Supplying extra carbon dioxide is a frequently used method to increase the yield of greenhouse horticultural crops. Depending on your location, the amount of carbon dioxide in the outdoor air is 350 parts per million. This amount is sufficient to grow plants, but if you place many plants together in a greenhouse, the carbon dioxide level decreases because all plants use carbon dioxide for photosynthesis. By adding CO2 (CO2 enrichment) it is possible to increase the photosynthetic potential of the crops, especially on sunny days.
CO2 Enrichment

CO2 enrichment in greenhouses
CO2 enrichment in greenhouses allows crops to utilize their photosynthetic potential. Enriching the air with CO2 can be done by burning natural gas or liquid CO2.