Extended Experimental Studies of Reactive Halogen Chemistry in the Polar Troposphere (HALOPOLE III)
Final Report Abstract
Within the scope of the HALOPOLE-III project, measurements of halogen radicals in the Arctic and Antarctic collected during the previous phase were analysed and interpreted in detail. A main focus was on the analysis of the long-term MAX-DOAS measurements from Neumayer Station and Arrival Heights, which are located at opposite sides of the Antarctic continent. Vertical profiles of BrO and aerosol extinction were retrieved for both stations for the entire data from the past 14 years using our in-house software HEIRPO. Using back-trajectory analysis in conjunction with sea ice maps, the influence of topographical, dynamical, and meteorological parameters of the air masses on BrO release prior to their arrival at the measurement site was investigated. It was found that BrO release is strongly linked to the presence of aerosol particles, confirming the importance of heterogeneous release and/or recycling of reactive bromine on airborne particles. Frequently observed elevated layers of BrO decoupled from the surface, and sustained by aerosols, probably provide a transport pathway of reactive bromine into the free troposphere where it has a potential impact on climate at least on a local scale. During polar spring, the presence of BrO is correlated to the residence time of the air mass over sea ice. During summer and autumn, however, BrO correlates with the residence time over the open ocean, indicating emission either from the water surface and/or from sea salt aerosols dispersed from the ocean surface. We were furthermore able to identify potential source areas for reactive bromine for both sites. Air enriched in BrO arriving at Neumayer mainly originates from the Weddell Sea west of the observation site, and from the marginal sea ice zone east of the station where new sea ice formation is frequently occurring in coastal polynya. An interesting finding is that air masses subject to katabatic winds over the Ronne-Larsen shelf ice, and to a smaller extent also from the distant Amery shelf ice, contain high levels of BrO. The source regions of BrO for air masses arriving at Arrival Heights are not as localised as for Neumayer. Arrival Heights is frequently subject to katabatic wind, and we were able to show that trans-continental transport of bromine enriched air from the Weddell Sea via the Antarctic plateau can occasionally lead to enhanced levels of BrO in the Ross Sea. Airborne measurements of reactive halogens in the Arctic, performed as part of the Bromex campaign around Barrow, Alaska, during the previous phase of the HALOPOLE project, were thoroughly analysed and interpreted in close collaboration with colleagues from the University of Fairbanks, Alaska, and the University of Michigan. Airborne observations of high levels of BrO far inland support the finding that the snowpack serves as a source for reactive halogens, probably after the deposition of saline particles. Furthermore, it has been shown that open lead interaction can terminate ozone depletion events by enhanced vertical mixing. Results from the Bromex flights also confirm that the presence of BrO layers aloft, sustained by aerosol particles, is a common phenomenon in the Arctic and in Antarctica. In support of our colleagues from the British Antarctic Survey, we analysed measurements from the Halley Bay station located in the coastal region and from the Kohnen Station on the Antarctic plateau. Elevated BrO during springtime is observed more frequently at Halley than at Neumayer, and similarly to Neumayer high BrO is often coincident with high levels of aerosols and frequently in layers aloft. The continuous presence of up to 10 ppt of BrO in the boundary layer over the Antarctic plateau as revealed from our measurements at Kohnen is surprising, and illustrates that long-range transport of bromine containing compounds is likely to be important for the release of reactive bromine. Our long-term measurements of the atmospheric composition at Neumayer Station using a novel LP-DOAS instrument provides a detailed insight into the release and transport mechanisms of halogen compounds, the chemical processes these compounds are involved in, as well as the importance of local meteorology and multiphase processes. In addition to the well-known bromine explosion events frequently observed after polar sunrise, which are dominated by transport of bromine enriched and ozone depleted air from the frozen ocean, we found that calm conditions with strong surface inversions lead to extremely high BrO amounts of up to 100 ppt with only little ozone depletion, leading to the conclusion that reactive bromine is emitted locally from the snow surface. Our LP-DOAS measurements at Neumayer confirm the findings from our measurement campaign at Scott Base during the second phase of HALOPOLE [Zielcke, 2015] that significant amounts of reactive chlorine are present in the Antarctic boundary layer, which are expected to have a strong impact on the oxidative capacity of the atmosphere and a high potential for destroying ozone. In fact, we were able to show that the observed ozone decrease can only be reproduced well by the known gas phase reactions if we consider both the BrO-BrO self-reaction cycle and the ClO-BrO cross reaction. The sources and release mechanisms for reactive chlorine, and the reasons for the sporadic occurrence of ClO remain unresolved questions that require further investigation. Regarding iodine monoxide, we were still not able to resolve the discrepancies existing between passive measurements (MAX-DOAS and satellite) showing significant levels of IO on the one hand, and active measurements (LP-DOAS and CE-DOAS) indicating no IO above the detection limit of about 2 ppt on the other hand. Frieß, U. and Nasse, J.-M. (2018) Tatort Antarktis – Wie Halogenverbindungen die Welt verändern, Ruperto Carola Ausg. 13, https://doi.org/10.17885/heiup.ruca.2018.13.23908
Publications
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Dependence of the vertical distribution of bromine monoxide in the lower troposphere on meteorological factors such as wind speed and stability, Atmos. Chem. Phys., 15, 2119-2137
Peterson, P. K., Simpson, W. R., Pratt, K. A., Shepson, P. B., Frieß, U., Zielcke, J., Platt, U., Walsh, S. J., and Nghiem, S. V.
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A case study of a transported bromine explosion event in the Canadian high arctic, J. Geophys. Res. Atmos., 121, 457– 477
Zhao, X., Strong, K., Adams, C., Schofield, R., Yang, X., Richter, A., Friess, U., Blechschmidt, A.‐M., and Koo, J.‐H.
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Intercomparison of aerosol extinction profiles retrieved from MAX-DOAS measurements, Atmos. Meas. Tech., 9, 3205-3222
Frieß, U., Klein Baltink, H., Beirle, S., Clémer, K., Hendrick, F., Henzing, B., Irie, H., de Leeuw, G., Li, A., Moerman, M. M., van Roozendael, M., Shaiganfar, R., Wagner, T., Wang, Y., Xie, P., Yilmaz, S., and Zieger, P.
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The role of open lead interactions in atmospheric ozone variability, Elementa
Peterson, P.K., Pratt, K. A., Simpson, W. R., Nghiem, S. V., Pérez Pérez. L. X., Boone, E.J., Pöhler, D., Zielcke, J., General, S., Shepson. P.B., Frieß, U., Platt, U., and Stirm, B. H.
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Horizontal and vertical structure of reactive bromine events probed by bromine monoxide MAX-DOAS, Atmos. Chem. Phys., 17, 9291-9309
Simpson, W. R., Peterson, P. K., Frieß, U., Sihler, H., Lampel, J., Platt, U., Moore, C., Pratt, K., Shepson, P., Halfacre, J., and Nghiem, S. V.
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Observations of bromine monoxide transport in the Arctic sustained on aerosol particles, Atmos. Chem. Phys., 17, 7567-7579
Peterson, P. K., Pöhler, D., Sihler, H., Zielcke, J., General, S., Frieß, U., Platt, U., Simpson, W. R., Nghiem, S. V., Shepson, P. B., Stirm, B. H., Dhaniyala, S., Wagner, T., Caulton, D. R., Fuentes, J. D., and Pratt, K. A.
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Polar nighttime chemistry produces intense reactive bromine events. Geophysical Research Letters, 45, 9987– 9994
Simpson, W. R., Frieß, U., Thomas, J. L., Lampel, J., and Platt, U.
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Springtime Bromine Activation over Coastal and Inland Arctic Snowpacks, ACS Earth and Space Chemistry 2 (10), 1075-1086
Peterson, P.K., Pöhler, D, Zielcke, J, General, S, Frieß, U., Platt, U, Simpson, , W.R., Nghiem, S. V., Shepson, P. B., Stirm, B. H., and Pratt, K.A.
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Intercomparison of MAX-DOAS vertical profile retrieval algorithms: studies using synthetic data, Atmos. Meas. Tech., 12, 2155-2181
Frieß, U., Beirle, S., Alvarado Bonilla, L., Bösch, T., Friedrich, M. M., Hendrick, F., Piters, A., Richter, A., van Roozendael, M., Rozanov, V. V., Spinei, E., Tirpitz, J.-L., Vlemmix, T., Wagner, T., and Wang, Y.
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Recent improvements of Long-Path DOAS measurements: impact on accuracy and stability of short-term and automated long-term observations, Atmos. Meas. Tech. Discuss.
Nasse, J.-M., Eger, P. G., Pöhler, D., Schmitt, S., Frieß, U., and Platt, U.
