Do trees emit CO2 at night? I thought it was that they inhale CO2 and emit oxygen - 24/7.

Just like all multicellular life, trees are oxygen breathers, using oxygen in burning sugar compounds to make ATP (energy). During the day, photosynthesis happens and CO2 consumption outstrips production, but during the night the tree still needs to breathe oxygen, so emits small amounts of CO2, as well as other waste gasses.

Having said that, "tree" is not a scientific grouping and encompasses a very wide range of tree-like things, many of which have different ways of dealing with the day/night cycle, so the specifics vary but that's the general gist:)