I doubt you were able to notice a difference between the co2 level in China and the the level in your own home. The difference is incredibly small only about 200 parts per million.
The fine particle pollution on the other hand is very noticable. It is important to not confuse one with the other.
In Shanghai, you can see easily the milky haze over the city, the smog is so thick that you may mistake the smog for an overcast day.