The oxygen in the atmosphere (21%) is the result of photosynthesis.

The nitrogen in the atmosphere (78%) is the result of denitrification.

The carbon dioxide in the atmosphere (0.0407%, or 407 ppm, August 2017 seasonal average) is the result of respiration.

The juvenile carbon dioxide [that not yet recycled through the biosphere] originated in volcano eruptions.

The balance between photosynthesis [by green plants] and respiration [by animals] determines the concentration of carbon dioxide in the atmosphere.

Uncommon sense:  The sense behind global warming

This concentration has been going up at an average rate of about 2 ppm per year in the past 5 years.