In a word Density. Assuming you mean ppm is ppm(w) by weight. This is why it is ALWAYS a good idea to state what basis you are using for your ppm measurement. vpm = ppm(v).
Ian
Ian Verhappen
Syncrude Canada Ltd.
PO Bag 4009, MD 0032
Fort McMurray, Alberta T9H 3L1
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At Different pressures gases have different densities so the number of mulecules can vary per volume. The conversion would be a simple ratio if you know the number of particles per a given volume at a given pressure and the pressure you are using.