Given this diagram, I would imagine the CO2 levels would be the same as in the water.
For example, in the case of CO2 injection, the levels of CO2 in the water are higher. CO2 will off gas from the water into the atmosphere and into the bell shaped jar. When the levels of CO2 in the jar reaches the same level as the water, then it will not increase anymore.
Of course, CO2 will be diffusing from the water into the atmosphere as well as into the jar.
As I understand it, we use the term "ppm", but, really, that's technically totally inaccurate - we do *NOT* aim for 30 parts per million. And the CO2 solubility of water is far less than that of air.
And that's as much as I know about water chemistry, and I'm sure I explained it badly. But there are some posts on it, written by people how actually know what they're talking about.