I am working on a project about pollution emission, but I am not a chemist. I am seeing the units ppmC, ppmC3 and ppmC6 almost everyday, and I have trouble understanding what they stand for. The only definition I get is "Parts per million hydrocarbon concentration expressed as methane (for ppmC)/propane (for ppmC3)/hexane (for ppmC6)". I have no trouble with the meaning of "part per million" at all, what I don't understand is whether using, for example ppmC3 means only propane concentration is measured? or is it something else? Is there a method to tell apart the methane, propane and hexane contribution to the hydrocarbon concentration? Thanks.