Methane: Trace Greenhouse Gas in Dry Air

Steps:

• Identify dry air composition: mainly N2, O2, Ar, CO2, with traces like methane (CH4) at ~1.8 ppm.
• Recall greenhouse gases: absorb infrared radiation, trapping heat and causing warming.
• Evaluate options: select natural trace gas in dry air that directly contributes to warming.
• Confirm methane as potent GHG from natural sources like wetlands.

Why B is correct:

• Methane (CH4) is a greenhouse gas with global warming potential 28-36 times CO2 over 100 years, per IPCC definitions.

Why the others are wrong:

• A: Carbon monoxide (CO) is not a greenhouse gas; it oxidizes to CO2 but mainly impacts health via toxicity.
• C: Nitrogen oxide (NOx) forms ozone (a GHG) but is minor/direct contributor and mostly anthropogenic.
• D: Sulfur dioxide (SO2) forms cooling aerosols in atmosphere, countering warming via reflection.

Final answer: B