trees take co2[carbon dioxide] in the day[or otjer wise sunny] to photosynthesis, during this they give out o2 [oxygen] which we breath in [respiration] to give out co2, plants respire small amounts of o2 at night [its their only energy source with no sun] co [carbon monoxide] will kill just about anything, also polution creates clouds of acid rain due to sulphur in power plant emissions and what not, acid rain erodes trees/soil and stop the sun reaching plants, we need plants to live