Optimal estimation of the soil uptake rate of molecular hydrogen from the Advanced Global Atmospheric Gases Experiment and other measurements

Abstract

Hydrogen (H2), a proposed clean energy alternative, warrants detailed investigation of its global budget and future environmental impacts. The magnitudes and seasonal cycles of the major (presumably microbial) soil sink of hydrogen have been estimated from high‐frequency in situ AGAGE H2 observations and also from more geographically extensive but low‐frequency flask measurements from CSIRO and NOAA‐GMD 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. The global production rate of H2 is estimated to be 103 ± 10 Tg yr−1 with only a small estimated interannual variation. Soil uptake (84 ± 8 Tg yr−1) represents the major loss process for H2 and accounts for 81% of the total destruction. Strong seasonal cycles are deduced for the soil uptake of H2. The soil sink is a maximum over the northern extratropics in summer and peaks only 2 to 3 months earlier in the Northern Hemisphere than in the Southern Hemisphere. Oxidation by tropospheric OH (18 ± 3 Tg yr−1) accounts for 17% of the destruction, with the remainder due to destruction in the stratosphere. The calculated global burden is 191 ± 29 Tg, indicating an overall atmospheric lifetime of 1.8 ± 0.3 years. Hydrogen in the troposphere (149 ± 23 Tg burden) has a lifetime of 1.4 ± 0.2 years.