Beijing's heavy smog derived from straw-burning?

As Beijingers wonder what caused the recent bout of heavy smog, an environmental watchdog in adjacent Henan Province said on Thursday farmers burning fields of straw is to blame.

The Beijing Meteorological Observatory issued a yellow alert for smog on Wednesday, which is forecast to continue for the next three days. Meanwhile, neighboring Hebei Province issued a more severe orange smog alert.

The choking smog reduced visibility to less than 1,000 meters in Beijing on Thursday morning, pushing the municipal traffic control commission to close several highways linking Beijing with northern and southern provinces.

The bout of smog has been reported to be the worst in north China since July. It was monitored to have started from Henan Province since Tuesday, when the provincial meteorological observatory issued a yellow smog alert.

The Henan provincial environmental protection bureau said pollution discharged from the 7,893 straw burning sites it has monitored in the past week has accumulated in the air, which lacked flow.

Farmers in north China region traditionally burn straw after autumn harvest, and plough to dig the ashes into earth, which they believe fertilizes farmland.

People in Beijing, however, are not quite convinced burning straw is the chief cause of the smog. Burning straw was far more prevalent 20 years ago in north China's rural areas at a time when smog was rarely heard of in Beijing.

A pollution law which will see large northern cities coordinate enforcement efforts has been promised by governments of Beijing, Tianjin and Hebei. Many believe this will be a key solution to treat the smog problem.

The weather departments in Beijing, Hebei and Henan have forecast a cold air front from the north would arrive on Saturday to disperse the stagnant smog.