A31B-0053 0800h
Modeling the Photochemical Origins of Deuterium Enrichment in Stratospheric H$_{2}$
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.
A31B-0054 0800h
Meridional trends in concentration and hydrogen isotope ratio of atmospheric H$_{2}$
Measurements of H$_{2}$ concentration and hydrogen isotope ratio were made on air samples collected on Pacific Ocean transects between Seattle, WA and Antarctica during Nov-Dec, 1998, May-June, 2002, and December-March 2004. Both in situ and grab sample measurements indicate a maximum in H$_{2}$ mixing ratio in the equatorial region near 600ppbv with mixing ratio decreasing poleward to $\sim$540ppbv in the high latitude southern hemisphere and to $\sim$520ppbv in the mid latitude northern hemisphere. Isotopic analyses reveal that this trend in mixing ratio is mirrored by a minimum in $\delta$D-H$_{2}$ in the equatorial region with values near 80$\permil$ (versus VSMOW), increasing poleward to $\sim$150$\permil$ in the high latitude southern hemisphere and to $\sim$125$\permil$ in the mid latitude northern hemisphere. The poleward increase in $\delta$D with decreasing mixing ratio appears to be stronger in gradient in the southern hemisphere than in the north as a result of the increased importance of the OH sink relative to the soil sink. Additionally, the importance of photochemical H$_{2}$ sources, enriched in deuterium, in the southern hemisphere relative to primary sources, depleted in deuterium, contributes to this observed gradient. However, in either hemisphere these data indicate that the poleward increase in $\delta$D with decreasing H$_{2}$ mixing ratio is too large to be due to sink effects and potentially too large to be explained by the distribution of known sources (i.e., photochemical hydrocarbon oxidation and surface sources biomass burning and fossil fuel combustion). Results suggest the presence of a very deuterium enriched source at high latitudes, potentially from stratospheric-tropospheric exchange.
A31B-0055 0800h
Modeled Predictions of the Inter-Annual and Inter-Hemispheric Variability of Deuterium in H$_{2}$
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.
A31B-0056 0800h
The Distribution of Molecular Hydrogen in the Lower Troposphere
The National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) has measured the distribution of molecular hydrogen using a cooperative air sampling network since the early 1990s. The global H$_{2}$ distribution and budget were examined by Novelli et al. (JGR, 104, 30427, 1999) using network data through 1997. Here we expand on the earlier paper to examine effects of the 1997/1998 wildfires on H$_{2}$ distributions, evaluate its decadal changes, and review the H$_{2}$ budget in terms of coincident measurements with carbon monoxide (CO). While CO and H$_{2}$ share many common sources, CO is predominately destroyed through reaction with OH while H$_{2}$ has a strong soil sink. Both gases exhibited anomalously high mixing ratios in 1998 which we attribute to emissions from extensive tropical and boreal wildfires. Despite their common sources, correlations between CO and H$_{2}$ were only significant during the northern winter and early spring. This relationship reflects their anthropogenic source combined with the absence of a significant sink. From a globally smoothed surface we extracted estimates of decadal changes in hydrogen. Average rates of change between 1992 and 2003 were small [$\pm$ 0.5 ppb yr$^{-1}$ or less ($<$ 0.1% yr$^{-1}$)] but showed both seasonal and latitudinal features. The eleven year time series showed greatest rates of change in the high latitudes of the Northern Hemisphere during winter. The changes tended to be negative in the Northern Hemisphere while slightly positive in the extra-tropical and polar Southern Hemisphere. These changes are discussed in terms of the distribution of H$_{2}$ sources and sinks.
http://www.cmdl.noaa.gov/ccg/index.html
A31B-0057 0800h
Are Soil H2 Concentrations a Biologically Relevant Proxy for Soil Redox Status in Upland Tropical Soils?
Wet upland tropical and temperate rainforest soils may play a major role in production and consumption of H2, yet this redox-active gas has been nearly ignored by soil scientists. We explored whether belowground H2 concentrations could be used to quantify the amplitude and temporal extent of major pE shifts in upland soils by testing D. Lovley and F.H. Chapelle's "TEAP" (terminal electron accepting process) model in both field and laboratory settings. Using soils from the Luquillo Experimental Forest in Puerto Rico, we measured soil H2 concentrations along with levels of other redox-active chemical species (O2, N2O, CH4, CO2, CO) and developed a model to correlate this data, so that thereafter, [H2] might be used alone as an indicator of the predominant TEAP occurring in the soil. This method has not been used previously in un-saturated soils, and there is some concern that the heterogeneity of co-occurring TEAPs may make it difficult to use as a precise indicator of soil pE. However, the method appears to give a qualitative indication of the redox processes that are dominating at any given point in time.
A31B-0058 0800h
Observations of Molecular Hydrogen Uptake by Soils From a Mixed Conifer and Hardwood Forest in California
Soil uptake of molecular hydrogen accounts for 62-92% of H$_{2}$ loss from the atmosphere, but ecologically relevant observations of H$_{2}$ uptake rates and distribution are sparse. New observations are needed to identify the sensitivity of H$_{2}$ fluxes to ecological and environmental drivers such as soil moisture, temperature and organic matter content. Here we present observations of H$_{2}$, CO and CO$_{2}$ uptake and vertical distribution in soils in both relatively dry and wet sites from a mixed conifer and hardwood forest in the San Jacinto Mountains of Southern California. Our sites were located within the University of California James San Jacinto Mountains Reserve located at $33\deg$ N and $116\deg$ W. The soils in these sites are uniform, well drained fine grain sands to at least 30 cm depth. Observed rates of H$_{2}$ uptake by these soils range from -4.6 nmol/m$^{2}$/s to -12.41 nmol/m$^{2}$/s. An apparent correlation between H$_{2}$ uptake and CO$_{2}$ soil respiration fluxes exists across the sites. Vertical distribution of H$_{2}$ shows that at 5 cm depth (just below the surface litter layer) the concentration of H$_{2}$ is less than 50% of ambient atmospheric levels, and decreases with depth to less than 50 ppb (by 25 cm) at all sites. These observations suggest that the steady-state compensation level of H$_{2}$ within soils is low. Vertical gradients in H$_{2}$ profiles are at a maximum in the late-afternoon and minimum at night. Within the soil, diurnal variability is greatest in the upper 5 cm of soil, and inter-site variability is greatest above 10 cm.
A31B-0059 0800h
How Would a Switch to a Future Hydrogen Economy Influence the Atmosphere?
Switching from a fossil fuel to a hydrogen-based energy system is likely to cause significant changes in the magnitude and composition of anthropogenic emissions. On the `good' side, the hydrogen economy may achieve reductions in emissions of CO and NOx, improving air quality. On the `minus' side man-made emissions of H$_{2}$ could increase through leaks, influencing the oxidising capacity of the atmosphere and stratospheric ozone. Model simulations using a 2D interactive model of the atmosphere have been performed for a selection of hydrogen economy emission scenarios. Results suggest the impact on stratospheric ozone may not be as large a problem as first thought, but that unwelcome tropospheric effects may be significant. There are strong atmospheric trade-offs between hydrogen and other gases, particularly methane.
A31B-0060 0800h
Reaping Environmental Benefits of a Global Hydrogen Economy: How Large, Fow Soon, and at What Risks?
The Western world has taken an aggressive posture to transition to a global hydrogen economy. While numerous technical challenges need to be addressed to achieve this it is timely to examine the environmental benefits and risks of this transition. Hydrogen provides an efficient energy carrier that promises to enhance urban and regional air quality that will benefit human health. It could also reduce risks of climate change if large-scale hydrogen production by renewable or nuclear energy sources becomes viable. While it is well known that the byproduct of energy produced from hydrogen is water vapor, it is not well known that the storage and transfer of hydrogen is inevitably accompanied by measurable leakage of hydrogen. Unintended consequences of hydrogen leakage include reduction in global oxidative capacity, changes in tropospheric ozone, and increase in stratospheric water that would exacerbate halogen induced ozone losses as well as impact the earth's radiation budget and climate. We construct plausible global hydrogen energy use and leak scenarios and assess their impacts using global 3-D simulations by the Model for Ozone And Related Trace species (MOZART). The hydrogen fluxes and photochemistry in our model successfully reproduce the contemporary hydrogen cycle as observed by a network of remote global stations. Our intent is to determine environmentally tolerable leak rates and also facilitate a gradual phasing in of a hydrogen economy over the next several decades as the elimination of the use of halocarbons gradually reduces halogen induced stratospheric ozone loss rates. We stress that the leak rates in global hydrogen infrastructure and the future evolution of microbial soil sink of hydrogen that determines its current lifetime (about 2 years) are principal sources of uncertainty in our assessment.
A31B-0061 0800h
The Effect of Converting to a U.S. Hydrogen Fuel Cell Vehicle Fleet on Emissions and Energy Use
This study analyzes the potential change in emissions and energy use from replacing fossil-fuel based vehicles with hydrogen fuel cell vehicles. This study examines three different hydrogen production scenarios to determine their resultant emissions and energy usage: hydrogen produced via 1) steam reforming of methane, 2) coal gasification, or 3) wind electrolysis. The atmospheric model simulations require two primary sets of data: the actual emissions associated with hydrogen fuel production and use, and the corresponding reduction in emissions associated with reducing fossil fuel use. The net change in emissions is derived using 1) the U.S. EPA's National Emission Inventory (NEI) that incorporates several hundred categories of on-road vehicles and 2) a Process Chain Analysis (PCA) for the different hydrogen production scenarios. NEI: The quantity of hydrogen-related emission is ultimately a function of the projected hydrogen consumption in on-road vehicles. Data for hydrogen consumption from on-road vehicles was derived from the number of miles driven in each U.S. county based on 1999 NEI data, the average fleet mileage of all on-road vehicles, the average gasoline vehicle efficiency, and the efficiency of advanced 2004 fuel cell vehicles. PCA: PCA involves energy and mass balance calculations around the fuel extraction, production, transport, storage, and delivery processes. PCA was used to examine three different hydrogen production scenarios: In the first scenario, hydrogen is derived from natural gas, which is extracted from gas fields, stored, chemically processed, and transmitted through pipelines to distributed fuel processing units. The fuel processing units, situated in similar locations as gasoline refueling stations, convert natural gas to hydrogen via a combination of steam reforming and fuel oxidation. Purified hydrogen is compressed for use onboard fuel cell vehicles. In the second scenario, hydrogen is derived from coal, which is extracted from mines and chemically processed into a hydrogen rich gas. Hydrogen is transmitted through pipelines to refueling stations. In the third scenario, hydrogen is derived via electrolysis powered by wind-generated electricity that has been transmitted across the country to electrolyzers at distributed hydrogen refueling stations. If hydrogen is produced via the first scenario, total annual U.S. production of carbon dioxide (CO2) could be expected to decrease by approximately 900 million metric tons, or 16 percent of annual U.S. CO2 production from all anthropogenic sources. Under this scenario, compared with the conventional vehicle fleet, a fuel cell vehicle fleet would produce some additional CO2 emissions due to the electric power required for the compression of hydrogen, but less CO2 emissions on the road during vehicle operation. This scenario results in an additional methane leakage of approximately one million metric tons per year, or 4 percent of annual U.S. methane emissions from all anthropogenic sources.