HR: 0800h
AN: A31B-0053    [Abstracts]
TI: Modeling the Photochemical Origins of Deuterium Enrichment in Stratospheric H$_{2}$
AU: * Mar, K A
EM: katiemar@berkeley.edu
AF: Department of Chemistry, University of California, Berkeley, CA 94720 United States
AU: McCarthy, M C
EM: mmccarthy@sonomatech.com
AF: Sonoma Technology, Inc., 1360 Redwood Way, Suite C, Petaluma, CA 94954 United States
AU: Connell, P
EM: connell2@llnl.gov
AF: Atmospheric Sciences Division, Lawrence Livermore National Laboratory, Lawrence, CA 94550 United States
AU: Boering, K A
EM: boering@cchem.berkeley.edu
AF: Department of Chemistry, University of California, Berkeley, CA 94720 United States
AU: Boering, K A
EM: boering@cchem.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AB: The isotopic composition of hydrogen produced by photo-oxidation of methane, $\delta$D$_{h }$$_{\nu}$, is an important and at present poorly constrained term in the global isotope budget of H$_{2}$. In order to better understand the mechanisms that control the magnitude of $\delta$D$_{h }$$_{\nu}$, we use the Lawrence Livermore 2D chemical-radiative-transport model to simulate $\delta$D-H$_{2}$ in the stratosphere, which depends on both isotope effects in H$_{2}$ production (i.e., the magnitude of $\delta$D$_{h }$$_{\nu}$) and isotope effects in H$_{2}$ loss. Our results indicate that approximately 60$%$ of the observed enrichment in stratospheric hydrogen ($\delta$D-H$_{2}$) is due to removal of H$_{2}$ via oxidation, with isotope effects in the production of H$_{2}$ from methane accounting for the remaining enrichment. There are three major processes in H$_{2}$ production that affect the value of modeled $\delta$D$_{h }$$_{\nu}$: kinetic isotope effects in the oxidation of formaldehyde, branching in the oxidation pathway from CH$_{3}$D to HD (i.e., the relative rates of H:D abstraction in various oxidation steps), and isotope effects in formaldehyde photolysis. Using our model simulations in combination with NASA ER-2 aircraft observations of $\delta$D-H$_{2}$, we estimate the value for $\delta$D$_{h }$$_{\nu}$ in both the stratosphere and troposphere, and examine the variation of calculated $\delta$D$_{h }$$_{\nu}$ with altitude. Finally, we discuss the uncertainties in our estimate of $\delta$D$_{h }$$_{\nu}$ and suggest experimental and/or theoretical work needed to further reduce these uncertainties.
DE: 1040 Isotopic composition/chemistry
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0317 Chemical kinetic and photochemical properties
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting