I've found out how to make a part gradually transparent, either by tweening or through a loop, but how about a particleEmitter? I'm not sure how to make a particleEmitter go transparent through tweening (if possible) or a loop.
The Transparency property of a ParticleEmitter is a NumberSequence. I don't believe the TweenService supports number sequences, so you may have to use a loop. Assuming that you don't actually care to use the special features of the NumberSequence and want to use it like a normal Transparency property, you can just use emitter.Transparency = NumberSequence.new(number)
to set the transparency. The link I attached should tell you all need to know about the NumberSequence data type.