Question

how are smoking zones useful in prevention of air pollution

Answer 1

Smoking zones are special areas reserved for smokers only. In the regions outside smoking zones, they are not allowed to smoke in public. 

Thus the nonsmokers are relieved of the pain faced by smokers and from the passive smoking. Also some action can be taken by the government or the organization to collect the smoke from the smoking zone properly. Then the smoke can be redirected so that it does not harm the air. The smoke could be chemically treated too.