...and here's a NOISE FILTER applied to the MM FOG Layer...- this creates lots of nice HEAVY DUST particles in the air- and again you can see how it "respects" the 3-D DEPTH LAYER effect- and is not just a 2-D effect...

I also added this to my image using variations of SCREEN, PINLIGHT and LUMINOSITY- don't be afraid to try different light filtering techniques- all of them can be pretty interesting...