Molecular hydrogen in the troposphere: Global distribution and budget

Molecular hydrogen (H-2) has been measured since 1989 in air samples collected using a globally distributed sampling network. Time series from 50 locations are used to better define the distribution and recent changes of H-2 in the remote lower troposphere. These data show that the globally averaged H-2 mixing ratio between 1991 and 1996 was about 531 +/- 6 parts per billion (ppb). Hydrogen exhibited well-defined seasonal cycles in each hemisphere, with similar seasonal maxima (530-550 ppb). However, in the Northern Hemisphere the seasonal minimum was 70 ppb deeper than in the Southern Hemisphere (similar to 450 and 520 ppb, respectively), resulting in similar to 3% more H-2 in the south than in the north. With these data we have reevaluated the global H-2 budget. Methane oxidation is the largest source of H-2 to the troposphere, and soil uptake accounts for much of its sink. The global annual turnover is estimated as similar to 75 Tg H-2 yr(-1). The annual turnover, combined with a calculated tropospheric burden of 155 Tg, indicates a lifetime of similar to 2 years. While our understanding of the global distribution of the sources and sinks of H-2 is Still incomplete, the lower annual minimum in the north may be reasonably attributed to hemispheric asymmetry in uptake by soils. The seasonal cycles in the two hemispheres show unusual similarities: the northern and the southern seasonal maxima and minima were offset by only a few months. We suggest that the seasonal cycle in the Southern Hemisphere is dominated by H-2 emissions from biomass burning.