How to implement global fog image effect if depth texture not support by graphic card

Hi guys, I want to use the vertical fog effect. In unity, the global fog image effect looks fantastic. But many machine in office with integrated graphics can not support depth texture. For example; Inter G31/G33.

I try to use particle system, but it effects the frame count too much if it cover the whole scene. Is there way to implement the global effect if the graphic card not support depth texture.

Sorry for my English. Thank You.