Particles looking Boxy in distance

I'm trying to apply a mysterious looking animated fog effect over certain regions of my game, and I've used particles to do so.

Only issue I seem to be having is when I move the camera away, or the particles are far off - they look incredibly boxy.

If you look at the screenshot, note how clean the fog looks at the bottom, and how pixelated it looks at the top.

I'm pretty sure t$$anonymous$$s is somet$$anonymous$$ng to do with LOD - is there some way I can have more control over it - for instance to remove the semi-transparent smudges around the particles?