BEIJING, Oct. 6 (Xinhua) -- Heavy fog in parts of north China on Sunday closed down expressways and airports, hindering the return of holiday travelers toward the end of the golden week.
In Beijing, heavy smog was into a second day. Visibility was under 1,000 meters downtown and even lower in the suburbs.
Early on Sunday, Beijing traffic police closed six interprovincial expressways, including roads to Harbin in the northeast, Shanghai to the south, Tianjin and Hebei Province.
Visibility at Beijing's Capital International Airport on the northeastern outskirts was 250 to 500 meters. Nearly 44 percent of the departing flights were delayed or canceled, and dozens of incoming flights were forced to land in neighboring cities or return to where they came.
Sunday is the sixth day of China's National Day holiday week and the smog has forced many holidaymakers to postpone their return.
Beijing's sky remained misty even after the weather bureau removed the heavy fog alarm in the afternoon.
As of 2 p.m., all 35 monitoring centers across the city indicated heavy air pollution at the highest level of six, according to the Beijing Municipal Environmental Monitoring Center.
The density of PM 2.5 (airborne particles of less than 2.5 microns), exceeded 200 micrograms per cubic meter, the center said.
The National Meteorological Center (NMC) said the severe fog in the north China region began on Friday and was expected to persist until Tuesday. The last round of smog in Beijing ended after a significant rainfall on Oct. 1.
Heavy fog will persist in the provincial regions of Hebei, Henan, Shaanxi, Shanxi, Shandong and Tianjin, according to NMC's forecast for Sunday and Monday.
Authorities in Tianjin closed all 14 expressways in the municipality.
In neighboring Hebei Province, 13 expressways were closed Sunday.
The smog is caused by high humidity and an absence of wind, said Wang Hua, chief weatherman with Beijing Meteorological Center.
"The conditions allow pollutants to stay in the air," he said.
This weather is typical of Beijing in late fall, before high winds sweep away most of the pollutants, said Wang Zifa, a researcher with the Institute of Atmospheric Physics of the Chinese Academy of Sciences.
"The city is heavily populated and surrounded by mountains on three sides," he said. "As a result, pollutants tend to linger in the air for 40 percent of year."
Beijingers used to be proud of their "golden days of fall", when it was neither too hot nor too cold, and the sky appeared a rare azure blue. Golden Week which started on Oct. 1, appeared vastly different from days gone by. Except for Oct. 1 and 2, most sightseers could not see the Forbidden City from the top of a hill at Jingshan Park, only 2 km away.
Besides the weather, experts say burning of coal, exhaust from motor vehicles and industrial emissions are to blame for the massive amount of particulates in Beijing's air.
Beijing's environment quality report last year showed the city had 20.69 million people and 5.2 million vehicles.
Meanwhile, the north China region covering Beijing, Tianjin and Hebei, with a cluster of chemical, steel, power and cement plants, burns more than 350 million tonnes of coal a year. Annual sulfur dioxide emissions averaged 8.5 tonnes per square km, 3.7 times the national average level of 2.3 tonnes.
To cut emissions, Beijing authorities said Saturday the city would build four power stations by the end of 2014 to replace coal with natural gas in heating and power generation.
In another development, Beijing unveiled a package of measures last month to curb vehicle emissions over the next five years as part of the capital's increasing efforts to improve air quality.
The package is part of an action plan by the municipal government to reduce PM 2.5 density by 25 percent or more by 2017. The municipal government will restrict the number of new cars on the road each year from January 2014.
By 2017, the number of vehicles in the city is expected to be no more than 6 million, according to the package. Statistics from Beijing Traffic Management Bureau show the city had 5.35 million vehicles by the end of July.