GP is likely trolling, but CO2 does work in the night the same as in the day. It keeps in infrared radiation emitted by the earth and things on the surface, like you. You won't have a temperature increase, but you'll have less cooling.

In that example, you'd need some structure to keep it in, since the parcel of air will eventually mix with its neighbors, and you can't breathe in the CO2. The structure itself would provide a greater greenhouse effect than the CO2 since it basically would be a greenhouse. But if you tested a windowed shelter filled with CO2 and one with normal air, Mythbusters-style, I suspect you'd find a measurable difference.