I would recommend: "Car exhaust emissions are a serious problem in China." - You should say "Car exhaust emissions" instead of "The car exhaust emissions", since you're referring to car exhaust in general (and not some specific emissions that the reader already knows about). - "Serious" by itself sounds a little odd in this context. If you mean to say they are a bad problem, I would say "a serious problem".

The amount of car exhaust emissions is alarming/ serious in China