The Isotopic Component of the Atmospheric Hydrogen Budget:  Project Summary

 

       The objective of the proposed research is to substantially improve our current understanding of the atmospheric H2 budget using isotopic measurements.  Although molecular H2 hasn’t received much attention until recently (Novelli et al., 1999; AGU Session in Fall, 1999), hydrogen deserves a closer look for a couple of important reasons.  First, the H2 budget is directly tied to the cycling of CH4, CO, and the non-methane hydrocarbons (NMHC) via formaldehyde (HCHO).  Photolysis of HCHO is the largest source of H2.  Since H2 shares many of the same sources as CO and a similar time trend in tropospheric concentrations since the 1980s, the H2 budget can constrain the CO budget (Khalil and Rasmussen, 1999), which in turn exerts a primary control on tropospheric OH levels.  Second, the global hydrogen budget will likely change substantially as society begins moving away from fossil fuel use.  For example, even a small 1% leakage from a hydrogen-based energy industry would double the current sources of atmospheric H2 (Wuebbles et al., 1997).

       Recently, a global H2 budget has been constructed based on atmospheric H2 concentration measurements made using the NOAA/CMDL sampling network (Novelli et al., 1999).  Novelli et al. concluded that soil uptake accounts for 75% of the global H2 sink although acknowledging that the global H2 budget is balanced only to about ±50% despite the addition of the new atmospheric H2 data. Novelli et al specifically recommend that isotopic measurements of H2 could significantly improve the H2 budget, echoing the earlier sentiments of Ehhalt and Volz (1976) and Ehhalt et al. (1989).

       Over the last four years we developed a method for remote collection of air and cryogenic concentration of H2 that allowed the D/H of atmospheric H2 at ambient concentrations to be measured to ±3 ‰ precision (Gerst and Quay, 2000).  Our new results indicate the dD of tropospheric H2 is 130 ‰ (n=25).  These new data are substantially improved over earlier dD measurements that relied on liquefied air collections mostly in urban locations or with poor collection efficiencies. 

       These new measurements raise a very important question.  Why is the dD of atmospheric H2 so enriched if the dD of the known sources (biomass burning and fossil fuel combustion) is deuterium depleted (‑200 to –300 ‰) and the fractionation during soil uptake of H2 is small (~-60 ‰)?  If soil uptake is the dominant H2 sink, as Novelli et al. (1999) indicate, then the dD of H2 in air should be close (within ~70 ‰) to the dD of the sources, assuming the H2 budget is near a steady-state.  In contrast, the dD of atmospheric H2 is 300-400 ‰ more enriched than the dD of the H2 emitted from fossil fuel and biomass combustion.  The only source for which we have no isotopic information is photochemically produced H2.  Only if the dD of the H2 produced via this pathway is substantially enriched can the isotopic composition of atmospheric H2 be reconciled with a soil-dominated sink.  Alternately, if the photochemical H2 source is deuterium depleted, then reaction of H2 with OH must be the dominant H2 sink.  Resolving this issue is the focus of our proposed research.  These issues are discussed in Gerst and Quay (2001).

       Our proposed research is comprised of four activities. First, we will verify our initial dD measurements of atmospheric H2 by measuring a biweekly time series at a coastal (marine air) site in Washington state and meridional transects of marine air during biannual oceanographic cruises between Seattle and Antarctica. Second, we plan to measure the D/H of H2 produced by photolysis of HCHO by measuring the D/H of HCHO in air and, via a collaboration with Carl Brenninkmeijer (Max Planck Institute, Mainz), the hydrogen isotope fractionation effect during H2 production during photolysis of HCHO.  Third, we plan to measure the D/H of H2 produced during biomass burning using air samples collected during field burning experiments in southwestern US and, possibly, Siberia via a collaboration with Wei Min Hao (USFS).  Finally, we intend to extend our initial measurements of the isotope fractionation effect during soil uptake of H2 to other soil types. We expect that our proposed isotopic measurements will result in a much better constrained budget for atmospheric hydrogen.  Specifically, we expect to decrease the uncertainty in the dominant H2 loss via soil uptake by a factor of five from the current ±50% to ±10%. This improved state of knowledge is important to achieve now as the role of H2 in the energy industry and, consequently, the anthropogenic source of H2 will increase in the future.

 

References:

 

Ehhalt, D.H. and A. Volz, 1976,Coupling of the CH4 with the H2 and CO cycle: isotopic evidence. In: Symposium on microbial production and utilization of gases (H2, CH4, CO). eds. Schlegel et al., p. 23-33, Akademie der Wissenschaften, Gottingen, Germany.

Ehhalt, D.H., et al., 1989, The kinetic isotope effect in the reaction of H2 with OH. J. Geophys. Res. 94: 9831-9836.

Gerst, S. and P.D. Quay. 2001. The deuterium component of the global molecular hydrogen cycle, J. Geophys. Res. 106: 5021-5037.

Gerst, S. and P.D. Quay. 2000. The deuterium content of atmospheric molecular hydrogen: method and initial measurements. J. Geophys. Res. 105:  21066-21080.

Khalil, M.A.K. and R.A. Rasmussen, 1999, The trends and global budget of atmospheric hydrogen, Fall AGU Meeting, San Francisco.

Novelli, P.C., et al., 1999, Molecular hydrogen in the troposphere: Global distribution and budget. J. Geophys. Res. 104: 30427-30444.

Wuebbles, D. et. al., 1997, Emissions and budgets of radiatively important atmospheric constituents, In: Engineering Response to Global Climate Change, R Watts (ed) CRC Lewis Publishers.