ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-3455-2010 Pressure dependence of the deuterium isotope effect in the photolysis of formaldehyde by ultraviolet light Nilsson E. J. K. 1 Andersen V. F. 1 Skov H. 2 Johnson M. S. 1 Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, Copenhagen, Denmark National Environmental Research Institute, Department of Atmospheric Environment, University of Aarhus, Denmark 14 04 2010 10 7 3455 3462 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.net/10/3455/2010/acp-10-3455-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/3455/2010/acp-10-3455-2010.pdf

The pressure dependence of the relative photolysis rate of HCHO vs. HCDO has been investigated for the first time, using a photochemical reactor at the University of Copenhagen. The dissociation of HCHO vs. HCDO using a UVA lamp was measured at total bath gas pressures of 50, 200, 400, 600 and 1030 mbar. The products of formaldehyde photodissociation are either H<sub>2</sub> + CO (molecular channel) or HCO + H (radical channel), and a photolysis lamp was chosen to emit light at wavelengths that greatly favor the molecular channel. The isotope effect in the dissociation, <i>k</i><sub>HCHO</sub>/<i>k</i><sub>HCDO</sub>, was found to depend strongly on pressure, varying from 1.1 + 0.15/&minus;0.1 at 50 mbar to 1.75&plusmn;0.10 at 1030 mbar. The results can be corrected for radical channel contribution to yield the kinetic isotope effect for the molecular channel; i.e. the KIE in the production of molecular hydrogen. This is done and the results at 1030 mbar are discussed in relation to previous studies at ambient pressure. In the atmosphere the relative importance of the two product channels changes with altitude as a result of changes in pressure and actinic flux. The study demonstrates that the δD of photochemical hydrogen produced from formaldehyde will increase substantially as pressure decreases.

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