Unexpected atmospheric chemistry may explain an air pollution mystery

Air pollution is a major cause of death around the world, and its intricate chemistry is still a puzzle. Exposure to the fine particles in haze killed an estimated 3 to 4.3 million people in 2015 (Lancet 2017, DOI: 10.1016/S0140-6736(17)32345-0. Researchers know a lot about how to control haze production, and their work has led to life-saving emissions restrictions around the world. But making further progress is challenging because atmospheric chemists still don’t fully understand how haze forms.

New research on the chemistry of haze around Beijing provides some much needed insight. This work, which points to a critical role for the pollutant black carbon, could support new regulation to further improve air quality in China and other industrializing countries (Proc. Natl. Acad. Sci. U.S.A. 2020, DOI: 10.1073/pnas.1919343117).

“Understanding this chemistry will help solve the problems” of persistent haze in China and in other places including India, says Renyi Zhang, an atmospheric chemist at Texas A&M University who is one of the study’s leaders.

China’s efforts to improve air quality by regulating power plants and other emitters are paying off. Since restrictions went into effect in 2013, regions including Beijing have experienced more clean air days. But severe haze events still happen, and moderately hazy days—which can cause health problems and have potential effects on the climate—have not decreased in frequency.

In China, one of the primary precursors to haze is sulfur dioxide, which is emitted by coal-burning power plants. SO₂ reacts in the atmosphere to produce sulfates, one of haze’s main components. China’s emissions regulations have effectively reduced levels of SO₂ from about 50 ppb to just a few ppb, Zhang says. So atmospheric chemists have been surprised by the persistence of moderate and severe haze events. Most chemical explanations for why these events have continued don’t fully account for the conditions in Beijing, Zhang says. For example, aqueous chemistry that can happen in and on fog particles can produce sulfate haze, but the air over Beijing is not moist enough.

So Zhang and his collaborators, including Mario Molina, a University of California, San Diego chemist who won the 1995 Nobel Prize in Chemistry for his work on the ozone hole, turned to another pollution player: black carbon. Emissions of these dark, charcoal-like particles have not decreased in China like SO₂ has. “We speculated that black carbon may serve as a catalyst,” Zhang says.

Zhang and his team simulated in the lab the chemistry, temperature, and humidity of the air in Beijing to investigate this idea. It was a good hunch: In the presence of NO₂ and ammonia, black carbon catalyzed the formation of sulfates—even at relatively low levels of SO₂. First, NO₂ reacts on the surface of black carbon particles to form nitrous acid (HONO). Ammonia stabilizes this surface interaction, enabling HONO to oxidize SO₂ to sulfates.

This basic set of reactions probably occurs around the world, Zhang says. But ammonia and black carbon levels in China are relatively high. Ammonia volatilizes from fertilizer applied by the country’s burgeoning agriculture industry. Black carbon is a byproduct of home-heating systems that use coal or firewood, which are common outside of China’s major cities; it’s also a product of diesel fuel combustion.

Beyond the public health implications, Zhang says this research may help illuminate the influence of haze particles on the climate. There’s some evidence that haze has a cooling effect, since many types of aerosols reflect sunlight. However, darker colored particles—black carbon–based ones in particular—absorb heat, potentially offsetting or outweighing the cooling effect. Zhang says his work suggests a net heating effect from this black carbon–catalyzed haze.

Given the global health burden of air pollution, “modeling particulate matter is one of the great challenges of the 21st century,” says Russell Dickerson, an atmospheric chemist at the University of Maryland. He says Zhang and Molina’s work is interesting from a basic chemistry perspective, but it also provides “actionable intelligence.” Dickerson notes that it’s already well known that regulating emissions of the pollutants at work in the novel sulfate cascade—particularly black carbon—will have direct public health benefits. The new research “adds urgency to policies that should already be on the way,” he says.