Seasonal and Interannual Variations of Atmospheric H2 in the Troposphere a Link of Climate Change

Abstract

Molecular hydrogen (H2) is the second most abundant reduced gas in the atmosphere. Like the most abundant reduced gas, CH4, atmospheric H2 is attacked by atmospheric oxidizing agent, hydroxyl radical, at a similar frequency. However, its lifetime (~1.4 years) is much shorter than CH4 (~10 years) due to its overwhelming destruction by soils. Major sources are photochemical oxidation of hydrocarbons (~50%) and combustion processes. Hydrogen economy in future may change the current atmospheric H2 cycle associated with the unavoidable leakage of H2 in the infra-structure. To assess the impact of the hydrogen economy to atmospheric environment in future, it is necessary to evaluate the present atmospheric H2 cycle as accurately as possible. Here we present atmospheric hydrogen cycles in the troposphere. Recent developments in continuous flow isotope ratio mass spectrometry allow us to obtain sufficiently precise data even using small volumes of air (0.4 L). Free tropospheric air samples have been collected with the CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) Boeing 767 over Europe and Africa during 3 flights in May, July, and December to investigate temporal, spatial variations of H2 abundance. The D/H ratio as well as the H2 mixing ratio shows a clear seasonality in both hemispheres. The seasonal variation of H2 is similar to that observed in the remote marine boundary layer. However, the D/H ratio varies out of phase with the H2 concentration in the northern hemisphere, while varying in phase in the southern hemisphere. This suggests that the source or sink processes regulating the H2 mixing ratios are different in each hemisphere. We also investigate the southern hemispheric H2 cycle using D/H stable isotope analyses of H2 and CH4in 29 samples of air collected from 1992 and 2004 at the Australian Cape Grim baseline air monitoring station and stored in the unique air archive in Aspendale. A harmonic-polynomi