No abstract

[1] The distribution and atmospheric budgets for molecular hydrogen and its deuterium component dD are simulated with the GEOS-Chem global chemical transport model and constrained by observations of H 2 from the NOAA Climate Monitoring and Diagnostics Laboratory network and dD observations from ship and ground stations. Our simulation includes a primary H 2 source of 38.8 Tg a À1 (22.7 Tg a À1 from fossil and biofuels, 10.1 Tg a À1 from biomass burning, 6.0 Tg a À1 from the ocean) (where a is years) and a secondary photochemical source from photolysis of formaldehyde of 34.3 Tg a À1 . The simulated global tropospheric mean H 2 is 525 ppbv, with a tropospheric burden of 141 Tg and tropospheric lifetime of 1.9 a. Uptake by enzymes in soils accounts for 75% of the H 2 sink, with the remainder due to reaction with OH. The model captures the observed latitudinal, vertical, and seasonal variations of H 2 . For dD we find that a photochemical source signature from methane and biogenic volatile organic compound oxidation of 162% yields a global mean atmospheric dD of 130%, consistent with atmospheric observations. The model captures the observed latitudinal gradient in dD, simulating a 21% greater enrichment in the Southern Hemisphere because of the predominance of isotopically depleted fossil fuel emissions in the Northern Hemisphere. We find that stratospheric-tropospheric exchange results in 37% enrichment of tropospheric dD. Our simulation provides new simultaneous constraints on the H 2 soil sink (55 ± 8 Tg a À1 ), the ocean source (6 ± 3 Tg a À1 ), and the isotopic signature for photochemical production (162 ± 57%).Citation: Price, H., L. Jaeglé, A. Rice, P. Quay, P. C. Novelli, and R. Gammon (2007), Global budget of molecular hydrogen and its deuterium content: Constraints from ground station, cruise, and aircraft observations,

Observations of atmospheric methane (CH 4 ) since the late 1970s and measurements of CH 4 trapped in ice and snow reveal a meteoric rise in concentration during much of the twentieth century. Since 1750, levels of atmospheric CH 4 have more than doubled to current globally averaged concentration near 1,800 ppb. During the late 1980s and 1990s, the CH 4 growth rate slowed substantially and was near or at zero between 1999 and 2006. There is no scientific consensus on the drivers of this slowdown. Here, we report measurements of the stable isotopic composition of atmospheric CH 4 ( 13 C/ 12 C and D/H) from a rare air archive dating from 1977 to 1998. Together with more modern records of isotopic atmospheric CH 4 , we performed a time-dependent retrieval of methane fluxes spanning 25 y (1984-2009) using a 3D chemical transport model. This inversion results in a 24 [18,27] Tg y −1 CH 4 increase in fugitive fossil fuel emissions since 1984 with most of this growth occurring after year 2000. This result is consistent with some bottom-up emissions inventories but not with recent estimates based on atmospheric ethane. In fact, when forced with decreasing emissions from fossil fuel sources our inversion estimates unreasonably high emissions in other sources. Further, the inversion estimates a decrease in biomass-burning emissions that could explain falling ethane abundance. A range of sensitivity tests suggests that these results are robust.atmospheric methane | greenhouse gas emissions | methane isotopic composition | methane trends | Bayesian inversion C onsiderable research since the 1970s has established the role of methane (CH 4 ) in climate, as an infrared active gas, and as a chemically reactive species affecting hydroxyl radical, ozone, and carbon monoxide in the troposphere and chlorine, ozone, and water vapor in the stratosphere. At a globally averaged mixing ratio of 1,800 ppb, the abundance of CH 4 in the atmosphere has more than doubled since the industrial revolution as a result of population growth, agricultural practices, and fossil fuel use (1). The rise in CH 4 concentration is considered to contribute 0.48 Wm −2 of the 2.83 Wm −2 radiative forcing by wellmixed greenhouse gases since 1750 (2). Including indirect effects from CH 4 emissions roughly doubles its effective radiative forcing. Its global warming potential (not including feedbacks) is 28 based on a 100-y time horizon, but 84 based on a 20-y timescale [global warming potential (GWP) is relative to CO 2 ], illustrating the potential of large changes in the burden of CH 4 to influence climate on short timescales (2).Both the decrease in the CH 4 growth rate and its interannual variability since 1984 are well documented by at least four global networks of atmospheric measurements; agreement between time series is excellent with some exceptions early on. Recently, a review and synthesis of the CH 4 budget (3) pointed to some consensus of measurement and modeling studies and their comparisons toward understanding temporal changes in the CH 4 budget....

Measurements of δ13C and δD of atmospheric CH4 from whole air samples collected in the upper troposphere and lower stratosphere aboard the NASA ER‐2 aircraft during the SOLVE (2000), POLARIS (1997), and STRAT (1996) campaigns are reported. Samples cover latitudes from 1°S to 89°N and altitudes from 11 to 21 km, providing CH4 mixing ratios that range from 1744 to 716 ppbv. Measurements of isotope ratios were made by continuous‐flow gas chromatography isotope ratio mass spectrometry which provides high‐precision analyses on 60 ml aliquots of air. These measurements comprise the first upper atmosphere isotopic CH4 data set to date using this technique and the most extensive with respect to latitude and season in any case. Values of δ13C‐CH4 on the V‐PDB scale range from −47.28‰ near the tropical tropopause to −34.05‰ in the high northern latitude stratosphere. Values of δD on the V‐SMOW scale range from −90.9‰ to +26.4‰. Correlations of isotope ratios with CH4 mixing ratios show enrichment in the heavy isotopes as CH4 mixing ratios decrease due to kinetic isotope effects associated with oxidation by reaction with OH, Cl, and O(1D). Empirical fractionation factors are found to be highly dependent on the range of CH4 mixing ratio considered, increasing with decreasing mixing ratio. Systematic nonlinearity in a Rayleigh fractionation model suggests a range of stratospheric fractionation factors, αCstrat = 1.0108 ± 0.0004 to 1.0204 ± 0.0004 (2σ) and αHstrat = 1.115 ± 0.008 to 1.198 ± 0.008 (2σ), from high to low CH4 mixing ratio, respectively. The variation in α over the range in mixing ratios reflect changes in partitioning between CH4 sink reactions in different regions of the stratosphere. In Part 1, these new high‐precision observations are discussed and compared with other stratospheric and tropospheric isotope measurements. In Part 2 [McCarthy et al., 2003] the observations are compared with 2‐D model results, and implications for the kinetic isotope effects for reactions with OH, Cl, and O(1D) are discussed.

We report δD and δ13C measurements of atmospheric CH4 from air samples collected from two locations in the United States. They are the mid continental site Niwot Ridge, Colorado (40°N, 105°W), and a Pacific coastal site receiving strong westerlies, Montaña de Oro, California (35°N, 121°W). Data from multiyear approximately bimonthly sampling provide information relating seasonal cycling of CH4 sources and sinks in background air, record long‐term trends in CH4 mixing and isotope ratio related to the atmospheric CH4 loading, and may indicate regional CH4 sources. At Niwot Ridge, δD‐CH4 averaged −93.1 ± 3.0‰ from 1999 to 2001, while δ13C‐CH4 averaged −47.22 ± 0.13‰ from 1995 to 2001 with distinct seasonal cycles in both isotope ratios. At Montaña de Oro, atmospheric CH4 was observed to be more depleted in 13C and D: Measured δD‐CH4 averaged −97.3 ± 3.7‰ from 2000 to 2001, while δ13C‐CH4 averaged −47.26 ± 0.17‰ from 1996 to 2001, and seasonal cycles were larger than those observed at Niwot Ridge. Mixing ratios observed at Montaña de Oro were higher on average than at Niwot Ridge. At both sites, δ13C‐CH4 was found to correlate poorly with mixing ratio, an indication that varying CH4 sources are partly responsible for the δ13C‐CH4 seasonal signal. In contrast, a strong anticorrelation exists between δD‐CH4 and mixing ratio, with maxima and minima approximately 6 months out of phase, indicating a sensitivity of δD to sink processes. The dual isotopic constraint to atmospheric CH4 seasonality implies that these midlatitude sites are annually influenced by a 13C‐enriched CH4 source(s) seasonally increasing in late spring and a 13C‐depleted CH4 source(s) seasonally increasing in late summer or early fall.