Atmos. Chem. Phys. Discuss., 9, 24029-24050, 2009
© Author(s) 2009. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Pressure dependent deuterium fractionation in the formation of molecular hydrogen in formaldehyde photolysis
E. J. K. Nilsson1, V. F. Andersen1, H. Skov2, and M. S. Johnson1
1Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, Copenhagen, Denmark
2National Environmental Research Institute, Department of Atmospheric Environment, University of Aarhus, Denmark

Abstract. The pressure dependence of the relative photolysis rates of HCHO and HCDO has been investigated using a new photochemical reactor at the University of Copenhagen. The relative photolysis rate of HCHO vs. HCDO under UVA lamp irradiation was measured at total pressures of 50, 200, 400, 600 and 1030 mbar. The relative dissociation rate kHCHO/kHCDO was found to depend strongly on pressure, varying from 1.1±0.1 at 50 mbar to 1.75±0.10 at 1030 mbar. The products of formaldehyde photodissociation are either H2+CO (molecular channel) or HCO+H (radical channel). The partitioning between the channels has been estimated using available values for the absorption cross section and quantum yield. As a result of the change in pressure with altitude the isotope effect for production of molecular hydrogen is found to change from a value of kH/kD=1.8±0.2 at the surface to unity at 50 km. The relative importance of the two product channels changes with altitude as a result of changes in both pressure and actinic flux. The study concludes that the δD of photochemical hydrogen produced in situ will increase substantially with altitude.

Citation: Nilsson, E. J. K., Andersen, V. F., Skov, H., and Johnson, M. S.: Pressure dependent deuterium fractionation in the formation of molecular hydrogen in formaldehyde photolysis, Atmos. Chem. Phys. Discuss., 9, 24029-24050, doi:10.5194/acpd-9-24029-2009, 2009.
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