Hydrogen isotope fractionation in the photolysis of formaldehyde
Item
Title (Dublin Core)
Hydrogen isotope fractionation in the photolysis of formaldehyde
Description (Dublin Core)
Experiments investigating the isotopic fractionation in the formation of H<sub>2</sub> by the photolysis of CH<sub>2</sub>O under tropospheric conditions are reported and discussed. The deuterium (D) depletion in the H<sub>2</sub> produced is 500(±20)‰ with respect to the parent CH<sub>2</sub>O. We also observed that complete photolysis of CH<sub>2</sub>O under atmospheric conditions produces H<sub>2</sub> that has virtually the same isotope ratio as that of the parent CH<sub>2</sub>O. These findings imply that there must be a very strong concomitant isotopic enrichment in the radical channel (CH<sub>2</sub>O+<i>h</i>ν → CHO+H) as compared to the molecular channel (CH<sub>2</sub>O+<i>h</i>ν → H<sub>2</sub>+CO) of the photolysis of CH<sub>2</sub>O in order to balance the relatively small isotopic fractionation in the competing reaction of CH<sub>2</sub>O 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 CH<sub>2</sub>O. When CH<sub>2</sub>O is in photochemical steady state, the isotope ratio of the H<sub>2</sub> produced is determined not only by the isotopic fractionation occurring during the photolytical production of H<sub>2</sub> (α<sub><i>m</i></sub>) but also by overall fractionation for the removal processes of CH<sub>2</sub>O (α<sub><i>f</i></sub>), and is represented by the ratio of α<sub><i>m</i></sub>/α<sub><i>f</i></sub>. Applying the isotopic fractionation factors relevant to CH<sub>2</sub>O photolysis obtained in the present study to the troposphere, the ratio of α<sub><i>m</i></sub>/α<sub><i>f</i></sub> varies from ~0.8 to ~1.2 depending on the fraction of CH<sub>2</sub>O that reacts with OH and that produces H<sub>2</sub>. This range of α<sub><i>m</i></sub>/α<sub><i>f</i></sub> can render the H<sub>2</sub> produced from the photochemical oxidation of CH<sub>4</sub> to be enriched in D (with respect to the original CH<sub>4</sub>) by the factor of 1.2–1.3 as anticipated in the literature.
Creator (Dublin Core)
Rhee, T. S.
Brenninkmeijer, C. A. M.
Röckmann, T.
Date (Dublin Core)
2018-01-15
Type (Dublin Core)
Text
Format (Dublin Core)
application/pdf
Identifier (Dublin Core)
10.5194/acp-8-1353-2008
https://acp.copernicus.org/articles/8/1353/2008/
Source (Dublin Core)
eISSN: 1680-7324
Language (Dublin Core)
eng
