Global budget of molecular hydrogen and its deuterium content:: Constraints from ground station, cruise, and aircraft observations

The distribution and atmospheric budgets for molecular hydrogen and its deuterium component delta D 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 H2 source of 38.8 Tg a(-1) (22.7 Tg a(-1) from fossil and biofuels, 10.1 Tg a(-1) from biomass burning, 6.0 Tg a(-1) from the ocean) (where a is years) and a secondary photochemical source from photolysis of formaldehyde of 34.3 Tg a(-1). 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 H2 sink, with the remainder due to reaction with OH. The model captures the observed latitudinal, vertical, and seasonal variations of H2. For dD we find that a photochemical source signature from methane and biogenic volatile organic compound oxidation of 162 parts per thousand yields a global mean atmospheric delta D of 130 parts per thousand, consistent with atmospheric observations. The model captures the observed latitudinal gradient in dD, simulating a 21 parts per thousand 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 parts per thousand enrichment of tropospheric delta D. Our simulation provides new simultaneous constraints on the H2 soil sink (55 +/- 8 Tg a(-1)), the ocean source (6 +/- 3 Tg a(-1)), and the isotopic signature for photochemical production (162 +/- 57 parts per thousand).