Trying to create a planet with sulphur dioxide (which is heavier than oxygen I think) on a lower level and oxygen above it. Is this possible? Need something that is horrible on the bottom and want mobile fungi [yep, they have legs] scrambling up to get away from it into the oxygen. (So annoying to have to always use fungi or bacteria when you are looking for an extremophile. Fungi have been done to death.)
Somewhat surprisingly, kind of yes. (If you ignore the issue of how you keep the SO2 from oxidizing to SO3, anyway.)
Usually, the answer to any question of the general form "can I have part of my atmosphere composed of X and a different part composed of Y" is "no, because gasses are all fully miscible. However, that stops being a problem if one of the components stops being a gas.
SO2 has a boiling point of -10C. Which means that at "high" altitudes, SO2 will condense. Now, that won't get rid of all of it, but it will significantly reduce the concentration of SO2 above the SO2 cloud level. So, you just need to make the planet cold enough that there is a significant amount of land above that altitude / below that temperature. -10C is fortunately well within the range of temperatures survivable by Earthlike life.
If you want to completely eliminate SO2 from the air above some point, you will need to go below -72C, which is far less compatible with macroscopic Earthlike life.