Hydrogen isotope fractionation in the photolysis of formaldehyde
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&l...
| Main Authors: | , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2008-03-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/8/1353/2008/acp-8-1353-2008.pdf |
| _version_ | 1819028123130265600 |
|---|---|
| author | T. S. Rhee C. A. M. Brenninkmeijer T. Röckmann |
| author_facet | T. S. Rhee C. A. M. Brenninkmeijer T. Röckmann |
| author_sort | T. S. Rhee |
| collection | DOAJ |
| description | 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(&plusmn;20)&permil; 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>&nu; &rarr; CHO+H) as compared to the molecular channel (CH<sub>2</sub>O+<i>h</i>&nu; &rarr; 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(&plusmn;0.08), which is equivalent to a 780(&plusmn;80)&permil; 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> (&alpha;<sub><i>m</i></sub>) but also by overall fractionation for the removal processes of CH<sub>2</sub>O (&alpha;<sub><i>f</i></sub>), and is represented by the ratio of &alpha;<sub><i>m</i></sub>/&alpha;<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 &alpha;<sub><i>m</i></sub>/&alpha;<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 &alpha;<sub><i>m</i></sub>/&alpha;<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&ndash;1.3 as anticipated in the literature. |
| first_indexed | 2024-12-21T05:53:21Z |
| format | Article |
| id | doaj.art-ef5be243dd304ad1a07db9cc9c4ed8cf |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-12-21T05:53:21Z |
| publishDate | 2008-03-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-ef5be243dd304ad1a07db9cc9c4ed8cf2022-12-21T19:13:54ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242008-03-018513531366Hydrogen isotope fractionation in the photolysis of formaldehydeT. S. RheeC. A. M. BrenninkmeijerT. RöckmannExperiments 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(&plusmn;20)&permil; 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>&nu; &rarr; CHO+H) as compared to the molecular channel (CH<sub>2</sub>O+<i>h</i>&nu; &rarr; 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(&plusmn;0.08), which is equivalent to a 780(&plusmn;80)&permil; 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> (&alpha;<sub><i>m</i></sub>) but also by overall fractionation for the removal processes of CH<sub>2</sub>O (&alpha;<sub><i>f</i></sub>), and is represented by the ratio of &alpha;<sub><i>m</i></sub>/&alpha;<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 &alpha;<sub><i>m</i></sub>/&alpha;<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 &alpha;<sub><i>m</i></sub>/&alpha;<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&ndash;1.3 as anticipated in the literature.http://www.atmos-chem-phys.net/8/1353/2008/acp-8-1353-2008.pdf |
| spellingShingle | T. S. Rhee C. A. M. Brenninkmeijer T. Röckmann Hydrogen isotope fractionation in the photolysis of formaldehyde Atmospheric Chemistry and Physics |
| title | Hydrogen isotope fractionation in the photolysis of formaldehyde |
| title_full | Hydrogen isotope fractionation in the photolysis of formaldehyde |
| title_fullStr | Hydrogen isotope fractionation in the photolysis of formaldehyde |
| title_full_unstemmed | Hydrogen isotope fractionation in the photolysis of formaldehyde |
| title_short | Hydrogen isotope fractionation in the photolysis of formaldehyde |
| title_sort | hydrogen isotope fractionation in the photolysis of formaldehyde |
| url | http://www.atmos-chem-phys.net/8/1353/2008/acp-8-1353-2008.pdf |
| work_keys_str_mv | AT tsrhee hydrogenisotopefractionationinthephotolysisofformaldehyde AT cambrenninkmeijer hydrogenisotopefractionationinthephotolysisofformaldehyde AT trockmann hydrogenisotopefractionationinthephotolysisofformaldehyde |