ABOUT 31 percent of PM2.5 (particulate matter) is caused by vehicle exhaust emissions, according to a research report on the city’s smog.

    Among other sources of PM2.5 — tiny particles or droplets in the air that are 2.5 microns or less in diameter — emissions from industrial plants and thermal power plants account for 24 percent and 18 percent respectively, the research said.

    The research, organized by the city’s environmental protection department, was done by associate professor, Huang Xiaofeng, from the Shenzhen Graduate School of Beijing University, aiming at studying the city’s PM2.5 data and carrying out effective ways to improve air quality.

    According to the research, 70 percent of the particulate matter is under 2.5 microns. However, China’s air pollution index only measures PM10. Shenzhen does not release PM2.5 data although the city has been monitoring fine particles. Between 2005 and 2011, the annual average concentration of the city’s PM2.5 was between 33 and 62 micrograms per cubic meter.

    There were more than 180 smoggy days in Shenzhen in 2004. The figure fell to 112 in 2011 and the annual average PM2.5 figures also dropped.

    Meanwhile, the research also showed pollution in winter was almost double that of summer.

    “The particulate matter is not harmful. However, the chemicals in the matter are potentially harmful,” said Pan Xiaochuan, a professor from Beijing University.

    (Wang Yuanyuan)