Pressure dependence of the deuterium isotope effect in the photolysis of formaldehyde by ultraviolet light 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
Received: 09 October 2009 – Published in Atmos. Chem. Phys. Discuss.: 12 November 2009 Abstract. 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 H2 + 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, kHCHO/kHCDO, was
found to depend strongly on pressure, varying from 1.1 + 0.15/−0.1 at 50 mbar
to 1.75±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.
Revised: 20 March 2010 – Accepted: 01 April 2010 – Published: 14 April 2010
Citation: Nilsson, E. J. K., Andersen, V. F., Skov, H., and Johnson, M. S.: Pressure dependence of the deuterium isotope effect in the photolysis of formaldehyde by ultraviolet light, Atmos. Chem. Phys., 10, 3455-3462, doi:10.5194/acp-10-3455-2010, 2010.