Use the noun troposphere when you're talking about the part of the atmosphere that's closest to the surface of the Earth.
The gases around our planet, which are affected by the Earth's gravity, make up the atmosphere. It is divided into three sections, the lowest of which is the troposphere. Our planet's weather is largely caused by whatever happens in the troposphere, which is known by scientists for its turbulent movement. The word troposphere comes from the Greek root tropos, "a turn or change."