Hydrogen isotope fractionation in the photolysis of formaldehyde

T. S. Rhee (1) , C. A. M. Brenninkmeijer (2) , T. Röckmann (3)

1 KOPRI - Korea Polar Research Institute
2 Atmospheric Chemistry Department [MPIC]
3 IMAU - Institute for Marine and Atmospheric Research [Utrecht]

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 + h? ? CHO + H) as compared to the molecular channel (CH₂O + h? ? 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 H₂ (?_m) but also by overall fractionation for the removal processes of CH₂O (?_f), and is represented by the ratio of ?_m/?_f. Applying the isotopic fractionation factors relevant to CH₂O photolysis obtained in the present study to the troposphere, the ratio of ?_m/?_f varies from ~0.8 to ~1.2 depending on the fraction of CH₂O that reacts with OH and that produces H₂. This range of ?_m/?_f 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.

Domaines: Océan, Atmosphère

T. S. Rhee, C. A. M. Brenninkmeijer, T. Röckmann. Hydrogen isotope fractionation in the photolysis of formaldehyde. Atmospheric Chemistry and Physics Discussions, 2007, 7 (4), pp.12715-12750. ⟨hal-00303076⟩