ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-12063-2010 Different photolysis kinetics at the surface of frozen freshwater vs. frozen salt solutions Kahan T. F. 1 2 Kwamena N.-O. A. 1 Donaldson D. J. 1 3 Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, M5S 3H6 Ontario, Canada Currently at Department of Chemistry, University of California Irvine, 92697 Irvine, California, USA Department of Physical and Environmental Sciences, University of Toronto, Canada 07 05 2010 10 5 12063 12077 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/10/12063/2010/acpd-10-12063-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/12063/2010/acpd-10-12063-2010.pdf

The presence of solutes such as sodium halide salts can greatly alter the physical nature of air-ice interfaces. In this work, we studied the effects of sodium chloride and sodium bromide on the photolysis kinetics of harmine, an aromatic organic compound, in aqueous solution and at the surface of frozen salt solutions. Harmine photolysis is much faster on pure ice surfaces than in aqueous solution, but the presence of NaCl or NaBr – which does not affect photolysis kinetics in solution – reduces the photolysis rate on ice. The rate decreases monotonically with increasing salt concentration; at the concentrations found in seawater, harmine photolysis at the surface of frozen salt solutions proceeds at the same rate as in aqueous solution. These results suggest that the brine excluded to the surfaces of frozen salt solutions is a true aqueous solution, and so it may be possible to use aqueous-phase kinetics to predict photolysis rates on sea ice. This is in marked contrast to the result at the surface of pure ice samples, where reaction kinetics are often not well-described by aqueous-phase processes.

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