Creating and Editing Particle Systems
Well, undoubtedly this is the way to go... This is a job I already did some time ago using the Alien Swarm particle editor and I don't mind to share it here 
These are the missing materials for the 'broken' particles:
BROKEN PARTICLES AND THEIR MISSING TEXTURES
Few considerations:
- Some of those .VMT files the particles need are inside the (/portal 2/portal2/) pak01_dir.VPK file, but others aren't.
- The .VTF files that those .VMTs listed above are pointing to are mostly missing though. Even thought there are still some of them inside the pak_dir01.VPK, they seem to be buggy. Examples: fire_particle_4.vtf and smoke1.vtf. Why? looks like Portal2 needs the previous versions of them (from HL2), the main difference to the ones shipped with the game is the texture's size and images' distribution inside them.
- All working materials needed (except those first 2 in the list) are attached to this post Just unzip the file under your /materials/ folder. There should appear a new folder called 'particle' under /materials/ with those required files. Once you restart the game the particles should be working (again, except the first 2).
- THESE ATTACHED FILES (again, plus the first 2 ones in that list >.<) are the ones to edit if you want some custom particles in your map, all you have to do is to pakrat them before publishing to the workshop. And due to the mentioned above, remember to respect the textures' file format/size in order to make them to work properly.
- Regarding those 2 first missing materials, a further research is needed in order to know what texture size is needed in order to make the particles to work properly. However, due to their purplecheckerboard-look, I think there are some already existing textures from other particles that could fit, it's just trial an error :p I'll do some further testing myself and post whatever I find.
- After putting these missing materials under your materials/particle folder, the broken entities 'func_dustmotes' or 'func_dustcloud' will be magically fixed
Cheers!
useful tools and stuff here on TWP 
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Damn O_o Good work Jose!
By size, do you mean some of them require a very specific texture size in terms of width/height (which would be kinda weird) or that the placement of the graphics inside the texture are important?
Edit: Seriously though... thanks 
Thanks, man! You too! 
Well, I know it might be weird... however, even not knowing at all how the textures for particles work, if I think about models and UV-mapping, and how the placement inside of the texture is used by the different parts on the model, I guess this should be similar to that... or not :p
This is what I mean for example:
useful tools and stuff here on TWP [spoiler]
[/spoiler]Yeah, that's what I thought. I know VTF supports multiple frames, but it seems like it would be more efficient to UV map small animations into one large texture atlas
Hmmmm.... sorry, but it seems that the func_dustcloud also needs some sprites not present in Portal2. Whenever I put it in a map, the console outputs 2 missing sprites.
I found them in old games and attached them for anyone interested. I can't see any difference/improvement in its appearance in game though >.<
They are meant to be into the materials/sprites folder
useful tools and stuff here on TWP [spoiler]
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