© Author(s) 2007. This work is licensed under a Creative Commons License. Atmospheric Chemistry and Physics Atmospheric deuterium fractionation: HCHO and HCDO yields in

Abstract. The formation of formaldehyde via hydrogen atom transfer from the methoxy radical to molecular oxygen is a key step in the atmospheric photochemical oxidation of methane, and in the propagation of deuterium from methane to molecular hydrogen. We report the results of the first in-vestigation of the branching ratio for HCHO and HCDO for-mation in the CH2DO + O2 reaction. Labeled methoxy rad-icals (CH2DO) were generated in a photochemical reactor by photolysis of CH2DONO. HCHO and HCDO concentra-tions were measured using FTIR spectroscopy. Significant deuterium enrichment was seen in the formaldehyde prod-uct, from which we derive a branching ratio of 88.2±1.1% for HCDO and 11.8±1.1 % for HCHO. The implications of this fractionation on the propagation of deuterium in the at-mosphere are discussed.