Atmos. Chem. Phys. Discuss., 7, 12715-12750, 2007
www.atmos-chem-phys-discuss.net/7/12715/2007/
doi:10.5194/acpd-7-12715-2007
© Author(s) 2007. This work is licensed under the
Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Hydrogen isotope fractionation in the photolysis of formaldehyde
T. S. Rhee1, C. A. M. Brenninkmeijer2, and T. Röckmann3
1Korea Polar Research Institute, Incheon, Korea
2Atmospheric Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany
3Institute for Marine and Atmospheric Research Utrech, Utrecht University, Utrecht, The Netherlands

Abstract. Experiments investigating the isotopic fractionation in the formation of H2 by the photolysis of CH2O under tropospheric conditions are reported and discussed. The deuterium (D) depletion in H2 produced is 500(±20)‰ with respect to the parent CH2O. We also observed that complete photolysis of CH2O under atmospheric conditions produces H2 that has virtually the same isotopic ratio as that of the parent CH2O. These findings imply that there must be a very strong concomitant isotopic enrichment in the radical channel (CH2O + → CHO + H) as compared to the molecular channel (CH2O + → H2 + CO) of the photolysis of CH2O in order to balance the relatively small isotopic fractionation in the competing reaction of CH2O with OH. Using a 1-box photochemistry model we calculated the isotopic fractionation factor for the radical channel to be 0.22(±0.08), which is equivalent to a 780(±80)‰ enrichment in D of the remaining CH2O. When CH2O is in photochemical steady state, the isotopic ratio of the H2 produced is determined not only by the isotopic fractionation occurring during the photolytical production of H2m) but also by overall fractionation for the removal processes of CH2O (αf), and is represented by the ratio of αmf. Applying the isotopic fractionation factors relevant to CH2O photolysis obtained in the present study to the troposphere, the ratio of αmf varies from ~0.8 to ~1.2 depending on the fraction of CH2O that reacts with OH and that produces H2. This range of αmf can render the H2 produced from the photochemical oxidation of CH4 to be enriched in D (with respect to the original CH4) by the factor of 1.2–1.3 as anticipated in the literature.

Citation: Rhee, T. S., Brenninkmeijer, C. A. M., and Röckmann, T.: Hydrogen isotope fractionation in the photolysis of formaldehyde, Atmos. Chem. Phys. Discuss., 7, 12715-12750, doi:10.5194/acpd-7-12715-2007, 2007.
 
Search ACPD
Discussion Paper
    XML
    Citation
    Final Revised Paper
    Share