Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 8 5 2008 10.5194/acp-8-1353-2008 http://www.atmos-chem-phys.net/8/1353/2008/ http://www.atmos-chem-phys.net/8/1353/2008/acp-8-1353-2008.html http://www.atmos-chem-phys.net/8/1353/2008/acp-8-1353-2008.pdf 1353 1366 2008-03-07 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 Utrecht, Utrecht University, Utrecht, The Netherlands 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)&permil; 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)&permil; 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 (αm) but also by overall fractionation for the removal processes of CH2O (αf), and is represented by the ratio of αm/αf. 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