ppm is literally parts per million.
It can be parts in kg / 1000000 kg or parts in g /1000000 g
In solids, that's how you work it out.
In solutions, you can say ppm == mg L-1
An example is in the 2007 paper I think
22 a), along the lines of this:
Calculate the volume of sulfur dioxide produced when a full tank of capacity 60kg of diesel is consumed at 25o C and 100kPa if the sulfur amount in diesel is 50ppm.
So - Allowed amount is 50ppm
So mass of sulfur allowed in the total amount of diesel is if you cut the 60kg into a million parts and take 50 of them (ppm does not have a fixed unit):
Mass sulfur = 50 / 10^6 x 60kg = 3 g
1 mol sulfur --> 1 mol SO2 ( S + O2 ---> SO2)
Vol. SO2 = 3 / 32.07 x 24.79 = 2.3L