Which gas is long-lived in the atmosphere with a long residence time?

• A. Water Vapour
• B. Carbon Dioxide
• C. Methane
• D. Ozone

Answer: (B)

This item tests how long different atmospheric gases stay in the air before being removed or transformed. Water vapour cycles quickly with weather—evaporating, condensing, and precipitating—so its residence time is measured in days to a couple of weeks, not centuries. Ozone also turns over relatively fast, especially in the troposphere, where lifetimes are days to weeks. Methane lasts longer than these, on the order of about a decade, but it is still removed faster than carbon dioxide. Carbon dioxide, however, lingers for centuries to thousands of years because its sinks—like ocean uptake, photosynthesis, and slower geological processes—operate on much longer timescales. That slow removal means CO2 accumulates and remains in the atmosphere far longer than the other gases listed.