Help with Particle Playground Cannon

I am using CS5, After Effects, Win 7. I have created an effect of someone sprinkling snow on a scene which loooks pretty good but I'd like the snow to fade to nothing over a period as it falls. I know I saw in another tutorial an option called something like dies or fades and a timespan selection. However, I cannot for the life of me find it. Can anyone help ?

Regards,

TL.