Final answer:
Most of the mass of radiatively active gases in Earth's atmosphere is found in the troposphere, which contains about 80% of the atmospheric mass and is crucial for the greenhouse effect.
Step-by-step explanation:
In Earth's atmosphere, the troposphere is where most of the mass of radiatively active gases reside. This layer extends up to approximately 12 km above sea level and contains roughly 80% of the Earth's atmospheric mass. The troposphere is also where most weather events originate, and it plays a significant role in the greenhouse effect due to the concentration of gases like carbon dioxide and water vapor that absorb and emit radiation.