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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