Isotopic fractionation of methane in the stratosphere and its effect on free tropospheric isotopic compositions

Abstract

Kinetic isotope effects (KIEs) for the oxidation of CH4 by OH, Cl, and O(¹D) were incorporated into a 2D model of the atmosphere to examine how sensitive the carbon isotopic composition of CH4 in the free troposphere is to three influences: realistic stratospheric chemistry and transport, tropospheric Cl chemistry, and use of new, experimentally‐determined KIEs for the OH and O(¹D) sinks. At steady‐state, our results indicate that stratospheric chemistry and transport enrich tropospheric δ13CH4 by 0.5 to 0.6‰, tropospheric Cl chemistry as modeled enriches δ13CH4 by ∼0.3‰, and the new KIE for the OH sink depletes modeled global δ13CH4 by >1‰. Ignoring the stratosphere (and Cl chemistry in general) in models therefore happens to partially compensate for use of the previously accepted KIE for OH when comparisons with observations of free tropospheric δ13CH4 are made. In contrast, including only one of these influences increases the error relative to including none. Thus, all 3 effects are of sufficient magnitude to warrant their inclusion in inverse models in order to reduce uncertainties in the CH4 budget.