Spheres are better for volumetric materials. With a torus the texture is less transparent where you are looking through one surface to another. What happens is the left and right edges of the torus look brighter or fuller than the middle. The easiest way to control dust and get it where you want it is postwork. You could try rendering just the ground a little brighter. Then use this image as the pallate for a 50% dense paintbrush in paintshop or photoshop. Even the big guys that use maya use add-in programs like After Effects rather than try to use the same render program so don't feel bad. If you really want to use bryce clouds its possible. It depends on your worldspace and model scale so I could probably not give you numbers you could just plug in.