You don’t need a stupid study.
There is virtually NO CO2 in the atmosphere.
No, there has to be enough for the plants to breathe in. Beyond that, 392 parts per million (0.0392%) is utterly minuscule.
There is virtually NO CO2 in the atmosphere
To visualize what your comment is saying, a green garden pea dropped in an empty 5 gal bucket fairly represents to ratio of CO2 to the entire atmosphere.
Anyways, CO2 is just fertilizer which plants use to produce more oxygen.