When is the best time to add co2?

I have an inline fan that cools 2k lights. I cant add the co2 while the fan is on cuz it will jus suck it all out. I want to add it either before the lights come on or right after they turn off. Which is better? Also, I know you can just get the air from outside to cool down the lights but the air outside is like 2 degrees F. Idk if thats too cold to cool down the lights. Will the lights be ok with the air from outside being so cold?

thanx sum good info.