Introduction
Conclusions
Hydrogen in the Atmosphere Today
The troposphere has an estimated 155 Tg of hydrogen gas, with approximately a two-year lifetime. This relatively short lifetime is accounted for by many sources of hydrogen gas and a few major sinks.
Sources
One of the major sources of H2 is the oxidation of methane. This produces an estimated amount of 26 Tg per year. Methane goes through many reactions to eventually produce hydrogen.
In the first step, methane reacts with a hydroxyl radical:
CH4 + OH → CH3 + H2O
methane and water molecules
The methyl radical then can react with oxygen in the atmosphere:
CH3 + O2 + M → CH3O2 + M
From here, CH3O2 can undergo several possible reactions:
CH3O2 + CH3O2 → 2 CH3O + O2 or CH3OH + HCHO + O2
CH3O2 + HO2 → CH3OOH + O2
CH3O2 + NO → CH3O + NO2
There are four hydrocarbon products here: CH3O, CH3OOH, CH3OH and HCHO. Formaldehyde (HCHO) can undergo photolysis. The product of the photolytic reaction will react to form hydrogen gas:
HCHO + hv → H + HCO → H2 + CO
formaldehyde
The rest of the hydrocarbons will undergo reactions that will eventually produce formaldehyde, which will form hydrogen in the above reaction:
CH3O + OH + O2 → HCHO + HO2 + H2O
CH3OOH + hv → CH3O + OH
CH3OOH + OH → CH3O2 + H2O or HCHO + H2O + OH
CH3OH + OH + O2 → HCHO + HO2 + H2O
Hydrocarbons other than methane can also oxidize to produce hydrogen gas. These molecules produce about 14 Tg hydrogen gas per year and include many organic molecules such as terpenes and aromatic hydrocarbons. H2 is also produced in combustion reactions. Studies have shown that during combustion, the ratio of H2 to CO produced falls within a range of 0.035 to 0.071. Therefore, by measuring the amount of CO produced in a combustion reaction, the amount of H2 produced can be calculated. Transportation produces around 5-20 Tg H2 per year, and non-transportation combustion sources produce around 0.4-6 Tg H2 per year. Also, biomass burning produces an additional 5-27 Tg H2. Finally, nitrogen fixing and oceans give off around 6 Tg total H2 per year.
A major sink of hydrogen gas is soil uptake. The amount of hydrogen in different locations around the world has been measured since 1989. There are seasonal variations in the amount of hydrogen in the atmosphere, and the measurements have shown that the Northern Hemisphere has a lower H2 minimum than the Southern Hemisphere. This reflects the greater area of land in the Northern Hemisphere. Another hydrogen sink is the formation of water from hydrogen gas and hydroxyl radicals:
H2 + OH → H2O + H
hydrogen molecule
This reaction accounts for the removal of around 20 Tg H2/year.
Source: Novelli, P.C., et al., Molecular hydrogen in the troposphere: Global distribution and budget. Journal of Geophysical Research, 1999. 104(D23): p. 30427-30444.
