Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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2009
10.5194/acpd-9-24029-2009
http://www.atmos-chem-phys-discuss.net/9/24029/2009/
http://www.atmos-chem-phys-discuss.net/9/24029/2009/acpd-9-24029-2009.html
http://www.atmos-chem-phys-discuss.net/9/24029/2009/acpd-9-24029-2009.pdf
24029
24050
2009-11-12
Pressure dependent deuterium fractionation in the formation of molecular hydrogen in formaldehyde photolysis
E. J. K. Nilsson
V. F. Andersen
H. Skov
M. S. Johnson
msj@kiku.dk
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
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
<i>k</i><sub>HCHO</sub>/<i>k</i><sub>HCDO</sub> 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 H<sub>2</sub>+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 <i>k</i><sub>H</sub>/</i>k</i><sub>D</sub>=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.
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