Smog is smoke plus fog. It is not comprised solely of CO2 but rather unburnt hydrocarbons, CO, CO2, SO2 and other chemicals. And although CO and unburnt hydrocarbon gases iaren't created in nature except through combustion, all the other gases are.
Smog is also only a localized phenomenon, not a climate event.
It was really a rhetorical question, as "smog" is only one example of man-made junk filling the atmosphere.
And it serves as an excellent example of how man can harm a localised area. But affecting the entire globe is a little beyond us at the moment, save for a nuclear winter of course.