Isotopic composition of H2 from CH4 oxidation in the stratosphere and the troposphere

Abstract

Enrichment in deuterium (D) of stratospheric H2 is investigated using new and published data. Applying a Rayleigh model to account for the isotopic fractionations in the photochemical chain, reactions from CH4 to the final product H2O via H2 reveals a strong enrichment of photochemically produced H2. This is consistent with previous studies, but the degree of enrichment obtained in this study appears to be 100-120‰ larger than the previous estimates on average. The discrepancy primarily stems from the former use of either an isotopic fractionation factor of H2 that does not take into account the effect of stratospheric transport or an H2 yield from CH2O photolysis that is not applicable to stratospheric conditions. We further investigate the same isotopic fractionation process under tropospheric conditions by considering the differences in both the species and amounts of oxidizing agents and the wavelengths that are effective in the photolysis of CH2O. These were not considered in the former studies when they derived the δD value under the tropospheric conditions. We thereby anticipate the δD value of H2 from photochemical oxidation of CH4 in the troposphere to be 190 (±50)‰. Copyright 2006 by the American Geophysical Union.