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.
But, 30 ppm of CO2 dissolved in water isn't in equilibrium with 30 ppm of CO2 in air. My education is lacking in this field, so I have no idea how to even start to solve this.
Air can have a varying amount of CO2 in it. If it is .03% CO2, it is 300 ppm, but water exposed to that air will contain more like 3 ppm of CO2. If the ppm in the water in equilibrium with the air above the water is a near constant percentage of the ppm in the air, this is an easy problem, but Murphy's Law seems to dictate that it isn't anywhere near a constant percentage.