Why do plants create an increase of CO2 in the dark and a decrease in the light?

Light-Dark reactions of the Calvin cycle...

Photosynthesis = Takes place with the availability of light and is much more efficient

Photorespiration = Takes place in the dark OR when CO2 levels are low in the air.

With sunlight, plants convert CO2, found in the air, into a sugar molecule. When there is no light or low amounts of CO2, plants use Photorespiration. One of the byproducts of photorespiration is CO2... thus increase of CO2 occurs in the DARK.

Frankly, the way you word the question there probably is no difference or an increase in light..... the key here is your word "CREATE". Plants are alive and the product of their metabolism is carbon dioxide. During the day with more metabolic activities, there should be an increase in carbon dioxide.

Of course the question really wants to know about carbon dioxide USAGE. Plants use carbon dioxide in photosynthesis... which occurs during the light hours. Thus the NET production of carbon dioxide to the atmosphere is less in the daytime, and more at night.