Hydrogen isotope fractionation in the photolysis of formaldehyde

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 the 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 isotope 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+hν → CHO+H) as compared to the molecular channel (CH₂O+hν → 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 isotope ratio of the H₂ produced is determined not only by the isotopic fractionation occurring during the photolytical production of H₂ (α_m) but also by overall fractionation for the removal processes of CH₂O (α_f), and is represented by the ratio of α_m/α_f. Applying the isotopic fractionation factors relevant to CH₂O photolysis obtained in the present study to the troposphere, the ratio of α_m/α_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 α_m/α_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.