campaigns using active longpath DOAS technique for studies of the reactive halogen species BrO, IO, OIO and I2 were conducted in different mid-latitudinal regions. Ground based measurements in the lower Arctic at the Hudson Bay yielded high levels of the BrO radical up to 35 ppt in the boundary layer, coinciding with nearly complete surface near ozone depletion. Two field campaigns investigated the appearance of RHS in the coastal environments of the German North Sea and the French Atlantic Coast, IO was the only halogen oxide found in both locations with concentrations reaching 7.7±0.5 ppt. BrO in the marine boundary layer was estimated to remain below 1.5 ppt, neither OIO nor I2 could be unequivocally identified in the spectra. Within a re-analysis of spectra taken during an earlier campaign in 1998 in Mace Head, Ireland up to 61±12 ppt I2 were detected at night during extraordinary low water levels. Most likely the I2 was emitted by laminaria algae inhabiting parts of the lower intertidal zone. Outlook/Future work Further studies will be performed within the EU project ”Marine Aerosol Production” (MAP). Main publications [Peters et al. , 2005], [Peters, 2005]
2.1. TROPOSPHERIC RESEARCH GROUP 21 2.1.3 Improvement of the Detection Limit of Active-DOAS-Measurements by use of fibre coupled light source Participating scientist André Merten Abstract Xenon-high-pressure lamps are commonly used in DOAS measurements. Unfortunately these lamps show strong spectral variability, which determines the minimum detectable optical density. A combination of a technical solution and a mathematical treatment to reduce the residual structures and therefore the detection limit was developed. Figure 2.3: Fibre light source prevents residual structures caused by a Xenon lamp. Background In active DOAS meausurements the minimum detectable optical density and thus the detection limit for trace gases are primarily determined by the spectral stability of the Xenonlight source. This is particularly important in spectral ranges where Xe-lines exist. Due to the large temperature gradient (several 1000 K/mm) and turbulent flow inside the arc these spectral structures strongly vary with time and across the arc of the Xe–high pressure lamp. When using a ’shortcut optics’, to remove this Xe-lines by a lamp reference, it is not guaranteed that the same area of the arc is imaged as is used in the measurement, causing strong residual structures which can be misinterpreted as optical densities. Imaging the lamp in a fibre and ’mode-mixing’ the light improves the situation, since all light leaving the fibre has the same spectral composition. Funding IALSI Methods and results After preliminary tests, a fibre coupled Xe-lamp was installed at the White cell (multireflection cell) at the EUPHORE smog chamberb (CEAM Valencia / Spain). Tests before and after the installation of this arrangement show a clear reduction of the residual structures and of the error in the concentration of detected trace gases. For a long path telescope it is even more complicated to run a short cut system that images the lamp arc in the same way as in the measurement mode, if the lamp is imaged directly into the telescope. The Xenon-lamp was replaced by a Xenon-fibre light source, which was mounted at the optical axis of the telescope, instead of a Newton-like telescope set-up, which makes the complicate alignment of the telescope easier. Figure 2.3 shows the results of a DOAS evaluation of NO2 and H2O from the old (left) and the new (right) set-up. In the left graph strong residual structures dominate the fit-result. In the right graphs (new set-up with fibre source), only very small residual structures are visible and the detection of weak water absorption is possible. Due to the easier alignment, effectively no loss of light occurred. Xenon lamps show also a spectral drift with time. If it is not possible to take lamp spectra at short time periods, residual structures arise. Fortunately the spectrum of the lamp does not change randomly, which can be determined by correlation analysis of lamp spectra. A linear model can describe the variation of the lamp spectrum. This was combined with the DOAS-analysis. A change in the lamp spectrum is now treated like an absorber and can removed by the fitting procedure. The necessary mathematical functions were integrated in a new evaluation software, developed to perform an efficient DOAS analysis. Outlook/Future work Using fibre optics offers the possibility to work with other light sources such as LED and to design a new type of long path telescopes.
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3.2. ICE AND CLIMATE 147 Preunkert,
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Forschungsstelle “Radiometrie”
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Institut für Umweltphysik Im Neuen
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