Although scientists have not agreed on an exact percentage of vehicle exhausts' contribution to fine particulate matters in big cities, they do agree it is a major source of pollution.

A recent study that said automobile exhaust accounts for less than 4 percent of Beijing fine particles triggered public concern as policies restricting vehicle purchase and use are being promoted nationwide.

Yet, the Chinese Academy of Sciences, which released the study, admitted later the figure was "substantially undervalued" as scientists are still unable to clearly distinguish the sources of some substances in PM2.5 - particulate matter with a diameter of 2.5 micrometers or less.