Moe you are correct, but I did find this.
CO2 ENRICHMENT GUIDE
Carbon dioxide (CO2) is used by plants in photosynthesis, or the conversion of water, atmospheric carbon dioxide and light in the plant's chloroplasts into food energy (simple carbohydrates), with oxygen as a byproduct. Resins and saps in the plants
stems and branches then transmit this food around the plant to promote growth, reproduction and prevention of disease.
Photosynthesis stops at night, thus plants do not use CO2 during the night, or lights-out stage. Although enrichment of the atmosphere during the night cycle will not harm the plants, efficient CO2 systems are regulated so that when the
lights go out, CO2 emissions stop.
Ambient air at sea level contains approximately 350-500 ppm of carbon dioxide. Higher altitudes and rural locations typically have a lower presence of CO2, while lowlands and urban areas have a higher presence. CO2 can be measured, in parts per million (ppm) of air, using an inexpensive device available in hydroponics supply catalogs and garden shops (approx US$20).
Carbon dioxide enrichment involves increasing the concentrations of CO2 to 4-5 times the normal atmospheric levels, to between 1200-1500 ppm in an enclosed space. Enrichment has been shown to promote faster growth, higher yields, and stronger, healthier plants. Levels higher than 2000 ppm have been shown to retard plant growth. Low levels of CO2 (below 200) have been show to halt vigorous growth, even when all other conditions are ideal. Because of this, any enclosed space requires replenishment of the internal CO2 as it is used by plants, either from
ventilation or from CO2 supplementation.