Breaking the Feedback Loop that Fuels China's Smog

The notorious smog that often shrouds Beijing is a complex brew of dust, soot and gases from smokestacks and tailpipes, trapped together by chemistry and the geography around China's capital.

And when water and nitrogen oxides mix with enough fine particles in the skies over Beijing, the result can be a kind of feedback loop that produces sulfur compounds and more floating particles, further fouling the air.

That's the conclusion of scientists who recently examined how a thick haze forms - and their findings may help China in its battle to curb the pollution that has risen with the country's rapid industrialization. Yafang Cheng, the project's lead researcher, said the results show how nitrogen oxides - released by power plants and most cars - fuels the smog.

"I think our results show how tightly coupled the processes involving gas phase, liquid phase and solid phase substances in our environment," said Cheng, an atmospheric chemist at Germany's Max Planck Institute of Chemistry. "The atmosphere is very complex, so there can be synergetic effects."

China's pollution problem is more than a nuisance. UN agencies blame it for more than one million premature deaths a year and warn it could shave two years off life expectancies in the world's most populous nation. It's fueling discontent at home and concerns abroad, said Jennifer Turner, who runs the China Environment Forum at the Woodrow Wilson Center, a Washington-based think tank.

"The public is watching, and the government sees it's bad for business," Turner said. "Companies don't want to invest and tourists don't want to come when you can't breathe."

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China is trying to cut the amount of smoke, soot and other pollutants put out by its factories and by the rapidly growing numbers of cars on its roads. But Beijing's problems in particular are complicated by geography. The city is flanked by mountains to its west and north, which produce periodic air inversions that trap smoke, dust and emissions from the city or from industrial zones to the south, Cheng said.

Working from air samples taken during a major smog outbreak in 2013, Cheng and her colleagues found that particles suspended in the air absorb water and trap sulfur dioxide - a common byproduct of burning coal, which still runs about three-quarters of China's power plants. That compound mixes with nitrogen dioxide, another common fossil-fuel emission, producing sulfate particles.

The more particles, the more water gets absorbed, and the process picks up speed. And it gets turbocharged when the concentrations of fine particulates - bits of dirt smaller than 2.5 millionths of a meter across - are at high concentrations.

World Health Organization standards warn against breathing air where those microscopic particles, shorthanded as PM2.5, exceed more than 10 millionths of a gram per cubic meter of air. But PM2.5 concentrations in Beijing routinely run into the triple digits - where the process Cheng described takes off.

"Under clean conditions, this reaction may not be so important," she said. "But when you've already accumulated a certain level of PM2.5, this reaction will start to self-amplify."

In addition, nitrogen oxides (NOx) are a chemical precursor to ground-level ozone, which can cause breathing problems and worsen lung ailments. So cracking down on NOx emissions could cut pollution sharply, Cheng said.

Turner said China has adopted emissions standards based on regulations in Europe, where diesel-powered cars are more common than in the United States. Beijing is trying to set tougher rules locally, the way California has tougher emissions controls than other U.S. states. China's latest Five-Year Plan – its guiding economic document -- calls for cutting sulfur and NOx emissions 15 percent by 2020.

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But that's a tough issue in a country that relies heavily on diesel engines, which put out more NOx than gasoline-powered cars, and where private auto ownership is ballooning. Most of China's efforts to date have focused on reducing particulate-matter counts. While the ruling Communist Party is setting tougher goals and warning officials that they'll be held accountable for environmental problems on their watch, local governments still lack the tools to put those plans into action, Turner said.

"Every now and then, when they need blue skies for APEC or whatever, Beijing orders all the factories shut down," she said, referring to the Asian economic summit the Chinese hosted in 2015. "That's not really an air pollution control plan. We've seen some results. Air quality is starting to get a bit better. But the process is going to take more time."

China is also the world's No. 1 source of the carbon emissions that fuel global warming. But under the Paris accord aimed at limiting climate change, it has promised to start reducing emissions by 2030 and has been pouring money into renewable energy. Those efforts may also help reduce pollution by cutting into the country's reliance on coal, said Eri Saikawa, an atmospheric chemist at Emory University in Atlanta who researches China's emissions.

"They've done a lot in terms of renewable energy, and in trying to create very stringent policies," Saikawa said. But while China has enormous capacity for wind, solar and hydroelectric power, coal-fueled power plants remain cheaper.

"They have to do something to make better use of the capacity they have and that's more of an economic problem or a problem related to employment," she said. "I think if they solve that problem there's going to have less coal-based emissions and that's going to have a pretty big impact on air pollution."

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