Hydrogen Isotope Fractionation during the destruction of formaldehyde in the atmosphere

Abstract

Due to the limited fossil energy sources and the climate impact of their final products CO2, hydrogen economy has been considered to be alternative. Large scale use of H2 as an energy carrier may perturb the current global H2 cycle. Molecular hydrogen in the atmosphere originates from direct emissions from the Earth surfaces and from airborne production by photolysis of formaldehyde. It is destroyed by the reactions with the OH radical in the atmosphere and with microbes in soils. Measurement of stable isotope ratio of H2 turns out to be useful to investigate atmospheric H2 cycle. Among the sources, photochemically produced H2 is uniquely enriched in deuterium compared to the isotopic ratio of atmospheric H2. Here we investigated isotopic fractionation of hydrogen species produced from photochemical reactions of CH2O. Experiments were conducted to determine the isotopic fractionation of the H2 produced by photolysis of CH2O in flask under atmospheric conditions. The H2 produced is depleted in deuterium by 500 ‰ with respect to the initial CH2O that does not undergo photolysis. We also found that complete photolysis of CH2O under atmospheric conditions produces H2 that has virtually the same isotopic ratios as that of CH2O itself. These findings imply that there must be a strong isotopic fractionation in the radical channel (CH2O + h&#61550

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

→ CO + H2). In the presentation we will discuss the results from the experiments focusing on how photochemical oxidation of CH4 may produce H2 enriched in deuterium.