203. Which of the following exhaust gas emissions from an LNG fueled diesel engine is the most potent greenhouse gas?

Methane is the correct answer; it possesses a significantly higher greenhouse warming potential per unit mass than the other listed exhaust components. Hydrogen sulfide and methyl mercaptan are primarily toxic pollutants, while carbon dioxide, though a major greenhouse gas overall, is less potent than methane on a per-mass basis. Unburned methane in LNG engine exhaust, known as methane slip, is a significant environmental concern due to its potent greenhouse effect.