# Atmospheric methane removal

Atmospheric methane removal is a category of potential approaches being researched to accelerate the breakdown of methane that is in the atmosphere, for the purpose of mitigating some of the impacts of climate change. With concentrations of atmospheric methane increasing twice as fast as carbon dioxide since 1750, methane is the second most impactful greenhouse gas.
Atmospheric methane has increased since pre-industrial times from 0.7 ppm to 1.9 ppm. From 2010 to 2019, methane emissions caused 0.5 °C (about 30%) of observed global warming. Global methane emissions approached a record 600 Tg CH$_4$ per year in 2017.

## Natural atmospheric methane sinks

Methane has a limited atmospheric lifetime, about 10 years, due to substantial methane sinks. The primary methane sink is atmospheric oxidation, from hydroxyl radicals (~90% of the total sink) and chlorine radicals (0-5% of the total sink). The rest is consumed by methanotrophs and other methane-oxidizing bacteria and archaea in soils (~5%).

## Potential approaches

Different methods to remove methane from the atmosphere include thermal-catalytic oxidation, photocatalytic oxidation, biological methanotrophic methane removal, concentration with zeolites or other porous solids, and separation by membranes.
Potential methods can be categorized by the underlying catalytic process, or the potential deployment form.

### Enhanced atmospheric methane oxidation

Enhanced Atmospheric Methane Oxidation is the concept of enhancing the overall oxidative methane sink in the atmosphere, through generating additional hydroxyl or chlorine atmospheric radicals.

#### Iron salt aerosols

Iron salt aerosols are one proposed method of enhanced atmospheric methane oxidation which involves lofting iron-based particles into the atmosphere (e.g. from planes or ships) to enhance atmospheric chlorine radicals, a natural methane sink. Winds over the Sahara raise dust into the troposphere and disperse it over the Atlantic. A 2023 study suggests that this has contributed to natural atmospheric methane oxidation.
Iron salt aerosols are being studied for the potential of iron(III) chloride (FeCl$_3$) to catalyze chlorine radical production. Chlorine atoms are produced by photolysis from the FeCl$_3$ stemming from iron-containing airborne dust aerosol particles in the oceanic boundary layer.
    FeCl$_3$ + hv → FeCl$_2$ + $^o$Cl
The chlorine atoms initiate methane oxidation:
    CH$_4$ + $^o$Cl → HCl + $^o$CH$_3$
The resulting methyl radical is unstable and oxidizes naturally to CO$_2$ and water:
    3.5O$_2$ + 2$^o$CH$_3$ → 2CO$_2$ + 3H$_2$O

##### Side effects of ferric chloride

Fine particles dispersed in the atmosphere can serve as cloud condensation nuclei and thereby cause marine cloud brightening
Eventually all FeCl$_3$ particles are washed out of the air and fall on land or water, where they dissolve into iron compounds and salt.
Iron salt aerosols may also therefore contribute to iron fertilization.

### Terrestrial methanotroph enhancement

Soil bacteria and archaea account for approximately 5% of the natural methane sink. Early research is going into how the activity of these bacteria may be able to be enhanced, either through the use of soil amendments, or introduction of selected or engineered methane-oxidizing bacteria.

### Catalytic engineered systems

Catalytic engineered systems are designed to pass air from the atmosphere, either passively or actively, through catalytic systems which leverage energy from the sun, an artificial light, or heat to oxidize methane. These catalysts include thermocatalysts, photocatalysts, and radicals produced artificially through photolysis (using light to break apart a molecule).