Obscuring particles with a layer
Since Trapcode Particular is applied to a 2D solid and the particles live in their own 3D world, other layers in the composition won't automatically obscure the particles. The Obscuration Layer provides a way to insert a 3D layer into the particles' world.

Step-by-step instructions on using the Obscuration Layer
Create a new comp (Ctrl/Cmd-N), make it 640*480 at 30 fps. 5 seconds long. Name it "Main".
Create a new comp-sized solid (Ctrl/Cmd-Y), name it "Particles". Make sure to click "Make Comp Size" before OK.
Apply Effect>Trapcode>Particular to the solid.

To be able to navigate easily in the scene, create a compostion camera:
Select Layer>New>Camera and use the 28mm preset.

Now create the layer that will obscure particles:
Create a 200*200 Red solid.
Switch it to 3D.

Try orbiting around the scene (press "c"). Notice that the particles are always behind the solid.

To fix this, move the solid below the particles layer in the TLW.
Set Visibility>Obscuration Layer to the solid.

Now try orbiting again and notice how the particles are correctly obscured by the solid. This obscuration is not antialiased so it does look a bit rough around the edges and it does not work with semi-transparent layers.

We have used a solid here, but it could be any type of layer except a text layer. The layer must have continous rasterize switched off.