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10.5194/acpd-7-12715-2007
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http://www.atmos-chem-phys-discuss.net/7/12715/2007/acpd-7-12715-2007.pdf
12715
12750
2007-08-29
Hydrogen isotope fractionation in the photolysis of formaldehyde
T. S. Rhee
rhee@kopri.re.kr
C. A. M. Brenninkmeijer
T. Röckmann
Korea Polar Research Institute, Incheon, Korea
Atmospheric Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany
Institute for Marine and Atmospheric Research Utrech, Utrecht University, Utrecht, The Netherlands
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 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 isotopic 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>hν</i> → CHO + H) as
compared to the molecular channel (CH<sub>2</sub>O + <i>hν</i> → 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 isotopic 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>m</sub>) but also by overall
fractionation for the removal processes of CH<sub>2</sub>O (α<sub>f</sub>), and is
represented by the ratio of α<sub>m</sub>/α<sub>f</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>m</sub>/α<sub>f</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>m</sub>/α<sub>f</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.
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