I can think of 2 ways that might work for you.
- You can weight paint the bottom part that you want the particles to come from.
a. then under vertex groups > density choose the weight paint group you just made.
- You can duplicate the face and hit "P" to seperate it from the mesh and just put the particle system on the new object you spererated. Then parent it to your main object (if it will be animated) and hide it with the emitter check box.