Present rate of increase is around 1-2 ppm per year. Effective during daylight hours because that is when photosynthesis is occurring. CO 2 injection that creates concentrations up to 2000 ppm illicit favorable plant growth response.
Upper level is crop-specific.
See examples with tomatoes, cucumbers, roses, lettuce in text. Higher CO 2 levels often create demand for more nutrients. Temperatures can be allowed to rise to higher levels if CO 2 injection present. Injected when vent fans are off and house is tight. CO 2 generators rely on the burning of LP or natural gas.