I use this all the time for smoke and fire.
Here is the expression:
[1920, time*-150]
Your offset will move upwards like smoke or fire.
The “1920” is the X position. It can be changed to any position but it remains static so it really doesn’t matter.
The “time*-150” is the rate and position your offset moves. Negative values go up (odd I know) and positive values go down. The 150 can be changed to affect the speed.
Hope this helps.