Though far fewer vehicles were on the roads during the National Day vacation in Beijing, the city was still blanketed by dense smog on some days, prompting many to doubt the conventional wisdom that the capital's pollution mainly comes from cars.

Though many Beijing residents went to the suburbs during the holiday, resulting in less traffic in urban areas, the Central Meteorological Station warned of heavy pollution on two consecutive days during the seven-day vacation.

"It's hard to define the relationship between smoggy days and automobiles because vehicles that fail to reach the emission standards in Beijing only account for less than 20 percent of total," said John Zeng, director of LMC Automotive Asia Pacific Forecasting.