How to affect density (alpha) of a pyro sim using particles' attribute "$AGE"

[foxtrotzulu]

Hi all,  a question about doing vapor trails (contrails).

I'm looking to do a contrail from an engine of a jet, and the method I'm using is 

1. Do a pyro sim, and cache out a single frame.

2. With POP network, birth particle points with attribute $AGE

3. Using copy SOP, copy the cached out single frame of the pyro smoke onto each point of the points from POP.

4. Create alpha attribute on the POP points using the expression I found searching on this forum on a point SOP, as answer provided by member Wolfwood;

1-smooth($AGE,2,3)

Now this works well on the Points, as the points fade out to 0 alpha and disappear after 3 seconds, but since I'm copying the static single pyro sim a (.bgeo) on to each of these points, I want the volume to have the same behavior; that is, after 3 seconds, completely disappear. Of course, since I'm copying the volume onto points, when points die after certain number of seconds(say, 6 seconds), the volume that has been copied onto the point also dies, but it's not gradual, rather, it just dies suddenly without slow fade to zero alpha.

So, the question is, what is the best way to transfer this slow fading of alpha attribute onto the volumes? I thought about using the expression on the material' density, but then the entire volume trail fades together, whereas I want the volume to start fading few seconds after they're born, that is, the volume furthest from the traveling jet's engine fades first.

I hope this makes sense and I would appreciate any insight. Thanks!

[3dome]

try using a volume vop/wrangle and multiply your density with that attibute you created