• Methane?(CH₄) is a simple?hydrocarbon
• It is present as a?gas?in the atmosphere, and underground, and is the main component of?natural gas?fossil fuel
• Methane can be produced by the?naturally occurring process?known as?methanogenesis?by organisms known as?methanogens
  • Before methanogenesis can occur, a series of bacteria convert organic matter into a compound called acetate, as well as carbon dioxide and hydrogen
  • A group of single-celled organisms called?archaeans?then carry out methanogenesis via two different mechanisms:

CO₂?+ 4H₂?→ CH₄?+ 2H₂O

CH₃COOH → CH₄?+ CO₂

• • CH₃COOH is acetic acid, a compound that can be formed from acetate
• Archaeans carry out methanogenesis in a range of environments
  • Waterlogged mud
    • E.g. in naturally occurring wetlands, or in man-made rice fields
  • The?guts of ruminant mammals?such as cattle
  • Landfill sites?containing organic matter such as food waste
  • Anaerobic digesters?used for the break down of organic waste
• Depending on the location of methanogenesis, the methane is either?released directly into the atmosphere?or into the surrounding soil
  • Methane can?accumulate in the ground, but it can gradually make its way to the surface where it is released into the atmosphere
• Human activities?such as livestock farming and landfill disposal of waste food are leading to an?increase in the release of methane into the atmosphere
  • Note that anaerobic digesters are sealed units for the production of biogas, and the methane produced is collected and used for fuel

• The lifetime of methane gas once it reaches the atmosphere is around 10-12 years
• This is because methane is?oxidised?in the atmosphere
  • This oxidation involves a reaction with molecules called?hydroxyl radicals?(·OH)
    • Hydroxyl radicals are?highly reactive?molecules that form in the presence of sunlight from other gases in the atmosphere e.g. nitrous oxides
  • Hydroxyl radicals?react with methane?in a series of reactions that produce?carbon dioxide and water
• Methane oxidation?keeps levels of methane in the atmosphere?relatively constant
  • Recent increases in atmospheric methane have led to concern that levels of hydoxyl radicals in the atmosphere may be declining