How to keep the sharp?

[shoujiaan]

That issue bother me for a long time. I'd like to create a fragment leading a long smoke tail down to the earth, but I found I can't keep the sharp of smoke. If I use particle age scale size, in the end of the animation I will got the straight thick smoke. If I use the object distant to scale size, at the beginning of the animation the smoke have a big head. How I keep the sharp? I have tried use long partile age to trick, it works but the side effect is that I lose the contrl of the density. Does anyone can help me?