Consider the following set-up: a spacecraft radiator is placed inside a vacuum chamber with transparent walls, on the surface of the Earth. The radiator acts as a heatsink touching one of the walls of the vacuum chamber.
Can that wall of the vacuum chamber then passively cool down below ambient temperature of the surrounding air?
The idea is that air does not interact with the insulated spacecraft radiator, which emits energy into the surroundings. Since the vacuum chamber is transparent, radiation does not heat its walls, but leaves through the atmosphere. In fact, the radiator could emit energy through the atmospheric transparency window.
If possible, this would be rather a useful device. This article sounds like it could be done.