CHANGE to Volume Emitter

Go to the Emitter node and under Basic Emitter Attributes, change the Emitter Type to Volume. Afterwards, under Volume Emitter Attributes, change the Volume Shape to Sphere from Cube. Under the same tab, set the Volume Sweep to 180. You should have a hemisphere that’s emitting particles at 180 degrees. Take the Emitter and rotate it to make sure that the flat side of it is parallel to the ground, where you can then place it. This will give you the basic ‘shape’ of the explosion as it interacts with the scene.