Fading out particle system from one side I am trying to animate a part of a forest being cleared and being replaced by a building. I was wondering how I would animate the tree particles in the area fading out in transparency from the top of the image to the bottom, with the bottom trees fading out last. The area around the clearing is a river. I have created a separate particle system with only the trees that I want to disappear. This particle system is hidden in the  picture. I was looking into using the distance from an empty on the top side with no success yet. Thank you.
I apologize if I did not explain the problem thoroughly. I found a rather simple way to achieve the effect. I first made a gradient and mapped it to a plane.



By setting the particles to use the uv map of the plane and checking "From Instancer", I used the gradient to determine the factor used for transparency.



By animating the location of the gradient I achieved this sweeping effect.