Optimal Estimation of the Soil Uptake Rate of Molecular Hydrogen from AGAGE and Other Measurements

Hydrogen (H2), a proposed clean energy alternative, warrants detailed investigation of its global budget and future environmental impacts. The major (presumably microbial) soil sink of hydrogen has been estimated from high frequency in situ AGAGE H2 observations and also from geographically extensive flask measurements from CSIRO and CMDL using the Kalman filter in a two-dimensional (2-D) global transport model. Hydrogen mole fractions exhibit well-defined seasonal cycles in each hemisphere with their phase difference being only about 3 months. Strong seasonal cycles are deduced for the soil uptake of H2. Photolysis of formaldehyde (HCHO) is the major immediate atmospheric source of H2 and the global production rate of H2 is estimated to be about 61.9 Tg/year. The multi-year average global sink is estimated as 61.8 Tg/year. Soil uptake (42.3 Tg/year) represents the major loss process for H2 and accounts for 68% of the total destruction. Oxidation by OH (17.8 Tg/year) accounts for 29% of the destruction, with the remainder due to destruction in the stratosphere. The soil sink is a maximum over the northern extratropics in summer. The calculated global burden is 186 Tg, indicating an overall atmospheric lifetime of about 3.0 years. Hydrogen in the troposphere (148 Tg burden) has a lifetime of about 2.4 years.