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Abstract. Molecular hydrogen (H2) 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 (-450 and 520 ppb, respectively), resulting in -3 % more H2 in the south than in the north. With these data we have reevaluated the global H2 budget. Methane oxidation is the largest source of H2 to the troposphere, and soil uptake accounts for much of its sink. The global annual turnover is estimated as -75 Tg H2 yr -i. The annual turnover, combined with a calculated tropospheric burden of 155 Tg, indicates a lifetime of-2 years. While our understanding of the global distribution of the sources and sinks of H2 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 H2 emissions from biomass burning.

Abstract. The Total Carbon Column Observing Network (TCCON) produces precise measurements of the column average dry-air mole fractions of CO 2 , CO, CH 4 , N 2 O and H 2 O at a variety of sites worldwide. These observations rely on spectroscopic parameters that are not known with sufficient accuracy to compute total columns that can be used in combination with in situ measurements. The TCCON must therefore be calibrated to World Meteorological Orga-