China’s horrific haze: New sources need control

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China’s horrific haze: New sources need control
Photo of Olympic stadium which can hardly be made out from the thick haze that shrouds it and everything around.
Beijing’s “Bird’s Nest” stadium, built for the 2008 Olympics, was obscured during the “airpocalypse” of early 2013.
Ru-Jin Huang and Jun-Ji Cao

Dust, soot and filth from farms, industry, traffic and power plants took most of the blame for China’s air-pollution episode in the beginning of 2013, when astronomic levels of particulate pollution afflicted 800 million people.

Pollution was 40 times worse than World Health Organization safe-air standards.

“Particulates,” despite the name, are usually droplets of liquid rather than solid particles.

But a new analysis of four Chinese cities (Beijing, Shanghai, Guangzhou and Xi’an) traces much of the problem to “secondary” chemicals that can form particulates after a chemical reaction in the atmosphere.

The study focused on particulates that are less that 2.5 microns in effective diameter, which are more dangerous than larger particles because they can lodge deep in the lungs. In the cities, 30 to 77 percent of these particles formed from secondary chemicals.

Photo of the Olympic stadium with blue sky visible through light cloud cover. The stadium is easily discernible.
On a clear day, you can see forever — or at least, the Olympic stadium.
Ru-Jin Huang and Jun-Ji Cao

A 2012 report from the World Health Organization estimated global mortality due to air pollution at 7 million annually, with hotspots in Africa, South Asia and China.

This week, the New York Times reported that, “People living in northern China were informed by a team of American, Chinese and Israeli researchers that they should expect to live much shorter lives — a full 5.5 years shorter — than their countrymen to the south” due to air pollution.

Even though China has pledged to reduce primary sources of air pollution by 25 percent by 2017, “It’s not enough to only think about the primary sources,” says André Prévôt, an atmospheric chemist at the Paul Scherrer Institute, Villigen, Switzerland, who lead the new analysis, in Nature. “Don’t just think about the black stuff coming out of chimneys; these [secondary] gases are not a small part of the particulate pollution.”

The Prevot study looked at:

* Inorganic secondary pollutants, such as nitrogen oxides (from traffic, big power plants and industry); sulfur dioxide (from coal burned for heat, industry and power); and ammonium (from cow manure and farm fertilizer).

* Volatile organic compounds (VOCs), a wide range of easily dispersed chemicals that are difficult to trace and tend to be more unhealthy than the inorganics.

Map of China describing major pollutants and their sources as percent of the total for large cities in the east and to the north.
In Beijing, primary pollutants from burning coal, and secondary organic and inorganic pollutants each accounted for about one-quarter of air pollution. In Shanghai, secondary sources accounted for nearly three-quarters of particulate pollution.
Credit: Andre Prevot

Prevot says it was known that secondary sources added to pollution in the summer, but his article’s reviewers doubted this could occur in winter, when colder temperatures and diminished sunlight both slow chemical reactions. He successfully argued otherwise, he says. “We happen to be the only group in world that has done experiments at low temperatures to reflect these conditions, and we know that in winter, biomass emissions, gasoline car emissions” can cause just as much of the particulate pollution as primary pollutants.

Because the chemical reactions can start within hours, even in winter, Prevot says, “even two or three days, in these very cold conditions, there is enough chemistry going on to produce secondary particulates.”

Primary & Secondary pollutants in China

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Credits: Cars: Michael Davis-Durchat, Coal Plants: South China Morning Post, Croplands: China Daily

Solar radiation alone could drive the reactions, Prevot says, but given the high humidity, the reactions could be taking place in clouds, or at night.

What good is this?

We wondered how a new focus on secondary pollutants could help nations battle air pollution, and Prevot’s message was simple: Widen the target for controls. “The Chinese government has put a lot of effort on primary emissions, but it’s obvious that’s not the only source.”

Although many secondary pollutants originate, like primary pollutants, in vehicles, farms, power plants and industry, clamping down on primary pollutants will not automatically cut the secondaries, Prevot says. “It depends on what technology is used. If it’s just a particulate filter on a coal plant, the gases that come out are still the same.”

Moreover, power plants that install pollution controls on sulfur dioxide may “neglect the VOC emissions that also form secondary particulates,” which tend to have greater toxicity, Prevot says. “We think governments should emphasize the VOC sources.”

And the problem is not just in China, he adds. “Europe takes too little care about domestic wood burning, a major source of VOCs.”

– David J. Tenenbaum

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Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer


  1. High secondary aerosol contribution to particulate pollution during haze events in China, Ru-Jin Huang et al, Nature, 19 Sept. 2014.
  2. What China is doing toclear the air and fight climate change.
  3. Chinese companies already use an internal price on carbon and many more are
  4. China seeks to fight smog by brainstorm: All ideas welcome.
  5. Harnessing the Tibetan Sun: Solar cookers to offset health concerns and indoor pollution.