HR: 0800h
AN: A31B-0055    [Abstracts]
TI: Modeled Predictions of the Inter-Annual and Inter-Hemispheric Variability of Deuterium in H$_{2}$
AU: * Rahn, T
EM: trahn@lanl.gov
AF: Los Alamos National Lab, EES-6 MS D-462, Los Alamos, NM 87545 United States
AU: Horowitz, L
EM: larry.horowitz@noaa.gov
AF: GFDL/NOAA, Princeton University PO Box 308, Princeton, NJ 08542 United States
AB: As with other trace gases, the isotopic content of molecular hydrogen (H$_{2}$) has the capacity to shed light on the details of its sources and sinks and how these may vary over time. Over the past several years, a number of studies have focused on quantifying the deuterium (HD) content (expressed as $\delta$D) of the various H$_{2}$ sources and the fractionations associated with H$_{2}$ oxidation, yet there is still a want for records of $\delta$D in atmospheric H$_{2}$ at any significant spatial or temporal resolution. It is well established that concentration can vary by more than 10% seasonally, that there are strong latitudinal H$_{2}$ gradients and that the Northern (NH) and Southern (SH) Hemispheres have unique seasonal cycles related to different distributions of sources and sinks. Although high frequency measurements of $\delta$D of atmospheric H$_{2}$ have yet to be made, there are now enough measurements of HD in the sources and of fractionation during soil uptake and atmospheric oxidation to perform elementary modeling exercises that predict the expected seasonal and inter-hemispheric $\delta$D variability. We used the 3-D Chemical-Transport Model MOZART (Horowitz {\it et al}, 2003) to find best fits for the 10-year monthly means of the spatially diverse CMDL flask-sampling record of H$_{2}$ concentration (Novelli {\it et al}, 1999). Average monthly NH and SH burdens were then calculated, as were source terms for anthropogenic emissions, biomass burning, oxidation of methane and non-methane hydrocarbons, nitrogen fixation in soils, and ocean super saturation and loss terms for atmospheric oxidation of H$_{2}$ by OH and dry deposition in soils. The monthly NH and SH averages were then combined in a 3-box model (NH, SH and stratosphere) with measured values of $\delta$D of H$_{2}$ sources and fractionation due to H$_{2}$ loss. Among the details revealed by this exercise are the following: 1) While the HD minimum corresponds directly with the concentration maximum in both the NH and SH, the HD maximum precedes the concentration minimum in both hemispheres, in the SH by as much as 3 months. This isotopic phase shift can largely be attributed to the fact that the dominant source terms, biomass burning and methane oxidation have different seasonality and very different HD content. 2) The annually averaged $\delta$D in the SH is almost 20 $\permil$ greater in the SH than in the NH (varying seasonally from 5 (Oct) to 30 $\permil$ (Apr)), in large part due to the fact that the relative proportion of H$_{2}$ oxidation by OH (large fractionation) to dry deposition on soils (modest fractionation) is much greater in the SH than the NH and secondarily to lower HD depleted anthropogenic emissions in the SH. 3) The modeled seasonal amplitude in the NH is $\sim$18 $\permil$ and in the SH $\sim$10 $\permil$, both significant variabilities that should be readily verifiable with reasonably precise records. We note that the results of this exercise are for H$_{2}$ in the free troposphere of the NH and SH and that surface measurements of $\delta$D like those of H$_{2}$ concentration should be expected to exhibit variability in accordance with location. We also note that records of H$_{2}$ concentration have shown large inter-annual deviations from the averages described here and that, because the various H$_{2}$ sources have $\delta$D terms that vary by as much as several hundreds of per mil, high resolution records of $\delta$D in H$_{2}$ may help reveal the origin of these anomalies. Novelli, P.C. {\it et al}, {\it J. Geophys Res., 104}, 1999. Horowitz, L. W. {\it et al}, {\it J. Geophys. Res., 108}, 2003.
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting