system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we measure ppm by volume for gases all the time and not by weight like water and soil?
gpt
Likely because the molar volume of gases is pretty much the same for everything. So if you take 1 mol ethanol and water, ecaporate it and have the vapor and gas ethanol at the same pressure and temperature they will have the same volume.

For liquids and solids its different for each compound and element, using weight here makes more sense. As gas has an easy determined amount of particles we can use ppm for better quantification of components.