5.2 Methane

At room temperature and standard pressure, methane is a colorless, odorless gas. In the atmosphere it is a „well mixed” greenhouse gas due to its relatively long lifetime in the atmosphere, and it belongs to the group of long-lived greenhouse gases (LLGHGs). Once emitted, CH4 remains in the atmosphere for approximately 8.4 years before removal. Methane is a potent GHG hence being of great interest among atmospheric scientists.

5.2.1 Sources and sinks of atmospheric methane

Sources

Methane has biogenic and non-biogenic (geological) natural sources. The most important biogenic source is anaerobic decomposition on wetlands, where methane is produced through anaerobic decomposition in soils and sediments. Warm temperatures and moist environments are especially favourable for methane production.

Slow decomposition rates generally cause nontidal wetlands to accumulate carbon as dead soil organic matter and/or peat; in other words, these systems act as an atmospheric CO2 sink. This source is highly sensitive to temperature so that changes in the global climate toward warmer conditions could, depending on the moisture changes, lead to significant increases in CH4 emissions and a positive feedback causing further warming. A significant drying of current wetland areas however would lead to diminished CH4 emissions and most likely to a net release of CO2 to the atmosphere.

Microorganisms breaking down difficult to digest material in the guts of ruminant livestock and termites produce methane that is then released during defecation. Non biogenic sources of methane are permafrost, glaciers, and ice cores. Large amounts of methane is stored in permafrost areas being released when melting. Frozen soils can be a source that slowly releases methane trapped in frozen environments as global temperatures rise.

Mass burning of organic matter releases huge amounts of methane into the atmosphere. Additional natural sources include termites, oceans, vegetation and CH4 hydrates. Methane is stored in large amounts in deep ocean deposits in form of methane hydrate. Methane hydrate is a solid form of methane combined with water under low temperatures (less than 25°C) and moderate pressure (greater than 3−5 MPa). These circumstances can be found around 300−500 m depth. With increasing temperature or decreasing pressure the methane hydrate stored in deep oceans can be released in large amounts calling scientists' attention to warming climate.

Sinks

Hydroxyl radical (·OH) in the atmosphere is the largest sink for atmospheric methane as well as one of the most significant sources of water vapor in the upper atmosphere

CH4 + ·OH → ·CH3 + H2O.

(R5.9)

Because tropospheric reactions with ·OH is the main sink of CH4 the abundance of ·OH regulates lifetime of methane. Reducing quantities of atmospheric ·OH as a result of biomass

burning therefore increases lifetime of methane. Isoprene, an organic carbon compound emitted by vegetation competes with methane for ·OH, therefore isoprene increases lifetime of CH4 by lowering ·OH concentration.

CH4 is lost to the stratosphere when it reacts with ·OH, Cl and O(1D). Methane oxidation is a source of water in the stratosphere

CH4 + O(1D) → ·CH3 + ·OH.

(R5.10)

Water vapor is important being a greenhouse gas and stratospheric ozone chemistry. Some of the methane in soils is metabolized by methanotrophs (methane-consuming microorganisms). Biological CH4 oxidation in drier soil is carried out by microorganisms that use methane in their metabolism (metanotrophs) under aerobic conditions, producing carbon dioxide and water

CH4 + 2O2 → CO2 + 2H2O.

(R5.11)

Under anaerobic condition methane is oxidized in the following reaction combining with nitrate/nitrites

CH4 + NO3/NO2 + H+ → CO2 + N2 + H2O.

(R5.12)

Methane also reacts with natural chlorine gas in the atmosphere to produce chloromethane and hydrochloric acid (HCl). This process is known as free radical halogenations.

CH4 + Cl2 → CH3Cl + HCl

(R5.13)

Table 5.1: Atmospheric sources and sinks of methane (IPCC AR4 2007)

 

 

Emission

[Tg(CH4) yr–1]

 

Total sources 

582

 

Imbalance 

+1

Sinks

Soils 

30

Tropospheric OH 

511

Stratospheric loss 

40

Total sink 

581