Hydrogen isotope effect of the photolysis of formaldehyde to the molecular and radical channels

Abstract

A series of experiments was carried out to investigate the hydrogen isotope fractionation during the photolysis of CH2O. Deuterium content of the H2 produced by the photolysis of CH2O was determined by a recently developed technique with continuous-flow isotope ratio mass spectrometry. The deuterium (D) of the H2 produced is depleted by 500(±20) ‰ with respect to the parent CH2O. We also observed that complete photolysis of CH2O under tropospheric 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 + h&#61550

→ CHO + H) as compared to the molecular channel (CH2O + h&#61550

→ 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(±90) ‰ enrichment in D of the remaining CH2O. This isotopic effect is significantly larger than the result obtained from the experiments in the EUPOHORE reaction chamber by Feilberg et al. (Relative tropospheric photolysis rates of HCHO and HCDO measured at the European Photoreactor facility, J. Phys. Chem., 2007).