Answer: You probably know that the overall level of concentration of carbon dioxide is around 0.0314% in uncontaminated air.

Carbon dioxide is around 1.5 times more dense than air. So, put simply, there is more of it at ground level because it is heavier than air. This is related to the exchange of gases between plants and animals, i.e. plants use carbon dioxide in photosynthesis and animals produce carbon dioxide as a waste product during respiration.