Several years ago scientists were surprised to discover that - in addition to the well-known ozone hole in the stratosphere - the ozone in the troposphere, the lowest layer of the atmosphere, can become completely depleted during polar springtime. Parallel to the loss in ozone, the concentration of reactive bromine oxide increases significantly above the sea ice and near the coasts (see Fig. 1). This tropospheric ozone hole has been attributed to the so-called bromine explosion, a chain reaction triggered by bromide ions from sea salt. Up to now it has not been understood how this bromide in the slightly alkaline seawater can be transformed into gaseous substances, since the chemical reactions of the bromine explosion can only take place in acidic solutions.
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 Fig. 1: Distribution of near-surface bromine oxide during the polar spring of 2006 (left: April 2006, right: September 2006). The distribution was calculated from measurements of the satellite instrument SCIAMACHY and shows strongly elevated bromine oxide concentrations above the sea ice near the coasts. |
Scientists from the Max Planck Institute for Chemistry (Mainz), the Institute for Environmental Physics (Bremen) and the Institute for Physical Oceanography (Hamburg) have now confirmed a new and convincing hypothesis by means of computer simulations with the atmospheric chemistry model "MECCA": When cracks form in the ice in spring (Fig. 2), liquid sea water becomes exposed to the extremely cold air. A new, thin ice layer quickly forms at the surface. This is overlaid by a layer of brine contining the soluble salts. In the extreme cold, less soluble salts such as sodium sulphate and calcium carbonate remain in the ice. Salt particles - aerosols - become uplifted be wind currents at the cracks in the sea ice. The important point here is that the aerosols produced in this way contain almost no carbonate. In the ocean, carbonate buffers the pH-value, keeping it in the alkaline range. The carbonate-poor aerosol can, however, easily become acidic, thus triggering the bromine explosion. Another important fact favouring the chain reaction is the shift in the chemical equilibrium under cold conditions. This influences the oxidation capacity of the near-surface air layer and destroys large amounts of ozone. In addition, it promotes the deposition of mercury in the polar regions.
Contrary to the ozone hole in the stratosphere, this ozone depletion has no direct influence on our health. The new findings on the reactions can, however, improve our understanding of the climate system, and they open new questions with regard to the future composition of the atmosphere under conditions of melting Arctic sea ice or progressive acidification of the oceans.
Original publication:
Ralf Sander, John Burrows, and Lars Kaleschke
Carbonate precipitation in brine - a potential trigger for tropospheric ozone depletion events
Atmos. Chem. Phys., 6, 4653-4658, 2006
www.atmos-chem-phys.org/acp/6/4653
Further information:
Dr. Rolf Sander
Max-Planck Institut für Chemie, Mainz
Tel.: 06131-305449
E-Mail: sander@mpch-mainz.mpg.de
Prof. John Burrows
Institut für Umweltphysik, Universität Bremen
Tel.: 0421-2184548
E-Mail: burrows@iup.physik.uni-bremen.de
Prof. Lars Kaleschke
Zentrum für Marine und Atmosphärische Wissenschaften
Institut für Meereskunde, Universität Hamburg
Tel.: 040 - 42838 6518
E-Mail: lars.kaleschke@zmaw.de
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 Fig. 2: Sea ice in the Fram Strait north of Spitsbergen. Sea ice serves as insulation between the relatively warm ocean and the cold atmosphere. The mass and heat exchange at the ice-free channels is evident in the photo: Sea smoke arises from the channels. Photo from approximately 500 m altitude, 1998. |
