2.5. Hydrogen

Beginning in 1989 the mole fraction of molecular hydrogen (H₂) has been measured in samples of air collected as part of the CMDL air sampling network. Only glass flasks with Teflon O rings were used. The Reduced Gas Detector used for CO measurements was also used to determine H₂. The analysis included a single calibration gas with approximately 500 ppb H₂ that was compared to each sample. Multiple calibration standards were maintained through frequent intercomparisons, and although the mole fractions assigned to these standards were based upon a somewhat arbitrary scale, the standards were all internally consistent. A set of H₂-in-air standards was recently prepared by gravimetric techniques in collaboration with the CMDL Nitrous Oxide and Halocarbon (NOAH) Group. These provide a range of mole fractions between 480 and 600 ppb. All working standards were recalibrated against the gravimetric standards. The archived peak height responses from all air samples analyzed since the beginning of the program were then used to recalculate sample H₂ mole fractions using the new values assigned to the standards. The gravimetric scale was about 6% higher than the preliminary working scale. The data set now provides H₂ time series from 48 sites used to study local, regional, and global scale distributions. Annual average mole fractions from network sites are given in Table 2.10. The north-to-south gradient of increasing annual mean mole fractions is unusual; many gases with anthropogenic sources (such as CO₂, CO, and CH₄) have higher levels in the industrialized northern hemisphere.

TABLE 2.10. Provisional Annual Mean H₂ Mole Fractions From Land Sites

There is about 3% more H₂ in the southern troposphere than in the north. Figure 2.19 presents the times series from two sites representative of the northern and southern hemispheres: BRW located at 71°N, and Cape Grim, Tasmania (CGO) at 41ºS. The large seasonal cycle found at BRW contrasts the much smaller variation at CGO. At BRW the mole fraction ranges from approximately 460 to 530 ppb with highest levels in late winter and early spring and lowest levels in fall. At Cape Grim the amplitude of the seasonal cycle is smaller with a maximum of 550 ppb and a seasonal minimum of about 520 ppb. The differences between the two sites, and similarly between the two hemispheres, are determined primarily by the strength of the seasonal drawdown. The two main sinks for H₂ are believed to be reaction with OH radicals and deposition at the surface through the activities of terrestrial microbes [Warneck, 1988 and references therein]. Since the amount of H₂ removed by OH is approximately the same in both hemispheres, biological uptake on northern continents is the likely cause of the observed differences between the hemispheres [also see Khalil and Rasmussen, 1989].

Fig. 2.19. Time series of hydrogen mole fractions: (Top) Point Barrow, Alaska, and (Bottom) Cape Grim, Tasmania.

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