Smog has plagued Beijing for days despite the fact that the city turned off its coal-fueled heating system on March 15. Following the example set by cities overseas, the capital has designed a system of wide ventilation corridors, which it hopes will ease overheating and air pollution, though experts are split over the plan's feasibility.

The early morning sun shines on buildings around Beijing's iconic China Central Television Tower on a smoggy March 1. Photo: CFP

Always on the lookout for ways to clear out the smog that all too often blankets China's cities, and hot on the heels of the introduction of giant mist cannons that wash the pollution out of the air, Chinese city governments are pondering a new way to freshen the air - building "urban wind passages."

In late February, the Beijing Municipal Commission of Urban Planning announced that the capital is going to build five 500-meter-wide ventilation corridors and more than 10 80-meter-wide corridors to allow breezes that cool and clean the city, the Beijing News reported.

On March 5, Yang Guanjun, head of the Department of Housing and Urban-Rural Development of Shaanxi Province in Northwest China, told reporters that his region also plans to introduce ventilation corridors.

Before Beijing's announcement, about 10 other cities, including Shanghai, Nanjing, Ji'nan, Hangzhou, Fuzhou and Zhengzhou, have proposed city ventilation research and building wind passages to dilute airborne pollutants and ease the Urban Heat Island (UHI) effect.

The UHI effect is when temperatures in urban areas rise to become markedly higher than those in surrounding areas.

The government of Guiyang, capital of Southwest China's Guizhou Province, has started to build a ventilation network in September last year.

Some forerunners like Stuttgart, Germany and Hong Kong have shown that these corridors can help deal with overheating and air quality problems, but some experts are still skeptical about their feasibility.

"I don't think they can play any part in introducing wind or treating air pollution. They can at most improve the landscape," Wang Yuesi, researcher at the Institute of Atmospheric Physics, Chinese Academy of Science (CAS) in Beijing, told the Global Times.

But Professor Edward Ng from the Chinese University of Hong Kong who helped design ventilation corridors in Hong Kong said that they are worth trying in Beijing.

"The efficiency of ventilation corridors depends a lot on geographic location and wind resources. Beijing is big, its buildings and population are very concentrated. But we will never know whether it works unless we try," Ng told the Global Times.

He pointed out that Tokyo has also created wind paths to improve its climate. "But ahead of the implementation, Beijing still needs to do adequate pre-project research and tests," said Ng.

Sick city

Following the Beijing planning authority announcement, controversy arose surrounding the latest anti-smog measure. Some have ridiculed the government for placing its hopes in wind, instead of cutting pollution.

"The building of a ventilation system mainly serves to relieve the UHI effect. If emissions are not reduced and the climate condition is unchanged, the ventilation corridors alone cannot tackle the smog problem," explained He Yong,  deputy director of the planning research department of the Beijing Municipal Institute of City Planning & Design, which has contributed to the design of the city's ventilation network.

"But it's certain that the network can be effective in improving the climate and assist dispersing concentrations of air pollutants," He was quoted as saying by the Xinhua News Agency.

However, Wang Yuesi from CAS thinks the effect will be too limited.

"In cities in northern China, northwest wind prevails in winter and southerly wind prevails in summer. In winter, the strong northerly wind can easily help clean the air without help of the corridors. But in summer, the winds are always slow and humid. If air pollution comes, the corridors may even help pollution spread around the city faster," Wang earlier told discovery.163.com, a science news website.

"Building so-called wind passages is just waste of money and manpower," he told the Global Times.

Zhou Fuduo, a retired urban planning professor from Zhejiang University who has contributed to Hangzhou's ventilation design, disagrees. "The wind passages in a city are just like meridians on a person. If the passages are obstructed, the city will get sick," he was quoted as saying by Southern Weekly.

Yu Zhuang, a professor from College of Architecture and Urban Planning, Huazhong University of Science and Technology, echoed Zhou's idea.

"For a large city where tall buildings are densely packed in, if there are no urban ventilation passages, the wind will make a detour," said Yu, who has been involved in the research and design of ventilation passages in Wuhan.

In 2006, he proposed that to reduce urban heat Wuhan government should establish ventilation corridors and protect them from being blocked. And the capital city of Central China's Hubei Province put the plan into force in 2010.

"I think the effect is good. The city is no longer one of China's big four 'ovens,'" Yu told the Global Times, referring to the group of cities that have the reputation of being the country's hottest.

According to Wuhan Evening News, the city had a cool summer in 2014 and 2015, with the average temperatures both 1 degree lower than same period in previous years. There were just eight summer days over 35 C last year, nearly half the average.

Overseas forerunners

Alan Robins, a professor of Environmental Fluid Mechanics from the University of Surrey in the UK, said many cities around the world are considering the benefits of opening-up dense city centers to allow better wind penetration.

"Better ventilation will reduce pollutant concentrations from local sources but make little or no change to pollutants transported from afar, so the answers depend on the cause and sources of pollution," Robins told the Global Times.

To seek greater use of natural ventilation and smarter design, he said a major new research project called MAGIC (Managing Air for Green Inner Cities) has just begun in England.

Ng cited Stuttgart as one of the best example of using ventilation corridors.

Lying in a valley, the industrial city's heat and pollution became dire in the 1970s. Gradually, the city begun to create large interconnected green areas and disallow new construction in ventilation corridors, policies which have been largely successful, according to the WWF.

"Officials need to change their conception of development and balance the land's economic and environmental benefits," Ng suggested. "As the problem of food and clothing has been solved, people have started to seek a more healthy life, and they demand clean air."

In an interview with Phoenix TV last week,  Xie Zhenhua, China's Special Representative on Climate Change affairs, also emphasized the necessity of urban ecological corridors.

"If it has no such corridors, the city must have design defects. The city of Beijing is characterized by urban sprawl, which is unscientific," said Xie.

In China, the urban land price keeps rising and income from land transfers and commercial development has become a major source of finance for local governments. Policymakers seldom prioritize green spaces.

Yu said the Wuhan climate map, which demarcates no-build zones in wetlands and ventilation passages, has been strongly resisted by local officials.

"They are eager to develop those zones," Yu said. "The only way to stop them is legislation, which city planning officials and experts are promoting."

In Stuttgart, supported by legislation, over 60 percent of the land is covered in green and the city has prevented over 60 hectares from being built on in recent years, said the WWF.

To improve wind conditions, the Hong Kong government officially adopted its Air Ventilation Assessment System in 2006 following the severe acute respiratory syndrome (SARS) outbreak which killed nearly 300 people in the city.

Ng said the system requires all major publicly funded development projects to undertake an assessment and make the results public, so as to ensure buildings do not hinder the city's ventilation.

Wind slowing down?

According to the Beijing planning commission, the five larger ventilation corridors in Beijing will mainly consist of parks, lakes, roads and green space, and nearly all run north to south as northerly winds prevail in the city.

Quoting unnamed designers for Beijing's ventilation network, Xinhua reported that massive demolition won't be necessary to create these corridors. But there are quite a few high-density neighborhoods blocking the passages, such as the Shilihe area in the southeast corner at the Third Ring Road, where demolition will be carried out slowly.

Some worry that the corridors might increase pollution in suburbs. The designers denied this, telling Xinhua that the pollutants will be diluted gradually or blown into the upper atmosphere.

But Yu said he believes that the planned design for five separate corridors in Beijing is not the optimal system.

Professor Ng agreed. "A complete ventilation system should be interconnected. Like a lung for the city, the system can only be effective when the main corridors, branches and smaller ones are all formed and work together," he noted.

However, Yang Xuexiang, one of the few experts who believe the wind slowing down contributes to the increasing presence of smog, said the ventilation corridors are useless when there's little wind.

"The wind in North China has slowed down, partly due to excessive exploitation of wind power and building of windbreakers, which could affect the spread of atmospheric pollutants," Yang, a retired professor from the College of Geo-exploration Science and Technology, Jilin University, told the Global Times.

A study published by the journal Nature Geoscience in 2010 found that the average annual surface wind speed in countries in mid-northern latitudes - including the US, China and Russia - had dropped by as much as 15 percent, from about 17 kilometers an hour to about 14 kilometers an hour, and that trees may be to blame.

Currently, to prevent seasonal sandstorms from coating Beijing in material from the Gobi desert, belts of trees have been planted around the city.

Yang argues that these trees should be replaced with shorter shrubs and other plants to allow the wind to blow faster.