EaseIn EaseOut at the top of an arch?

[Streetwise]

I hope I can explain this well enough. I've created a basic event that emits particles straight up, that then start to fall. The problem is when the particles get to the top of the arc, there doesn't seem to be a nice transition in speed before gravity takes over. They just start falling rather abruptly. Is there an adjustment to control this sort of thing?  

Thanks!

Dave