Most changes that take place in the atmosphere take place in the

The troposphere is the first layer above the surface and contains half of the Earth's atmosphere. Weather occurs in this layer.  Many jet aircrafts fly in the stratosphere because it is very stable. Also, the ozone layer absorbs harmful rays from the Sun. Meteors or rock fragments burn up in the mesosphere.The thermosphere is a layer with auroras. It is also where the space shuttle orbits. The atmosphere merges into space in the extremely thin exosphere. This is the upper limit of our atmosphere.

I'm not sure if this helps you but I would say most of the changes happen in the troposphere because that is where the weather occurs. Sorry if this isn't the answer you were looking for.