Air pollution chokes as smog migrates

BEIJING - People living in South China have been grumbling about gales blowing smog in from the north, as hazardous air chokes more parts of the country this winter.

In 2013, China's smog belt stretched from the developed regions of the Yangtze and Zhujiang river deltas and areas around Beijing and Tianjin, to other places where smog was seldom seen. The southernmost province of Hainan and autonomous region of Tibet recorded smoggy days.

Chinese Academy of Sciences issued a report last month about climate change, saying that besides local pollutants, "smog migration" was another serious problem.

In Beijing, 30 percent to 40 percent of the PM 2.5, airborne particles smaller than 2.5 microns in diameter, was caused by emissions, 20 percent to 30 percent from chemical conversion in the atmosphere, and the rest from smog movement, according to the report.

Meanwhile, according to statistics published monthly by the Ministry of Environmental Protection, Hebei province, around Beijing, is home to up to seven of the country's top 10 polluted cities.

Chen Changsheng, deputy director of the meteorological center of Jilin province, said the idea of "smog migration" originates from "advection fog" that usually appears in winter and moves with wind for a long period of time.

He explained that when advection fog emerges, local air particles like dust and secondary inorganic aerosols - sulfates and nitrates - will mingle with the mist from other places, and generate smog, which means it is hard to correctly attribute the causes.

"The wind field in middle latitude makes smog movement possible from North China to south in winter," said Zhao Chuanfeng, a professor with the College of Global Change and Earth System Science of Beijing Normal University.

He said that big particles like dust can drop to land in one or two days, but tiny ones always float in the air for much longer, moving with wind which forms smog migration.