Question: In my alternate history (1800s), I have troop hiking through tropical sub-Saharan Africa. For plot reasons, I have man-killing mosquitos, but I also have a plant that exudes CO2 and other attractants. These flowering plants are placed in the room at night to attract the mosquitos away from people. 

Evolution-wise, however, 'consuming' CO2 and giving off O2 is so deeply ingrained in a plant's DNA, it is hardly believable to be the other way around.

I'd easily hand-wave it away as some special scent, but the CO2 is critical to the plot. **Can a plant evolve to give off CO2?** Or rather, how difficult would that be to explain.

Answer: 
As has been pointed out, plants can give off CO2 at night. If you pick a [plant that only blooms at night](http://themysteriousworld.com/10-most-beautiful-night-blooming-flowers/), you'd have a plant that is using up much more energy at night, thus producing more CO2, and the scent of the flower could add to the insect repellent function. My mother used to grow these orchids in our home (back in my home-country) that would only bloom once every few weeks, and only at midnight and the bloom would be wilted by morning. They were so beautiful, when my mom knew they would be blooming, we'd stay up to see them.
You might also look into *photorespiration*, an error in photosynthesis which causes a plant to produce CO2 instead of O2. This only occurs when CO2 supply to the plants is low, which might happen when the stomata are closed to prevent water loss. Research suggests that photorespiration might have to do with nitrogen assimilation as well, meaning a plant in a nitrogen-poor environment might be more likely to photorespire.