Scrolling one comp to the next

The screenshot helps a bit but I still don't have any idea what you are doing in the layers so I don't have any suggestions for a fix. What does the animated null (layer 5) control? Are the 3 particle trails inside Comp 3? Why are layers 1 and 4 3D?  What do those layers contribute to the composition? I see FX applied to layers 1 and 3 but the only effect I see in the comp is what appears to be three copies of a CC Particle world Effect. What layer has the expression error and what is the expression trying to do. This project looks suspiciously like you are trying to follow a tutorial presented by an enthusiast that is not following efficient workflows or explain things correctly.

If I understand your question correctly you want to move the three smoke trails from left to right then have them stop when they hit a wall. 

I would approach the problem by creating a solid layer called Particles that is the same size as the comp, adding CC Particle World to the Solid, add a 3D null just above the Solid that I would use to control the emitter for particle world, add the following expressions to CC Particle world (use the attached animation preset) then animate the null and adjust the particle settings to get the first particle trail. The comp would look something like this and you should be able to see everything that I did to all layers:

Here is the animation preset that will tie the emitter of CC particle world to a 3D null just above the Particle layer: https://www.dropbox.com/s/n9rjxcj2qs062sg/Particle%20World%20Follow%20Null.ffx?dl=0

Just download it and save it to your Custom Presets folder then apply it to the solid you want to use for the particles.

Here are the expressions:

//Position X;
zro = [thisComp.width, thisComp.height];
pos = thisComp.layer(index - 1).transform.position;//emitter null
pos[0]/zro[0] - .5

//Position Y;
zro = [thisComp.width, thisComp.height];
pos = thisComp.layer(index - 1).transform.position//emitter Null
pos[1]/zro[0] - .25

//Position Z;
zro = [thisComp.width, thisComp.height];
pos = thisComp.layer(index - 1).transform.position;//emitter Null
pos[2]/zro[0] * .5

You can put as many copies of CC Particle World on a layer as you like if you check Composite on Original on the second copies. If you have more than one copy on a single layer then you need another 3D null and if you just add one number to the index + part of the expression you can end up with as many streams as you like on a single layer with the position of the emitter controlled by a null.

Either try my method or tell me what is going on in the layers. Select all pertinent layers and press UU to show me what you have done on the layers. If CC Particle world is in the nested comps (pre-comps) then show me a flow chart and tell me what is going on in those compositions and show me the layers.