We can compare either numbers of molecules, masses or volumes of gases when determining mixing ratio. Because the units cancel out, yet are important because the same amounts of gases produce different mixing ratios depending on whether we are comparing masses or volumes, we specify which by saying "parts per million volume" (ppmv) or parts per million mass" (ppmm). When we see neither specification, it's probably because the basis of comparison was a count of the numbers of molecules. For gases at a given pressure, the volume taken up by a gas sample will be directly proportional to the number of gas molecules in the sample , so in the absence of a volume or mass qualifier, assume the mixing ratio measurement is a volume measurement.