Need Help Setting Up an Emitter

The emission angle helped a lot. I already have the shape set to Line, but the particles birth is all over the place instead of each one starting from the same y-coordinate. How can I get them all starting from the same line, except maybe change the vertical scale from 0 to an increasing value over life.

No, I actually do mean the y-coordinates, since the line to be pulled apart is a horizontal line and the particles are to rise up from it. Even though the emitter shape type is Line, the particles are not originating from a single line. They're spread out as though the shape type were Rectangle.

Yes, I use a mask on another emitter earlier in the clip to keep the particles contained within a specific outline shape and it works great. But this one from Data Flow won't accept a mask for some reason. When I drag a mask onto it, it changes to a shape and the Convert to Mask option is disabled. Maybe I'll just create an emitter from scratch again. Here's a picture. You can see that the shape type is Line and the particles are not emitting from a "line" (red markup on the right). I'm wanting to remove all the particles below the white line so it looks like the line is "disintigrating" from the topside.

I can't get a mask to work on the emitter. Motion is only allowing one to be dropped onto the particle cell instead of the actual emitter. On my other effect I somehow got the mask on the emitter object instead of the particle cell.

I finally got a mask working by copying my other effect, swapping out the mask object, and swapping out the emitter object. The particles are now masked below the line, but I now also see that a mask approach will not work because the Outer Glow filter is still present on all the particles (including the masked ones), making the glow too large and an obvious mask edge is visible. So back to figuring out how to have the particles all birth from one single y-coordinate.

Figured it out. The emitter particle cell for Data Flow is "spots", which is a scattering of shapes rather than a single image, and that's what's making it look like the particles don't share the same y starting point. Each "particle" is actually several spread out spots. Time for me to get creative....

(Is there a way to create particles for an emitter so they look like the fragmented streaks of Data Flow?)

So after adjusting the Outer Glow settings a bit I arrived at this which I think looks pretty good. Rather than have the line "fade" out by keying the opacity, any ideas on how to make it look like particles leaving the line are actually pieces removed from the line itself? I'm thinking maybe another particle emitter (or replicator) over the line to mask bits of it out as the particles move away, so after a dozen frames or so the line ends up completely masked.

Thanks guys, your suggestions of duplicating the emitter with 0 speed is also what I was thinking to mask out the line with small bits over time until it's all masked out. Yes, the particles move upwards from the line, after the line has moved into place revealing my entrance to a video, ala Tron style:

The background is actually transparent for matting in an "electrified" animated hexagon grid as a background in FCPX. This project is my first time using Motion and Final Cut Pro, and thanks to the help here and various YouTube tutorial videos all the desired effects in my project are being realized as I had imagined them (or better).

Thanks, the masking approach to remove the line as particles move away from it isn't working out so well because of the Outer Glow applied to the line. Covering up the line means the line is still there behind the mask and consequently the glow remains intact. Better would be to "build" the line out of several small shapes and then progressively set their opacity to zero (so their individual glows will also "disappear"), but then each shape has its own separate glow which doesn't look the same as a glow around the whole line (glow "halos" are visible within the line from the separate shapes). Are there any clever ideas out there to have the glowing line look like its "decomposing" from the particles leaving it before I resort to simply fading the line out with its opacity setting?

fox_m, I really like your solution and photo, except that there are visible gaps in the green line, I have a Bulge filter applied to my line so its slightly thicker in the middle than at the ends, and I'd really prefer to have the departing particles streaked like in Data Flow (and my first photos) than as small bits. Which emitter did you use for that screenshot? I'd like to try something similar in white instead of green and see whether it's adaptable to how I'm hoping it would look in my video.

Here is the effect I'm trying to simulate in Motion. I won't use these actual frames because they're copyrighted by Disney, they're not transparent for working with my background, and I want to create my own effect which resembles this but not a blatant copy of it (original art rather than pirated).