Vehicle exhaust fumes have replaced industrial pollution as the biggest contributor to the worsening air quality in major Chinese cities, according to a top researcher.

Zhuang Guoshun has been studying trends in pollution levels since 1998, and his team has collected 20,000 samples of atmospheric aerosol using monitoring stations.

"Over the past 20 to 30 years, we've seen a rise in manmade particulate matter in the atmosphere," he said. "Pollution from nitrogen oxide has increased sharply, mainly caused by car emissions."