Introduction:
In urban areas, landfills are common waste disposal sites. As organic waste decomposes in landfills, various gases are released into the atmosphere. One of the most significant gases released from landfills in urban areas is methane.

Explanation:

Methane:
- Methane (CH4) is a potent greenhouse gas that contributes to climate change. It is released during the anaerobic decomposition of organic waste in landfills.
- When organic waste, such as food scraps and yard trimmings, is buried in a landfill, it undergoes decomposition in the absence of oxygen (anaerobic conditions).
- Under anaerobic conditions, microorganisms break down the organic matter in a process called anaerobic digestion. This digestion process produces methane as a byproduct.

Other gases:
- Nitrogen (N2) and oxygen (O2) are the two main components of the Earth's atmosphere and do not significantly contribute to landfill gas emissions.
- Hydrogen (H2) is not commonly released from landfills in significant amounts.

Importance of methane:
- Methane is considered a potent greenhouse gas, with a global warming potential 25 times greater than carbon dioxide (CO2) over a 100-year period.
- Landfills are one of the largest human-caused sources of methane emissions globally.
- Methane emissions from landfills contribute to climate change and can also pose safety risks, as methane is flammable and can potentially lead to explosions if not properly managed.

Conclusion:
In urban areas, landfills are major sources of methane emissions. Methane is released during the anaerobic decomposition of organic waste in landfills. It is a potent greenhouse gas that contributes to climate change. Proper landfill management practices, such as capturing and utilizing methane, can help reduce the environmental impact of landfill gas emissions.