Experimentelle Studien der reaktiven Halogenchemie in der polaren Troposphäre
Zusammenfassung der Projektergebnisse
The objective of the research activities within the HALOPOLE projects was the experimental investigation of the role of reactive halogen compounds in the chemistry of the polar boundary layer and the free troposphere based on measurements of halogen radicals by Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). Processes halogen radicals are involved in were studied by performing MAX-DOAS and longpath-DOAS measurements of tropospheric halogen compounds on various platforms. Ground-based long-term and intensive measurements in polar coastal areas and on ships in the Arctic sea provided unique information on the diurnal and seasonal variability of reactive halogens and on their sources and sinks in both polar hemispheres. The perhaps most striking finding of the research conducted within HALOPOLE was the detection of huge concentrations of iodine monoxide (IO) in the Antarctic snowpack. Mixing ratios inferred from MAX-DOAS measurements which collect not only skylight, but also light reflected by the snow, reach several tens of ppb (parts per billion), about three orders of magnitude more than usually found in the atmosphere. The measurements indicate that IO is confined to the snowpack interstitial air and the lowermost centimetres of the atmosphere above the snow surface. Here IO is expected to have a significant – if not dominating – impact on atmospheric chemistry. However, it is still unclear which processes lead to such a strong accumulation of reactive iodine in the snowpack, and how these large IO concentrations in the gas phase can be sustained. During a cruise of the Amundsen research vessel in the Canadian Arctic, simultaneous measurements of BrO, OClO, ozone and other trace gases were performed by longpath-DOAS and MAX- DOAS. This is the first time that direct measurements of bromine and chlorine radicals were performed offshore, in areas where the bromine activation occurs. The results demonstrate that halogen chemistry over the sea ice covered ocean is much more active than previously observed at coastal areas. The BrO diurnal variation shows a characteristic shape with a minimum around noon which might indicate the presence of chlorine radicals. A much stronger halogen activation directly above the sea ice than at coastal stations was also observed during our joint German/New Zealand field campaign in the vicinity of Scott Base, Antarctica. Here we were able to successfully perform measurements of BrO and IO with portable, fully autonomous MAX-DOAS instruments. During OASIS 2009, a major field campaign within the scope of IPY, simultaneous longpath- and MAX-DOAS measurements were performed in Barrow, Alaska. Based on the MAX-DOAS measurements, it was possible for the first time to determine the vertical profile of BrO, and also of the aerosol extinction. The measurements show that BrO is confined to the lowermost 200 m above the snow surface, and BrO enhancements coincide with blowing snow at high wind speeds. Both findings support the hypothesis that BrO release occurs at the surfaces of snow and aerosols.
Projektbezogene Publikationen (Auswahl)
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Enhanced tropospheric BrO over Antarctic sea ice in mid winter observed by MAX-DOAS on board the research vessel Polarstern, Atmos. Chem. Phys., 7, 3129-3142, 2007
Wagner, T., O. Ibrahim, R. Sinreich, U. Frieß, R. von Glasow, and U. Platt
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Active, passive and satellite borne spectroscopic measurements of tropospheric BrO during the OASIS 2009 campaign in Barrow, Alaska, 2009 AGU Fall Meeting, San Francisco, USA, 2009
Frieß, U., H. Sihler, T. Wagner, and U. Platt
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Iodine monoxide in the Antarctic snowpack, Atmos. Chem. Phys., 10, 2439-2456, 2010
Frieß, U., T. Deutschmann, B. S. Gilfedder, R. Weller, and U. Platt
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Observation of halogen species in the Amundsen Gulf, Arctic, by active long-path differential optical absorption spectroscopy, PNAS 2010
Pöhler, D., L. Vogel, U. Frieß, and U. Platt
