You're trolling, right? I can't imagine what your understanding of the atmosphere is when you think a tank of compressed CO2 would provide any heating effect at night.
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.
As you mentioned, a structure is required. The CO2 is useless. By the way, on a greenhouse, CO2 has no function regarding temperature.
The anecdote is to be illustrative and not really done literally, naturally, but if I have to explain that...