Dear jp.101;
No!, it’s not exactly one by one!
You need the density of the medium (air: ~1.20 kg/m3), because ppm is like % only given for the identical units: ppm(v/v), or ppm(wt/wt), etc., like %(v/v) or %(wt/wt).
For ppm you may read the 3. Paragraph on: http://en.wikipedia.org/wiki/Parts_per_million
Good Luck!
ARGOS++